During application by airplane excessive amounts of zinc phosphide were lost from the bait registered to control rat damage in Hawaiian sugarcane. The losses created unnecessary hazards and potentially reduced the efficacy of the control program. In a series of screening tests, alternate adhesives, adhesive concentrations, and bait mixing procedures were evaluated for zinc phosphide retention, acceptance by rats, phosphine residues in sugarcane and operational effectiveness. A formulation was developed that reduced zinc phosphide losses 32% during application, increased acceptance by rats, left residues in sugarcane below the established tolerance and equaled or exceeded the performance of the original formulation

Pank, Larry F.

English

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Proceedings of the 7th Vertebrate Pest Conference (1976) Vertebrate Pest Conference Proceedings collection March 1976 EFFECTS OF BAIT FORMULATIONS ON TOXICANT LOSSES AND EFFICACY Larry F. Pank U.S. Fish and Wildlife Service, Hilo, Hawaii Follow this and additional works at: https://digitalcommons.unl.edu/vpc7  Part of the Environmental Health and Protection Commons Pank, Larry F., "EFFECTS OF BAIT FORMULATIONS ON TOXICANT LOSSES AND EFFICACY" (1976). Proceedings of the 7th Vertebrate Pest Conference (1976). 37. https://digitalcommons.unl.edu/vpc7/37 This Article is brought to you for free and open access by the Vertebrate Pest Conference Proceedings collection at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Proceedings of the 7th Vertebrate Pest Conference (1976) by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. EFFECTS OF BAIT FORMULATIONS ON TOXICANT LOSSES AND EFFICACY LARRY F. PANK, Research Biologist, U.S. Fish and Wildlife Service, Hilo, Hawaii  96720 ABSTRACT:  During application by airplane excessive amounts of zinc phosphide were lost from the bait registered to control rat damage in Hawaiian sugarcane.  The losses created unnecessary hazards and potentially reduced the efficacy of the control program.  In a series of screening tests, alternate adhesives, adhesive concentrations, and bait mixing procedures were evaluated for zinc phosphide retention, acceptance by rats, phosphine residues in sugarcane and operational effectiveness.  A formulation was developed that reduced zinc phosphide losses 32% during application, increased acceptance by rats, left residues in sugarcane below the established tolerance and equalled or exceeded the performance of the original formulation. INTRODUCTION Infield aerial application of zinc phosphide-treated oat groats to control rats in Hawaiian sugarcane was registered in 1970, (EPA Reg. No. 10646-1). This bait was formulated with 2% zinc phosphide (94% A.I.), and 3% corn oil as an adhesive on crimped oat groats. Corn oil was selected as the adhesive because of its low cost, availability, biodegradable properties,   and good acceptance by the three pest species -- Polynesian rats, Rattus exulans; Norway rats,          R. norvegicus; and black rats, R. Rattus (Hilton, et al., 1972).  However, we discovered that up to 50% of the zinc phosphide was lost during the application of this bait (Hilton, Yauger, and Teshima, 1971).  The losses and drift of toxicant potentially posed a hazard and reduced efficacy of the treatment (Yauger, Teshima and Hilton, 1972). The objectives of this study were to develop a suitable replacement formulation with all     of the positive characteristics of the original (effective against the target species, negligible residues in cane 90 days after application, and effective reduction in cane damage), but alleviating the excessive losses of zinc phosphide during application. METHODS Screening Trials:  During these preliminary zinc phosphide retention studies, we subjectively evaluated 18 candidate adhesives, 2 to 6 adhesive concentrations, and 3 bait mixing procedures.  The test sequence included:  (l) hand mixing 200 g batches of each formulation, (2) air drying for 2 days, (3) sifting loose zinc phosphide fines through a 6  mesh/cm sieve onto white paper, and (4) visually ranking the loose fines from each formulation.  Formulations with the highest retention rates, compatible with existing mixing and application  equipment, were selected for further evaluation in simulated field blower trials. Simulated Field Blower Trials:  Batches of 9 kg of six candidate baits selected for advanced screening and two corn oil baits were formulated in a cement mixer, air dried for 2   days, sacked in plastic bags, and stored at room temperature.  After 7 days, 8 kg of each bait was passed through a shoulder-mounted powered blower and collected on cheesecloth; the remaining 1 kg was stored.  One kg of each formulation passed through the blower was reserved   for bioassays; the remaining 7 kg was distributed in covered 1 x 4 m soil flats for weathering trials. Formulations in each flat were then sprinkled with 3-8 cm of water; sprinkling was   repeated at 10-day intervals for 30 days.  