This paper presents a sampling procedure for estimating adult populations of the alfalfa blotch leafminer, Agromyza frontella (Rondani). The method is based on counts of the flies as they emerge from the soil following adult metamorphosis, taken in a series offurmel traps. Analysis of sampling variability showed that 80 traps per field will give adequate precision for life table studies in alfalfa. The pattern of counts was overdispersed but conformed to the negative binomial distribution

Harcourt, D. G

English

ValpoScholar

The Great Lakes Entomologist 
Volume 15 
Number 1 - Spring 1982 Number 1 - Spring 1982 Article 8 
April 1982 
Population Assessment During the Adult Stage of the Alfalfa 
Blotch Leafminer, Agromyza Frontella (Diptera: Agromyzidae) 
D. G. Harcourt 
Agriculture Canada 
Follow this and additional works at: https://scholar.valpo.edu/tgle 
 Part of the Entomology Commons 
Recommended Citation 
Harcourt, D. G. 1982. "Population Assessment During the Adult Stage of the Alfalfa Blotch Leafminer, 
Agromyza Frontella (Diptera: Agromyzidae)," The Great Lakes Entomologist, vol 15 (1) 
Available at: https://scholar.valpo.edu/tgle/vol15/iss1/8 
This Peer-Review Article is brought to you for free and open access by the Department of Biology at ValpoScholar. 
It has been accepted for inclusion in The Great Lakes Entomologist by an authorized administrator of ValpoScholar. 
For more information, please contact a ValpoScholar staff member at scholar@valpo.edu. 
1982 THE GREAT LAKES ENTOMOLOGIST 49 
POPULATION ASSESSMENT DURING THE ADULT STAGE 

OF 
THE ALFALFA BLOTCH LEAFMINER, 
AGROMYZA FRONTELLA 

(DIPTERA: AGROMYZIDAE)1 

D. G. Harcourt2 
ABSTRACT 
This paper presents a sampling procedure for estimating adult populations of the alfalfa 
blotch leafminer, Agromyza frontella (Rondani). The method is based on counts f the flies 
as they emerge from the soil following adult metamorphosis, taken in a series offurmel traps. 
Analysis of sampling variability showed that 80 traps per field will give adequate precision 
for life table studies in alfalfa. The pattern of counts was overdispersed but conformed to the 
negative binomial distribution. 
This paper is the third in a series dealing with sampling procedures for natural populations 
of the 
alfalfa blotch leaf miner, 
Agromyza frontella (Rondani) in fields of alfalfa. Earlier 
papers considered population assessment of the egg and larva! stages which occur on the 
plant (Harcourt and Birms 1980a), and the prepupal and pupal stages which are soil-borne 
(Harcourt and Binns 1980b). This article describes a method for estimating numbers of 
adults as they emerge from the soil, and the analysis of data leading to development of a 
reliable sampling plan for this stage. The technique is so designed that the data obtained can 
be integrated directly into life tables. 
The 
life history 
of the leafininer has been described by Guppy (1981). In eastern Ontario 
there are typically three generations a year. Briefly, the adults emerge from hibernation i  
early spring and deposit their eggs within the alfalfa leaflets. The larvae feed between the 
epidermal tissues, developing a characteristic blotch-type mine during the course of three 
instars. The mature larvae emerge from the min d leaflets and drop to the ground where they 
pupate just beneath the soil surface. The insect overwinters as a partially developed pupa 
that 
completes its development in early spring. Peaks of adult emergence occur in late May, 
early JUly, and early August. 
EXPERIMENTAL PLOTS 
The 
data were collected during 1980-1981 in conjunction with life table studies ofthe pest 
at 
two locations in eastern Ontario, the Central Experimental Farm, 
Ottawa, and the College 
of 
Agricultural Technology, Kemptville. The fields, each 
of which comprised a 0.4 ha (I ac) 
stand of 
pure Vernal alfalfa, were situated 
in flat terrain and contained an average of 440 
stems per m2 during the course of the study. Soil types ranged from clay loam to sandy loam. 
SAMPLING METHODS 
The adults were trapped as they emerged from the soil in each generation. Following 
harvest of 
the crop, 
an area measuring 30 x 30 m (100 x 100 ft) was marked off within each 
field and divided into four blocks of equal size. These were further subdivided into four 
equal plots and four traps were placed at random within each plot, making a total of 64. 
The traps comprised 22-cm diameter plastic funnels3 fitted with 24-oz screw-cap styrene 
jars as 
shown in Figure I. These were inverted over the crop stubble and secured by a nail 
IContribution No. 658. Ottawa Research Station, Agriculture Canada, Ottawa. 