Three randomly selected 10 g samples of each   formulation were taken:  (1) immediately after mixing, (2-3) after 7 and 87 days storage, (pre-blower sample), (4) immediately after blowing, and (5-7) after 10, 20, and 30 days weathering.  Zinc phosphide content was analyzed by the toluene-gas chromato-graphic procedure developed by   Robison and Hilton (1970) Bioassays:  The four most promising post-blower formulations from the simulated field trials were each bioassayed on 10 adult, wild trapped, rats of each species. The standard corn oil formulation and untreated oat groats were used as control formulations. Five of the individually caged rats in each test were prebaited with untreated oat groats daily for 3 days, five were not prebaited.  Based on average daily consumption, individuals of each species   (Polynesian, black, and Norway rats) were offered 5, 10 and 13 g of fresh bait daily for 3 days. 196 Bait consumption and mortality were recorded daily and rats were observed 7 more days for delayed reactions.  Laboratory chow ration and water were offered ad libitum during the test period.  ANOVA and multiple range tests were used to compare the data on consumption. Airplane Trials:  In this second series of field-oriented toxicant retention studies, three formulations yielding the most consistent results in previous experiments were again compared with the standard corn oil formulation.  Each formulation was prepared using procedures outlined for the Simulated Field Blower Trials.  Baits were applied by Stearman biplane equipped with a deflector-type fertilizer spreader (Nass, Hood and Lindsey, 1970) and collected from a 18 x 37 m tarp on the ground.  Pre- and post-application bait samples were collected and analyzed chemically to quantify losses of zinc phosphide during operational applications. Residues:  The most efficient formulation resulting from previous experiments (highest mortality in bioassays and maximum toxicant retention during application) was aerially applied  to 10-month old cane at the registered rate of 5.6 kg/ha.  Cane was hand harvested in three randomly located plots (3 m of cane row) one day before, immediately after, and 83 days after application.  The leaves were   raised and tied to the stems to hold any zinc phosphide particles lodged in the leaf axils.  Cane from each plot was ground up in an ensilage chopper,   mixed and a 2.3 kg sample was withdrawn.  Samples were analyzed for phosphine (Robison and Hilton, 1971) to determine if residues exceeded the established tolerance of 0.01 ppm 90 days after application. Operational Efficacy:  The effectiveness of the selected formulation in reducing rat damage was assessed in three 20 ha cane fields in which the registered formulation had been evaluated during the previous crop cycle.  All fields were planted to Variety 59-3775 and bordered by noncrop areas.  Each field was divided into an untreated reference block and a treated block.  Treated blocks were baited by aircraft at a rate of 5-6 kg of bait per ha when   the cane was 10, 13, 16, and 19 months old.  Rat damage was assessed at  harvest (22-23 months)   by the V-cut method (Hood, 1971) Fifty V-cut samples randomly taken in each reference and  treated block were examined to determine the number of rat damaged stalks per 100 five-foot   lengths of stalk.  Paired T-tests were used to determine if damage levels were significantly  different between the treated and reference areas for each formulation. RESULTS AND DISCUSSION Preliminary Screening:  Zinc phosphide losses from 18 different adhesives, each prepared   in 2-6 different concentrations and by 3 different mixing procedures, were compared during the sifting trials (Fig. 1).  The formulation producing the fewest zinc phosphide fines was  lecithin oil, followed in order, by baits formulated with Rhoplex AC-331 (Rohm and Haas Co.),   Dow Latex 512R (Dow Chemical Co.), drying oils (i.e., boiled linseed oil), hexadecanol (a   fatty alcohol), nondrying oils (i.e., macadamia nut oil) and fatty acids (i.e., lauric acid).     The highest concentration of each adhesive usually had the lowest losses; however, many of these remained tacky or soupy and were considered incompatible with the existing bait mixing and application equipment.  Of the mixing procedures tested:  1) predusting the zinc phosphide on   the oat groats before adding the adhesive, 2) adding an adhesive-zinc phosphide slurry to the  groats, or 3) adding the adhesive to the groats and postdusting the zinc phosphide (standard  procedure for the registered formulation), the first method had the lowest losses with nearly all adhesives. Retention rates for the six best formulations are compared with two corn oil formulations   in Figure 2.  All except the corn oil (3%) formulation were prepared by predusting the zinc phosphide on the oat groats and then adding the adhesive.  