20
ttawa Research Station, Agriculture Canada, Ottawa, Ontario, Canada 
KIA OC6. 

3Bow Plastics Limited. Granby, Quebec. 

1
Harcourt: Population Assessment During the Adult Stage of the Alfalfa Blotc
Published by ValpoScholar, 1982
50 THE GREAT LAKES ENTOMOLOGIST Vol. 15, No. I 
r---- 2 em hole 
T 
8cm I1 5cm 
em 
FUNNEL­ J 
~ 
---TAB 
em 
• ! 
Fig. I. Emergence traps used for A. fromella adults. The flies are trapped in ethylene glycol contained  
the lid of the jar. The union between lid and funnel is sealed with plastic glue. 
driven through the funnel tab. A thin layer of ethylene glycol was poured into the lid of each 
jar 
to trap and preserve the flies as they emerged. The basal area covered by the trap was 
0.034 
m2 (0.375 ft2). 
In preliminary studies during 1979, the funnel traps were compared for efficiency to larger 
pyramid traps (0.83 m2) previously used at Ottawa for emergence of the cereal leaf beetle, 
Oulema melanopus (L.), and its parasites (for description, see Gage and Haynes, (975). 
Data 
for 
17 sampling dates on which 32 traps of each size were compared showed that 
relative efficiency, defined as the reciprocal of variance per m2 , was 2.5 times greater for the 
smaller trap than for the larger. 
The 
flies were counted 
at weekly intervals during peak periods of emergence. At sam­
pling, the jars were removed and the flies separated from the liquid in the lids by means of a 
small art brush. An average of 1.5 min was required to count and remove the contents of 
each 
trap. In 
the two years, numbers 
of emerging adults were recorded on a total of 35 occasions. 
The 
totals for individual traps ranged from 0 to 
131 and means per population sample (of 64 
traps), from 1.0 to 31.5. 
STATISTICAL ANALYSIS 
Detection of tbe spatial pattern. Using th.e Fortran program of Gates & Ethridge (1972), the 
Poisson (S2 = m) and negative binomial (S2 = m + m2/k) distributions were fitted to the 35 
sets of data and tested by X". When Poisson distributions were fitted to the observed 
distributions, discrepancies between observed and expected values were significant in 27 
cases. However, the frequencies of all counts closely approximated the negative binomial 
2
The Great Lakes Entomologist, Vol. 15, No. 1 [1982], Art. 8
https://scholar.valpo.edu/tgle/vol15/iss1/8