The lecithin oil consisted of 4 parts lecithin and 1 part white oil.  Alcohol was used to dissolve the hexadecanol and then evaporated off after mixing bait. The Rhoplex was added as a 1:9 AC-33 to water dilution.  The boiled linseed and coconut and corn oil formulations required no special preparation. Simulated Field Blower Trails: The replicated analyses of percent zinc phosphide remaining in each formulation during the various sampling periods were consistent (S.E. < 0.10%);   therefore, results are expressed as the mean percent reduction in concentration (Figs. 2, 3). 1Use of trade names in this publication does not imply endorsement of commercial products by    the federal government.  197  Figure 2.  Zinc phosphide losses from 8 adhesive formulations during mixing and storage.                                             198 Figure 3.  Zinc phosphide losses from 8 adhesive formulations during simulated application and weathering trials. Zinc phosphide losses during the mixing process were excessive with 3% hexadecanol and the 3% corn oil formulations (Fig. 2).  Losses were related, respectively, to the evaporation of alcohol for the hexadecanol formulation on contact with the mixer wall and the postdusting of zinc phosphide on mixer walls already coated with corn oil.  Short-term storage losses were   lowest with the hexadecanol formulation.  The lecithin (3%) formulation had the lowest losses   during long-term storage (Fig. 2).  All formulations retained greater amounts of zinc phosphide than the registered corn oil formulation during the blower trials (Fig. 3).  The lecithin oil formulations lost only 2.1 percent of the zinc phosphide compared to losses of 27-1% with the corn oil formulation.  The lecithin oil formulations improved retention during application by 92   percent.  Linseed oil also substantially improved retention. Major losses of zinc phosphide from all formulations occurred during the first 10 days of artificial weathering (Fig. 3).  Cumulative losses over 30 days were highest with the 3% lecithin oil and 6% linseed oil formulations.  In contrast, concentrations of zinc phosphide on   the Rhoplex formulation declined very little after the first 10 days of weathering.  Zinc phosphide breaks down rapidly when in contact with moist Hawaiian soils (Hilton and Robison,   1972); therefore, nontarget hazards and residue levels associated with food-crop applications   are primarily related to the persistence of zinc phosphide on baits and decomposition of the oat groats.  Under these conditions, a bait that withstood weathering for 5-10 days and then rapidly broke down was preferred, however, none weathered in this manner.  In terms of residues and nontarget hazards, the 3% lecithin oil and 6% linseed oil formulations were the safest and the 8% Rhoplex formulation potentially the most hazardous. 199 Bioassay: The 3% lecithin oil, 6% linseed oil and 6% coconut oil formulations were selected for bioassays.  Due to the superiority of the lecithin oil formulation, three additional lecithin products (Alcolec S, SLV-42 and 211, American Lecithin Co.) were compared using procedures outlined in the preliminary screening trials.  Of these, a 2% Alcolec S formulation had less sifting losses than the 3% lecithin oil formulation and was included in the series. Mean day 1 consumption by all species was significantly less with formulated vs. untreated oat groats (Table 1).  Prebaiting increased consumption of nearly all formulations by all species.  The Alcolec S formulation was consumed in significantly (P<0.05) greater amounts by non-prebaited and prebaited Norway rats and, excluding the corn oil formulation, by prebaited black rats.  Mean consumption of all formulations over 3 days was proportionately similar to the first day for all species.  The Alcolec S, corn oil and lecithin formulations had the lowest percentage of noneaters and linseed oil had the highest.  Although the Alcolec S formulation offers some improvement in bait acceptance over the corn oil formulation, differences were of minor importance.  Of greater importance, the principal differences between adhesive formulations were related to zinc phosphide concentrations on the post-blower samples and are reflected in the amount of zinc phosphide consumed per rat (Table 1).  The LD50'S (95% C.L.) in mg/kg of zinc phosphide for Polynesian, black and Norway rats were 23.1 (6.3-33.4), 21.3 (19.3-23.9) and 27.0 (12.0-61.0), respectively. The number of LD50's consumed by all species was significantly (P<0.05) higher with the Alcolec S formulation than all others in four of six comparisons.  It was the only formulation producing 100 percent mortality among both non-prebaited and prebaited rats of all species (Table 1).  It was exceeded only by lecithin oil when fed to prebaited Polynesian rats. Airplane Trials:  The Alcolec S formulation reduced zinc phosphide losses 92 percent during aerial application when compared to the corn oil formulation (Table 2).  Retention of zinc phosphide with the Alcolec S formulation was also better than both the lecithin and linseed oil formulations.  The mixing time required to obtain a uniform bait with the Alcolec S formulation was approximately double that required for the corn oil formulation, and with repetitive mixing, residues tended to harden and accumulate in the mixer.  These disadvantages were considered acceptable when compared to the benefits of improved retention, acceptance and mortality.  Although storage losses were not determined for the Alcolec S formulation, they are expected to be similar to the lecithin oil formulation.  Operationally, bait is usually applied within 1 to 2 days of mixing thus minimizing the importance of storage losses. Residues:  No phosphine residues in sugarcane were detected (<0.001 ppm) one day before application.  Residues averaged 0.087 ppm (range = 0.072-0.104) immediately after the application of Alcolec S bait but dropped to 0.001 ppm 83 days later.  Residues immediately after application were attributed to bait lodged in the leaf axils.  Residues after 83 days were well below the established tolerance of 0.01 ppm phosphine 90 days after application. Efficacy:  Rat damage to sugarcane at harvest in the three Alcolec S-zinc phosphide-treated blocks was 68, 49 and 63 percent less than in the paired untreated blocks (Table 3).The corn oil formulation applied on the same treated blocks during the preceding crop cycle reduced damage 36, 43 and 69 percent, respectively.  A paired t-test showed that differences in damage between the treated and untreated blocks were significant (P<0.05) for both formulations.  Damage was reduced an average of 60 percent with the Alcolec S formulation and 49 percent with the corn oil formulation: however, we cannot show a significant difference between the two formulations (Table 3). CONCLUSION The zinc phosphide - Alcolec S formulation is an effective substitute for the original corn oil formulation--losses of zinc phosphide during operational application were reduced 92%.  Acceptance and mortality increased during laboratory bioassays with all species of rats, reductions in damage equalled or exceeded levels achieved with the corn oil formulation, and phosphine residues remained below the established tolerance.  Following this series of tests, the original corn oil registration was amended to substitute Alcolec S for operational use. 200                                                                             201Table 2. Zinc phosphide losses from four adhesive formulations during application by a irplane.  Concentra- Percent  zinc phosphide PercentAdhesive tion (%) Mixing order Pre-drop Post-drop loss Alcolec  S 2 Post-dust 1.90 1.83 4 Lecithin  oil 3  ″         ″ 1.90 1.54 19 Linseed oil 5  ″         ″ 1.88 1.25 34 Corn  oil 3 Pre-dust 1.83 0.95 48 Table 3. Comparative operational efficacy of the corn oil and Alcolec S formulations of z inc phosphide treated oat groats.    Mean   percent stalks   per (S.E.)V-cutdamaged sample Field Zn 3 P2 n Corn  oil n Alcolec  S 43B Treated Untreated 75 75 3.88(0.31)      6.94(0.39) 50 502.18(0.35) 6.73(0.73)  51A % reduction     Treated         Untreated 75 75     35.8 6.41(0.40) 11.20(0.61) 50 5067.6 4.10(0.41)   8.08(0.61)  61B % reduction Treated Untreated 75 75    42.7 2.72(0.27) 8.87(0.82) 505049.3 3.10(0.32) 8.32(0.53)         % reduction  Mean percent damage reduction  69.3            49.3  62.7 59.9 ACKNOWLEDGMENTS Contributors to this study include personnel of the Wildlife Damage Research Station, Hawaiian Sugar Planters' Association, Mauna Kea Sugar Company, Murrayair, Ltd., and especially R.A. Wilson, U.S. Fish and Wildlife Service, Denver Wildlife Research Center and H.W. Hilton, Hawaiian Sugar Planters’ Association. LITERATURE CITED HILTON, W.H., W.L. YAUGER and A. TESHIMA. 1971. Drift of zinc phosphide "fines" as a function of bait formulation.  In 1971 Annual Report Exp. Sta. Hawaiian Sugar Planters’ Ass'n. pp. 48.          ., W.H. ROBISON, A.H.TESHIMA and R.D. NASS. 1972. Zinc phosphide as a rodenticide for rats in  Hawaiian sugarcane.  In_ Proc. 14th Congress, Int. Soc. Sugar Cane Technol. New Orleans, La., Nov. 1971. pp 561 - 570.           ____., and   . 1972. Fate of zinc phosphide in the soil-water environment. J. Agr. and Food Chem., Vol. 20(6): 1209-1213.  HOOD, G.A. 1968. Estimating rat damage to sugarcane. In Proc. Hawaiian Sugar Technol. Conf.  27:40-44.  ROBISON, W.H. and H.W. HILTON. 1971.  Gas chromatography of phosphine derived from zinc phosphide in sugarcane. J. Agr. and Food Chem. 19(5)-875-878.  YAUGER, W.L., A.H. TESHIMA and W.H. HILTON. 1972. Zinc phosphide drift studies. In 1972 Annual Report, Exp. Sta. Hawaiian Sugar Planters’ Ass’n. pp. 39-40.                                                 202 