1982 THE GREAT LAKES ENTOMOLOGIST 51 
1000 
100 
N 
~ 
uJ 
(,) .. 
Z
::; 
0;; 
.
<! . 
> .... . 
10 
.. 
. 
 SLOPE = 1.333 
MEAN (m) 
Fig. 2. Variance-mean relationships for 35 counts of A. Jrontella adults taken in emergence traps. Each 
point plotted is based on a sample of 64 traps. 
Table I. Results of analysis of variancea for counts of emerging adults of the alfalfa blotch 
leafminer, 14 June 1980. 
Source of 
variation 
df Observed mean square F 
Blocks 3 0.066 0.316 
Plots within blocks 12 0.209 1.215 
Traps within plots 48 0.172 
aTransformed scale. 
series, and deviations between observed and expected values were significant in only one 
case. Individual k values ranged from 1.01 to 7.56. The common value ofk as determined by 
the regression method of Bliss (1958) was 1.42.
Variance-mean relationships for the 35 counts are illustrated in Figure 2. The over­
dispersed nature of the data is clearly shown by the plotted values, which depart noticeably 
from the 45° line of Poisson expectation to attain a slope of 1.33. 
3
Harcourt: Population Assessment During the Adult Stage of the Alfalfa Blotc
Published by ValpoScholar, 1982
52 	 THE GREAT LAKES ENTOMOLOGIST Vol. 15, No. I 
Table 
2. 
Estimates of the numbers of traps required for two confidence probabilities and a 
15% level of precision in sampling for adults of the alfalfa blotch leafminer. 
Confidence probability 
Sample Mean number per trap CV 
.80a .90b 
1980 1 	 9.9 1.2 83 50 83 2 	 7.0 0.7 64 30 49 
3 	 4.5 0.5 70 36 59 
4 	 14.2 ± I.l 55 22 36 
5 	 4.0 ± 0.4 64 30 49 
6 	1.7 ± 0.2 94 65 106 
7 	 12.1 ± 1.0 58 25 40 
8 23.0 ± 2.7 82 49 81 
9 	 4.5 0.6 90 59 97
10 
3.4 0.4 89 58 
95 
II 
28.8 3.6 
96 67 III 
12 31.5±3.1 78 44 73 
I3 20.0 ± 2.1 84 52 85 
14 	
6.5 
± 0.7 93 63 104
15 	
4.5 
± 0.6 99 72 118 
16 	
4.4 
± 0.5 90 59 97 
17 	
3.6 
± 0.3 65 31 51 
18 	
7.3 0.6 
66 32 52 
19 
2.3 0.2 
74 40 66 
20 1.1±0.1 109 87 143 
1981 
21 	
13.0 
± 1.4 88 57 93 
22 5.1 ± 0.6 88 57 93 
23 10.1 ± 1.0 76 42 69 
24 3.4 0.4 91 60 100 
25 9.2 0.8 71 37 61 
26 2.8 ± 0.3 89 58 95 
27 9.7 ± 1.4 113 93 154 
28 11.0 ± l.l 80 47 77 
29 	 7.1 ± 0.7 100 73 120 
30 	 6.7 ± 0.5 60 26 43 
31 	
3.7 0.5 
106 82 135 
32 	 7.7 0.8 79 46 75 
33 	 5.1 0.5 81 48 79 
34 	 1.3 ± 0.2 114 95 156 
35 	 1.0 ± 0.1 107 84 138
= 	 1.2816. 

1.6449. 

Analysis of variance. The statistical methods used in this study follow those of Harcourt 
and Binns 
(I980ab). 
The data were analysed using a nested analysis of variance (among 
blocks, among 
plots in 
blocks, and among traps in plots). In these analyses the data were 
stabilized by the 
transformation log (x 
+ I), where x is equal to the observed count. The 
analysis of 
variance is illustrated (Table I) using 
one of the sets of data. 
Analysis of 
the 35 
counts showed that variation between blocks and plots was significant 
in just two and seven cases, respectively. Although this implied that variation between traps 
4
The Great Lakes Entomologist, Vol. 15, No. 1 [1982], Art. 8
https://scholar.valpo.edu/tgle/vol15/iss1/8
1982 THE GREAT LAKES ENTOMOLOGIST 53 
Table 3. Levels of precision corresponding to certain values ofN, and I  for adults of the 
alalfa blotch leafminer. 
Ns 
Confidence probability 
(1 - a) Percent precision (p)a 
80 0.8 12 
0.9 15 
70 0.8 13 
0.9 17 
60 O.S 14 
0.9 18 
50 0.8 15 
0.9 20 
ap z CVvNs 
comprised most of the sampling variance, it also indicated that some heterogeneity occurred 
throughout the field. For this reason, it was deemed adequate to adopt a single stage 
sampling procedure. Hence, a sampling plan for estimating numbers of emerging adults 
should provide for a number of traps well spread out over the field. 
Sampling precision. Table 2 lists the estimates of population density together with their 
standard errors in the untransformed scale. Invariably. the latter were within 15% of the 
mean. 
OPTIMUM ALLOCA nON OF SAMPLING RESOURCES 
To 
avoid interpretive problems associated with presenting sampling 
recommendations in a 
transformed scale, raw data were used to investigate the sample size for predetermined 
confidence limits. The inter-trap coefficients of variation (CV) were derived as lOOs{m. 
These ranged from 55 to 114 (Table 2). 
Following Snedecor and Cochran (1967), the number of traps corresponding to a confi­
dence 
interval 
of width p% of the mean (m) at a probability level of 100 (l - a)% was 
calculated as 
Ns (z.. CV{p)2 
where CV is the percent coefficient 
of variation, p is the level of precision, and z.. is the 
1 a{2 significant value for the standard normal distribution. 
U sing this equation. values for Ns were obtained for confidence probabilities of 0.8 and 
0.9, and a 15% level of precision (Table 2). The data were then averaged to determine the 
precision corresponding to certain given values ofN s' 1 -a, and a CV of 84; the latter was 
chosen by taking its rounded arithmetic mean over dates. Precisions corresponding to sam­
ple sizes of 50,60,70, and SO traps, with confidence probabilities of 0.8 and 0.9 are given in 
Table 3. From these results, it is evident that an acceptable confidence probability (O.S) and 
level of precision (12%) for life table studies can b  obtained with 80 traps per field. To 
obtain a greater or lesser degree of precision, numbers of traps should be increased or 
decreased 
accordingly. 
DISCUSSION 
In determining the sample size for target insect stages, it is important to evaluate the costs of 
sampling. 
In these terms, only 120 min (=2h) would be required to count and remove the 
contents of 80 traps. However, where resources are limited o  less intensive data are re­
quired (as in surveys), a lower level of precision could be adopted; Table 3 shows that just 50
5
Harcourt: Population Assessment During the Adult Stage of the Alfalfa Blotc
Published by ValpoScholar, 1982
54 THE GREAT LAKES ENTOMOLOGIST Vol. 15, No. I 
traps would yield a population estimate that provides a confidence probability of 0.8 and a 
level of precision of 15%. 
In 
the foregoing statistical appraisal, untransformed data were used to set the sample size 
for specified precision. As a rule, sampling plans based on log-transformed data tend to 
underestimate the sampling requirement; therefore, in situations where log-transformed data 
are appropriate, a plan based on non-transformed data should ensure adequate precision. 
ACKNOWLEDGMENTS 
The author 
is grateful to 
L M. Cass for his considerable help on design of the trap and to 
1. 
S. Walker for collecting the field data. 
LITERATURE CITED 
Bliss, 
C. 1. 1958. The analysis of insect counts as negative binomial distributions. Proc. 10th 
Int. Congr. Entomol. 2:1015-1032. 
Gage, S. H. and L L Haynes. 1975. Emergence under natural and manipulated conditions 
of Tetrastichusjulis, an introduced larval parasite of the cereal leaf beetle, with reference 
to 
regional population management. Environ. Entomol. 4:425-434. 
Gates, C. E. and F. G. Ethridge. 
1972. A generalized set of discrete frequency distribution 
with Fortran program. J. Int. Assoc. Math. Geo. 4:1-24. 
Guppy, J. C. 1981. Bionomics of the alfalfa blotch 1eafminer, Agromyzajrontella (Diptera: 
Agromyzidae), in eastern Ontario. Canadian Entomol. 113:593-600. 
Harcourt, 
D. G. and 
M. R. Binns. 1980a. A sampling system for estimating egg and laryal 
populations Agromyza frontella (Rond.) (Diptera: Agromyzidae) in alfalfa. Canadian 
Entomol. 112:375-385. 
----. 1980b. 
Sampling techniques for the soil-borne stages 
of Agromyza frontella 
(Diptera: Agromyzidae). Gr at Lak s Entomol. 13: 159-164. 
Snedecor, G. W. and W. G. Cochran. 1967. Statistical methods. Iowa State Univ. Press. 
Ames. 6th ed. 
6
The Great Lakes Entomologist, Vol. 15, No. 1 [1982], Art. 8
https://scholar.valpo.edu/tgle/vol15/iss1/8