EFFECTS OF BAIT FORMULATIONS ON TOXICANT LOSSES AND EFFICACY

UNL | Libraries

University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
Proceedings of the 7th Vertebrate Pest 
Conference (1976) 
Vertebrate Pest Conference Proceedings 
collection 
March 1976 
EFFECTS OF BAIT FORMULATIONS ON TOXICANT LOSSES AND 
EFFICACY 
Larry F. Pank 
U.S. Fish and Wildlife Service, Hilo, Hawaii 
Follow this and additional works at: https://digitalcommons.unl.edu/vpc7 
 Part of the Environmental Health and Protection Commons 
Pank, Larry F., "EFFECTS OF BAIT FORMULATIONS ON TOXICANT LOSSES AND EFFICACY" (1976). 
Proceedings of the 7th Vertebrate Pest Conference (1976). 37. 
https://digitalcommons.unl.edu/vpc7/37 
This Article is brought to you for free and open access by the Vertebrate Pest Conference Proceedings collection at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Proceedings of the 7th 
Vertebrate Pest Conference (1976) by an authorized administrator of DigitalCommons@University of Nebraska - 
Lincoln. 
EFFECTS OF BAIT FORMULATIONS ON TOXICANT LOSSES AND EFFICACY 
LARRY F. PANK, Research Biologist, U.S. Fish and Wildlife Service, Hilo, Hawaii  96720 
ABSTRACT:  During application by airplane excessive amounts of zinc phosphide were lost from 
the bait registered to control rat damage in Hawaiian sugarcane.  The losses created 
unnecessary hazards and potentially reduced the efficacy of the control program.  In a series 
of screening tests, alternate adhesives, adhesive concentrations, and bait mixing procedures 
were evaluated for zinc phosphide retention, acceptance by rats, phosphine residues in 
sugarcane and operational effectiveness.  A formulation was developed that reduced zinc 
phosphide losses 32% during application, increased acceptance by rats, left residues in 
sugarcane below the established tolerance and equalled or exceeded the performance of the 
original formulation. 
INTRODUCTION 
Infield aerial application of zinc phosphide-treated oat groats to control rats in 
Hawaiian sugarcane was registered in 1970, (EPA Reg. No. 10646-1). This bait was formulated with 
2% zinc phosphide (94% A.I.), and 3% corn oil as an adhesive on crimped oat groats. Corn oil was 
selected as the adhesive because of its low cost, availability, biodegradable properties,   and 
good acceptance by the three pest species -- Polynesian rats, Rattus exulans; Norway rats,          
R. norvegicus; and black rats, R. Rattus (Hilton, et al., 1972).  However, we discovered that up 
to 50% of the zinc phosphide was lost during the application of this bait (Hilton, Yauger, and 
Teshima, 1971).  The losses and drift of toxicant potentially posed a hazard and reduced 
efficacy of the treatment (Yauger, Teshima and Hilton, 1972). 
The objectives of this study were to develop a suitable replacement formulation with all     
of the positive characteristics of the original (effective against the target species, 
negligible residues in cane 90 days after application, and effective reduction in cane 
damage), but alleviating the excessive losses of zinc phosphide during application. 
METHODS 
Screening Trials:  During these preliminary zinc phosphide retention studies, we 
subjectively evaluated 18 candidate adhesives, 2 to 6 adhesive concentrations, and 3 bait 
mixing procedures.  The test sequence included:  (l) hand mixing 200 g batches of each 
formulation, (2) air drying for 2 days, (3) sifting loose zinc phosphide fines through a 6  
mesh/cm sieve onto white paper, and (4) visually ranking the loose fines from each 
formulation.  Formulations with the highest retention rates, compatible with existing mixing 
and application  equipment, were selected for further evaluation in simulated field blower 
trials. 
Simulated Field Blower Trials:  Batches of 9 kg of six candidate baits selected for 
advanced screening and two corn oil baits were formulated in a cement mixer, air dried for 2   
days, sacked in plastic bags, and stored at room temperature.  After 7 days, 8 kg of each bait 
was passed through a shoulder-mounted powered blower and collected on cheesecloth; the 
remaining 1 kg was stored.  One kg of each formulation passed through the blower was reserved   
for bioassays; the remaining 7 kg was distributed in covered 1 x 4 m soil flats for weathering 
trials. Formulations in each flat were then sprinkled with 3-8 cm of water; sprinkling was   
repeated at 10-day intervals for 30 days.  Three randomly selected 10 g samples of each   
formulation were taken:  (1) immediately after mixing, (2-3) after 7 and 87 days storage, (pre-
blower sample), (4) immediately after blowing, and (5-7) after 10, 20, and 30 days weathering.  
Zinc phosphide content was analyzed by the toluene-gas chromato-graphic procedure developed by   
Robison and Hilton (1970) 
Bioassays:  The four most promising post-blower formulations from the simulated field 
trials were each bioassayed on 10 adult, wild trapped, rats of each species. The standard corn 
oil formulation and untreated oat groats were used as control formulations. Five of the 