Population Assessment During the Adult Stage of the Alfalfa Blotch Leafminer, \u3ci\u3eAgromyza Frontella\u3c/i\u3e (Diptera: Agromyzidae)

Valparaiso University

The Great Lakes Entomologist Volume 15 Number 1 - Spring 1982 Number 1 - Spring 1982 Article 8 April 1982 Population Assessment During the Adult Stage of the Alfalfa Blotch Leafminer, Agromyza Frontella (Diptera: Agromyzidae) D. G. Harcourt Agriculture Canada Follow this and additional works at: https://scholar.valpo.edu/tgle  Part of the Entomology Commons Recommended Citation Harcourt, D. G. 1982. "Population Assessment During the Adult Stage of the Alfalfa Blotch Leafminer, Agromyza Frontella (Diptera: Agromyzidae)," The Great Lakes Entomologist, vol 15 (1) Available at: https://scholar.valpo.edu/tgle/vol15/iss1/8 This Peer-Review Article is brought to you for free and open access by the Department of Biology at ValpoScholar. It has been accepted for inclusion in The Great Lakes Entomologist by an authorized administrator of ValpoScholar. For more information, please contact a ValpoScholar staff member at scholar@valpo.edu. 1982 THE GREAT LAKES ENTOMOLOGIST 49 POPULATION ASSESSMENT DURING THE ADULT STAGE OF THE ALFALFA BLOTCH LEAFMINER, AGROMYZA FRONTELLA (DIPTERA: AGROMYZIDAE)1 D. G. Harcourt2 ABSTRACT This paper presents a sampling procedure for estimating adult populations of the alfalfa blotch leafminer, Agromyza frontella (Rondani). The method is based on counts f the flies as they emerge from the soil following adult metamorphosis, taken in a series offurmel traps. Analysis of sampling variability showed that 80 traps per field will give adequate precision for life table studies in alfalfa. The pattern of counts was overdispersed but conformed to the negative binomial distribution. This paper is the third in a series dealing with sampling procedures for natural populations of the alfalfa blotch leaf miner, Agromyza frontella (Rondani) in fields of alfalfa. Earlier papers considered population assessment of the egg and larva! stages which occur on the plant (Harcourt and Birms 1980a), and the prepupal and pupal stages which are soil-borne (Harcourt and Binns 1980b). This article describes a method for estimating numbers of adults as they emerge from the soil, and the analysis of data leading to development of a reliable sampling plan for this stage. The technique is so designed that the data obtained can be integrated directly into life tables. The life history of the leafininer has been described by Guppy (1981). In eastern Ontario there are typically three generations a year. Briefly, the adults emerge from hibernation i  early spring and deposit their eggs within the alfalfa leaflets. The larvae feed between the epidermal tissues, developing a characteristic blotch-type mine during the course of three instars. The mature larvae emerge from the min d leaflets and drop to the ground where they pupate just beneath the soil surface. The insect overwinters as a partially developed pupa that completes its development in early spring. Peaks of adult emergence occur in late May, early JUly, and early August. EXPERIMENTAL PLOTS The data were collected during 1980-1981 in conjunction with life table studies ofthe pest at two locations in eastern Ontario, the Central Experimental Farm, Ottawa, and the College of Agricultural Technology, Kemptville. The fields, each of which comprised a 0.4 ha (I ac) stand of pure Vernal alfalfa, were situated in flat terrain and contained an average of 440 stems per m2 during the course of the study. Soil types ranged from