individually caged rats in each test were prebaited with untreated oat groats daily for 3 days, 
five were not prebaited.  Based on average daily consumption, individuals of each species   
(Polynesian, black, and Norway rats) were offered 5, 10 and 13 g of fresh bait daily for 3 days. 
196 
Bait consumption and mortality were recorded daily and rats were observed 7 more days for 
delayed reactions.  Laboratory chow ration and water were offered ad libitum during the test 
period.  ANOVA and multiple range tests were used to compare the data on consumption. 
Airplane Trials:  In this second series of field-oriented toxicant retention studies, three 
formulations yielding the most consistent results in previous experiments were again compared 
with the standard corn oil formulation.  Each formulation was prepared using procedures outlined 
for the Simulated Field Blower Trials.  Baits were applied by Stearman biplane equipped with a 
deflector-type fertilizer spreader (Nass, Hood and Lindsey, 1970) and collected from a 18 x 37 m 
tarp on the ground.  Pre- and post-application bait samples were collected and analyzed 
chemically to quantify losses of zinc phosphide during operational applications. 
Residues:  The most efficient formulation resulting from previous experiments (highest 
mortality in bioassays and maximum toxicant retention during application) was aerially applied  
to 10-month old cane at the registered rate of 5.6 kg/ha.  Cane was hand harvested in three 
randomly located plots (3 m of cane row) one day before, immediately after, and 83 days after 
application.  The leaves were   raised and tied to the stems to hold any zinc phosphide 
particles lodged in the leaf axils.  Cane from each plot was ground up in an ensilage chopper,   
mixed and a 2.3 kg sample was withdrawn.  Samples were analyzed for phosphine (Robison and 
Hilton, 1971) to determine if residues exceeded the established tolerance of 0.01 ppm 90 days 
after application. 
Operational Efficacy:  The effectiveness of the selected formulation in reducing rat 
damage was assessed in three 20 ha cane fields in which the registered formulation had been 
evaluated during the previous crop cycle.  All fields were planted to Variety 59-3775 and 
bordered by noncrop areas.  Each field was divided into an untreated reference block and a 
treated block.  Treated blocks were baited by aircraft at a rate of 5-6 kg of bait per ha when   
the cane was 10, 13, 16, and 19 months old.  Rat damage was assessed at  harvest (22-23 months)   
by the V-cut method (Hood, 1971) Fifty V-cut samples randomly taken in each reference and  
treated block were examined to determine the number of rat damaged stalks per 100 five-foot   
lengths of stalk.  Paired T-tests were used to determine if damage levels were significantly  
different between the treated and reference areas for each formulation. 
RESULTS AND DISCUSSION 
Preliminary Screening:  Zinc phosphide losses from 18 different adhesives, each prepared   
in 2-6 different concentrations and by 3 different mixing procedures, were compared during the 
sifting trials (Fig. 1).  The formulation producing the fewest zinc phosphide fines was  
lecithin oil, followed in order, by baits formulated with Rhoplex AC-331 (Rohm and Haas Co.),   
Dow Latex 512R (Dow Chemical Co.), drying oils (i.e., boiled linseed oil), hexadecanol (a   
fatty alcohol), nondrying oils (i.e., macadamia nut oil) and fatty acids (i.e., lauric acid).     
The highest concentration of each adhesive usually had the lowest losses; however, many of these 
remained tacky or soupy and were considered incompatible with the existing bait mixing and 
application equipment.  Of the mixing procedures tested:  1) predusting the zinc phosphide on   
the oat groats before adding the adhesive, 2) adding an adhesive-zinc phosphide slurry to the  
groats, or 3) adding the adhesive to the groats and postdusting the zinc phosphide (standard  
procedure for the registered formulation), the first method had the lowest losses with nearly 
all adhesives. 
Retention rates for the six best formulations are compared with two corn oil formulations   
in Figure 2.  All except the corn oil (3%) formulation were prepared by predusting the zinc 
phosphide on the oat groats and then adding the adhesive.  The lecithin oil consisted of 4 
parts lecithin and 1 part white oil.  Alcohol was used to dissolve the hexadecanol and then 
evaporated off after mixing bait. The Rhoplex was added as a 1:9 AC-33 to water dilution.  The 
boiled linseed and coconut and corn oil formulations required no special preparation. 
Simulated Field Blower Trails: The replicated analyses of percent zinc phosphide remaining 
in each formulation during the various sampling periods were consistent (S.E. < 0.10%);   
therefore, results are expressed as the mean percent reduction in concentration (Figs. 2, 3). 
1Use of trade names in this publication does not imply endorsement of commercial products by    
the federal government. 
 