clay loam to sandy loam. SAMPLING METHODS The adults were trapped as they emerged from the soil in each generation. Following harvest of the crop, an area measuring 30 x 30 m (100 x 100 ft) was marked off within each field and divided into four blocks of equal size. These were further subdivided into four equal plots and four traps were placed at random within each plot, making a total of 64. The traps comprised 22-cm diameter plastic funnels3 fitted with 24-oz screw-cap styrene jars as shown in Figure I. These were inverted over the crop stubble and secured by a nail IContribution No. 658. Ottawa Research Station, Agriculture Canada, Ottawa. 20ttawa Research Station, Agriculture Canada, Ottawa, Ontario, Canada KIA OC6. 3Bow Plastics Limited. Granby, Quebec. 1Harcourt: Population Assessment During the Adult Stage of the Alfalfa BlotcPublished by ValpoScholar, 198250 THE GREAT LAKES ENTOMOLOGIST Vol. 15, No. I r---- 2 em hole T 8cm I1 5cm em FUNNEL­ J ~ ---TAB em • ! Fig. I. Emergence traps used for A. fromella adults. The flies are trapped in ethylene glycol contained  the lid of the jar. The union between lid and funnel is sealed with plastic glue. driven through the funnel tab. A thin layer of ethylene glycol was poured into the lid of each jar to trap and preserve the flies as they emerged. The basal area covered by the trap was 0.034 m2 (0.375 ft2). In preliminary studies during 1979, the funnel traps were compared for efficiency to larger pyramid traps (0.83 m2) previously used at Ottawa for emergence of the cereal leaf beetle, Oulema melanopus (L.), and its parasites (for description, see Gage and Haynes, (975). Data for 17 sampling dates on which 32 traps of each size were compared showed that relative efficiency, defined as the reciprocal of variance per m2 , was 2.5 times greater for the smaller trap than for the larger. The flies were counted at weekly intervals during peak periods of emergence. At sam­pling, the jars were removed and the flies separated from the liquid in the lids by means of a small art brush. An average of 1.5 min was required to count and remove the contents of each trap. In the two years, numbers of emerging adults were recorded on a total of 35 occasions. The totals for individual traps ranged from 0 to 131 and means per population sample (of 64 traps), from 1.0 to 31.5. STATISTICAL ANALYSIS Detection of tbe spatial pattern. Using th.e Fortran program of Gates & Ethridge (1972), the Poisson (S2 = m) and negative binomial (S2 = m + m2/k) distributions were fitted to the 35 sets of data and tested by X". When Poisson distributions were fitted to the observed distributions, discrepancies between observed and expected values were significant in 27 cases. However, the frequencies of all counts closely approximated the negative binomial 2The Great Lakes Entomologist, Vol. 15, No. 1 [1982], Art. 8https://scholar.valpo.edu/tgle/vol15/iss1/81982 THE GREAT LAKES ENTOMOLOGIST 51 1000 100 N ~ uJ (,) .. Z::; 0;; .<! . > .... . 10 .. .  SLOPE = 1.333 MEAN (m) Fig. 2. Variance-mean relationships for 35 counts of A. Jrontella adults taken in emergence traps. Each point plotted is based on a sample of 64 traps. Table I. Results of analysis of variancea for counts of emerging adults of the alfalfa blotch leafminer, 14 June 1980. Source of variation df Observed mean square F Blocks 3 0.066 0.316 Plots within blocks 12 0.209 1.215 Traps within plots 48 0.172 aTransformed scale. series, and deviations between observed and expected values were significant in only one case. Individual k values ranged from 1.01 to 7.56. The common value ofk as determined by the regression method of Bliss (1958) was 1.42.Variance-mean relationships for the 35 counts are illustrated in Figure 2. The over­dispersed nature of the data is clearly shown by the plotted values, which depart noticeably from the 45° line of Poisson expectation to attain a slope of 1.33. 