197 
 Figure 2.  Zinc phosphide losses from 8 adhesive formulations during mixing and storage. 
                                            198 
Figure 3.  Zinc phosphide losses from 8 adhesive formulations during simulated application and 
weathering trials. 
Zinc phosphide losses during the mixing process were excessive with 3% hexadecanol and the 
3% corn oil formulations (Fig. 2).  Losses were related, respectively, to the evaporation of 
alcohol for the hexadecanol formulation on contact with the mixer wall and the postdusting of 
zinc phosphide on mixer walls already coated with corn oil.  Short-term storage losses were   
lowest with the hexadecanol formulation.  The lecithin (3%) formulation had the lowest losses   
during long-term storage (Fig. 2).  All formulations retained greater amounts of zinc phosphide 
than the registered corn oil formulation during the blower trials (Fig. 3).  The lecithin oil 
formulations lost only 2.1 percent of the zinc phosphide compared to losses of 27-1% with the 
corn oil formulation.  The lecithin oil formulations improved retention during application by 92   
percent.  Linseed oil also substantially improved retention. 
Major losses of zinc phosphide from all formulations occurred during the first 10 days of 
artificial weathering (Fig. 3).  Cumulative losses over 30 days were highest with the 3% 
lecithin oil and 6% linseed oil formulations.  In contrast, concentrations of zinc phosphide on   
the Rhoplex formulation declined very little after the first 10 days of weathering.  Zinc 
phosphide breaks down rapidly when in contact with moist Hawaiian soils (Hilton and Robison,   
1972); therefore, nontarget hazards and residue levels associated with food-crop applications   
are primarily related to the persistence of zinc phosphide on baits and decomposition of the oat 
groats.  Under these conditions, a bait that withstood weathering for 5-10 days and then rapidly 
broke down was preferred, however, none weathered in this manner.  In terms of residues and 
nontarget hazards, the 3% lecithin oil and 6% linseed oil formulations were the safest and the 
8% Rhoplex formulation potentially the most hazardous. 
199 
Bioassay: The 3% lecithin oil, 6% linseed oil and 6% coconut oil formulations were selected 
for bioassays.  Due to the superiority of the lecithin oil formulation, three additional 
lecithin products (Alcolec S, SLV-42 and 211, American Lecithin Co.) were compared using 
procedures outlined in the preliminary screening trials.  Of these, a 2% Alcolec S formulation 
had less sifting losses than the 3% lecithin oil formulation and was included in the series. 
Mean day 1 consumption by all species was significantly less with formulated vs. untreated 
oat groats (Table 1).  Prebaiting increased consumption of nearly all formulations by all 
species.  The Alcolec S formulation was consumed in significantly (P<0.05) greater amounts by 
non-prebaited and prebaited Norway rats and, excluding the corn oil formulation, by prebaited 
black rats.  Mean consumption of all formulations over 3 days was proportionately similar to the 
first day for all species.  The Alcolec S, corn oil and lecithin formulations had the lowest 
percentage of noneaters and linseed oil had the highest.  Although the Alcolec S formulation 
offers some improvement in bait acceptance over the corn oil formulation, differences were of 
minor importance.  Of greater importance, the principal differences between adhesive formulations 
were related to zinc phosphide concentrations on the post-blower samples and are reflected in the 
amount of zinc phosphide consumed per rat (Table 1).  The LD50'S (95% C.L.) in mg/kg of zinc 
phosphide for Polynesian, black and Norway rats were 23.1 (6.3-33.4), 21.3 (19.3-23.9) and 27.0 
(12.0-61.0), respectively. The number of LD50's consumed by all species was significantly 
(P<0.05) higher with the Alcolec S formulation than all others in four of six comparisons.  It 
was the only formulation producing 100 percent mortality among both non-prebaited and prebaited 
rats of all species (Table 1).  It was exceeded only by lecithin oil when fed to prebaited 
Polynesian rats. 
Airplane Trials:  The Alcolec S formulation reduced zinc phosphide losses 92 percent during 
aerial application when compared to the corn oil formulation (Table 2).  Retention of zinc 
phosphide with the Alcolec S formulation was also better than both the lecithin and linseed oil 
formulations.  The mixing time required to obtain a uniform bait with the Alcolec S formulation 
was approximately double that required for the corn oil formulation, and with repetitive mixing, 
residues tended to harden and accumulate in the mixer.  These disadvantages were considered 
acceptable when compared to the benefits of improved retention, acceptance and mortality.  
Although storage losses were not determined for the Alcolec S formulation, they are expected to 
be similar to the lecithin oil formulation.  Operationally, bait is usually applied within 1 to 2 
days of mixing thus minimizing the importance of storage losses. 
Residues:  No phosphine residues in sugarcane were detected (<0.001 ppm) one day before 
application.  Residues averaged 0.087 ppm (range = 0.072-0.104) immediately after the application 
of Alcolec S bait but dropped to 0.001 ppm 83 days later.  Residues immediately after application 
were attributed to bait lodged in the leaf axils.  Residues after 83 days were well below the 
established tolerance of 0.01 ppm phosphine 90 days after application. 
Efficacy:  Rat damage to sugarcane at harvest in the three Alcolec S-zinc phosphide-treated 
blocks was 68, 49 and 63 percent less than in the paired untreated blocks (Table 3).The corn oil 
formulation applied on the same treated blocks during the preceding crop cycle reduced damage 
36, 43 and 69 percent, respectively.  A paired t-test showed that differences in damage between 
the treated and untreated blocks were significant (P<0.05) for both formulations.  Damage was 
reduced an average of 60 percent with the Alcolec S formulation and 49 percent with the corn oil 
formulation: however, we cannot show a significant difference between the two formulations 
(Table 3). 
CONCLUSION 
The zinc phosphide - Alcolec S formulation is an effective substitute for the original corn 
oil formulation--losses of zinc phosphide during operational application were reduced 92%.  
Acceptance and mortality increased during laboratory bioassays with all species of rats, 
reductions in damage equalled or exceeded levels achieved with the corn oil formulation, and 
phosphine residues remained below the established tolerance.  Following this series of tests, the 
original corn oil registration was amended to substitute Alcolec S for operational use. 
200
 
 
 
                                                                          201
Table 2. Zinc phosphide losses from four adhesive formulations during application by 
a irplane. 
 Concentra- Percent  zinc phosphide Percent
Adhesive tion (%) Mixing order Pre-drop Post-drop loss 
Alcolec  S 2 Post-dust 1.90 1.83 4 
Lecithin  oil 3  ″         ″ 1.90 1.54 19 
Linseed oil 5  ″         ″ 1.88 1.25 34 
Corn  oil 3 Pre-dust 1.83 0.95 48 
Table 3. Comparative operational efficacy of the corn oil and Alcolec S formulations of 
z inc phosphide treated oat groats. 
   Mean   percent 
stalks   per 
(S.E.)
V-cut
damaged 
sample 
Field Zn 3 P2 n Corn  oil n Alcolec  S 
43B Treated 
Untreated 
75 
75 
3.88(0.31)      
6.94(0.39) 
50 
50
2.18(0.35) 
6.73(0.73) 
 
51A 
% reduction 
    Treated         
Untreated 
75 
75 
    35.8 
6.41(0.40) 
11.20(0.61) 
50 
50
67.6 
4.10(0.41)   
8.08(0.61) 
 
61B 
% reduction 
Treated 
Untreated 
75 
75 
   42.7 
2.72(0.27) 
8.87(0.82) 
50
50
49.3 
3.10(0.32) 
8.32(0.53) 
 
 
 
 
    % reduction  
Mean percent damage reduction 
 69.3            
49.3 
 62.7 
59.9 
ACKNOWLEDGMENTS 
Contributors to this study include personnel of the Wildlife Damage Research Station, 
Hawaiian Sugar Planters' Association, Mauna Kea Sugar Company, Murrayair, Ltd., and 
especially R.A. Wilson, U.S. Fish and Wildlife Service, Denver Wildlife Research Center and 
H.W. Hilton, Hawaiian Sugar Planters’ Association. 
LITERATURE CITED 
HILTON, W.H., W.L. YAUGER and A. TESHIMA. 1971. Drift of zinc phosphide "fines" as a 
function of bait formulation.  In 1971 Annual Report Exp. Sta. Hawaiian Sugar 
Planters’ Ass'n. pp. 48. 
         ., W.H. ROBISON, A.H.TESHIMA and R.D. NASS. 1972. Zinc phosphide as a 
rodenticide for rats in  Hawaiian sugarcane.  In_ Proc. 14th Congress, Int. Soc. 
Sugar Cane Technol. New Orleans, La., Nov. 1971. pp 561 - 570. 
          ____., and   . 1972. Fate of zinc phosphide in the soil-water environment. 
J. Agr. and Food Chem., Vol. 20(6): 1209-1213.  
HOOD, G.A. 1968. Estimating rat damage to sugarcane. In Proc. Hawaiian Sugar Technol. 
Conf.  27:40-44.  
ROBISON, W.H. and H.W. HILTON. 1971.  Gas chromatography of phosphine derived from zinc 
phosphide in sugarcane. J. Agr. and Food Chem. 19(5)-875-878.  
YAUGER, W.L., A.H. TESHIMA and W.H. HILTON. 1972. Zinc phosphide drift studies. In 1972 
Annual Report, Exp. Sta. Hawaiian Sugar Planters’ Ass’n. pp. 39-40. 
 
                                               202 


During application by airplane, excessive amounts of zinc phosphide were lost from the bait registered to control rat damage in Hawaiian sugarcane. The losses created unnecessary hazards and potentially reduced the efficacy of the control program. In a series of screening tests, alternate adhesives, adhesive concentrations, and bait mixing procedures were evaluated for zinc phosphide retention, acceptance by rats, phosphine residues in sugarcane and operational effectiveness. A formulation was developed that reduced zinc phosphide losses 92% during application, increased acceptance by rats, left residues in sugarcane below the established tolerance, and equaled or exceeded the performance of the original formulation

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Effects of bait formulations on toxicant losses and efficacy

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EFFECTS OF BAIT FORMULATIONS ON TOXICANT LOSSES AND EFFICACY

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