3Harcourt: Population Assessment During the Adult Stage of the Alfalfa BlotcPublished by ValpoScholar, 198252 	 THE GREAT LAKES ENTOMOLOGIST Vol. 15, No. I Table 2. Estimates of the numbers of traps required for two confidence probabilities and a 15% level of precision in sampling for adults of the alfalfa blotch leafminer. Confidence probability Sample Mean number per trap CV .80a .90b 1980 1 	 9.9 1.2 83 50 83 2 	 7.0 0.7 64 30 49 3 	 4.5 0.5 70 36 59 4 	 14.2 ± I.l 55 22 36 5 	 4.0 ± 0.4 64 30 49 6 	1.7 ± 0.2 94 65 106 7 	 12.1 ± 1.0 58 25 40 8 23.0 ± 2.7 82 49 81 9 	 4.5 0.6 90 59 9710 3.4 0.4 89 58 95 II 28.8 3.6 96 67 III 12 31.5±3.1 78 44 73 I3 20.0 ± 2.1 84 52 85 14 	6.5 ± 0.7 93 63 10415 	4.5 ± 0.6 99 72 118 16 	4.4 ± 0.5 90 59 97 17 	3.6 ± 0.3 65 31 51 18 	7.3 0.6 66 32 52 19 2.3 0.2 74 40 66 20 1.1±0.1 109 87 143 1981 21 	13.0 ± 1.4 88 57 93 22 5.1 ± 0.6 88 57 93 23 10.1 ± 1.0 76 42 69 24 3.4 0.4 91 60 100 25 9.2 0.8 71 37 61 26 2.8 ± 0.3 89 58 95 27 9.7 ± 1.4 113 93 154 28 11.0 ± l.l 80 47 77 29 	 7.1 ± 0.7 100 73 120 30 	 6.7 ± 0.5 60 26 43 31 	3.7 0.5 106 82 135 32 	 7.7 0.8 79 46 75 33 	 5.1 0.5 81 48 79 34 	 1.3 ± 0.2 114 95 156 35 	 1.0 ± 0.1 107 84 138= 	 1.2816. 1.6449. Analysis of variance. The statistical methods used in this study follow those of Harcourt and Binns (I980ab). The data were analysed using a nested analysis of variance (among blocks, among plots in blocks, and among traps in plots). In these analyses the data were stabilized by the transformation log (x + I), where x is equal to the observed count. The analysis of variance is illustrated (Table I) using one of the sets of data. Analysis of the 35 counts showed that variation between blocks and plots was significant in just two and seven cases, respectively. Although this implied that variation between traps 4The Great Lakes Entomologist, Vol. 15, No. 1 [1982], Art. 8https://scholar.valpo.edu/tgle/vol15/iss1/81982 THE GREAT LAKES ENTOMOLOGIST 53 Table 3. Levels of precision corresponding to certain values ofN, and I  for adults of the alalfa blotch leafminer. Ns Confidence probability (1 - a) Percent precision (p)a 80 0.8 12 0.9 15 70 0.8 13 0.9 17 60 O.S 14 0.9 18 50 0.8 15 0.9 20 ap z CVvNs comprised most of the sampling variance, it also indicated that some heterogeneity occurred throughout the field. For this reason, it was deemed adequate to adopt a single stage sampling procedure. Hence, a sampling plan for estimating numbers of emerging adults should provide for a number of traps well spread out over the field. Sampling precision. Table 2 lists the estimates of population density together with their standard errors in the untransformed scale. Invariably. the latter were within 15% of the mean. OPTIMUM ALLOCA nON OF SAMPLING RESOURCES To avoid interpretive problems associated with presenting sampling recommendations in a transformed scale, raw data were used to investigate the sample size for predetermined confidence limits. The inter-trap coefficients of variation (CV) were derived as lOOs{m. These ranged from 55 to 114 (Table 2). Following Snedecor and Cochran (1967), the number of traps corresponding to a confi­dence interval of width p% of the mean (m) at a probability level of 100 (l - a)% was calculated as Ns (z.. CV{p)2 where CV is the percent coefficient of variation, p is the level of precision, and z.. is the 1 a{2 significant value for the standard normal distribution. U sing this equation. values for Ns were obtained for confidence probabilities of 0.8 and 0.9, and a 15% level of precision (Table 2). The data were then averaged to determine the precision corresponding to certain given values ofN s' 1 -a, and a CV of 84; the latter was chosen by taking its rounded arithmetic mean over dates. Precisions corresponding to sam­ple sizes of 50,60,70, and SO traps, with confidence probabilities of 0.8 and 0.9 are given in Table 3. From these results, it is evident that an acceptable confidence probability (O.S) and level of precision (12%) for life table studies can b  obtained with 80 traps per field. To obtain a greater or lesser degree of precision, numbers of traps should be increased or decreased accordingly. DISCUSSION In determining the sample size for target insect stages, it is important to evaluate the costs of sampling. In these terms, only 120 min (=2h) would be required to count and remove the contents of 80 traps. However, where resources are limited o  less intensive data are re­quired (as in surveys), a lower level of precision could be adopted; Table 3 shows that just 505Harcourt: Population Assessment During the Adult Stage of the Alfalfa BlotcPublished by ValpoScholar, 198254 THE GREAT LAKES ENTOMOLOGIST Vol. 15, No. I traps would yield a population estimate that provides a confidence probability of 0.8 and a level of precision of 15%. In the foregoing statistical appraisal, untransformed data were used to set the sample size for specified precision. As a rule, sampling plans based on log-transformed data tend to underestimate the sampling requirement; therefore, in situations where log-transformed data are appropriate, a plan based on non-transformed data should ensure adequate precision. ACKNOWLEDGMENTS The author is grateful to L M. Cass for his considerable help on design of the trap and to 1. S. Walker for collecting the field data. LITERATURE CITED Bliss, C. 1. 1958. The analysis of insect counts as negative binomial distributions. Proc. 10th Int. Congr. Entomol. 2:1015-1032. Gage, S. H. and L L Haynes. 1975. Emergence under natural and manipulated conditions of Tetrastichusjulis, an introduced larval parasite of the cereal leaf beetle, with reference to regional population management. Environ. Entomol. 4:425-434. Gates, C. E. and F. G. Ethridge. 1972. A generalized set of discrete frequency distribution with Fortran program. J. Int. Assoc. Math. Geo. 4:1-24. Guppy, J. C. 1981. Bionomics of the alfalfa blotch 1eafminer, Agromyzajrontella (Diptera: Agromyzidae), in eastern Ontario. Canadian Entomol. 113:593-600. Harcourt, D. G. and M. R. Binns. 1980a. A sampling system for estimating egg and laryal populations Agromyza frontella (Rond.) (Diptera: Agromyzidae) in alfalfa. Canadian Entomol. 112:375-385. ----. 1980b. Sampling techniques for the soil-borne stages of Agromyza frontella (Diptera: Agromyzidae). Gr at Lak s Entomol. 13: 159-164. Snedecor, G. W. and W. G. Cochran. 1967. Statistical methods. Iowa State Univ. Press. Ames. 6th ed. 6The Great Lakes Entomologist, Vol. 15, No. 1 [1982], Art. 8https://scholar.valpo.edu/tgle/vol15/iss1/8

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Population Assessment During the Adult Stage of the Alfalfa Blotch Leafminer, \u3ci\u3eAgromyza Frontella\u3c/i\u3e (Diptera: Agromyzidae)

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