Eggs and larvae of the imported cabbageworm, Pieris rapae, were much more abundant in broccoli interplanted with nectar-producing plants than in broccoli monoculture. More diamondback moth larvae, Plutella xylostella, occurred in broccoli interplanted with or adjacent to nectar-producing plants than in broccoli monoculture. Density of cabbage looper larvae, Trichoplusia ni, was similar among the three types of broccoli plantings. For Cotesia rubecula, established in Michigan after introduction from Yugoslavia, pupae were more numerous in broccoli interplanted with nectar-producing plants than in other plots. High parasitism rates of diamondback moth, mainly by Diadegma insulare, were observed in every plot, but there were no differences in parasitism of diamondback moth between the treatments. Results indicate that the interactions between pests, parasitoids and nectar-producing plants are complex and may be different for each species

Ayers, G. S

Grafius, E. J

Stehr, F. W

Zhao, J. Z

English

ValpoScholar

The Great Lakes Entomologist 
Volume 25 
Number 4 - Winter 1992 Number 4 - Winter 
1992 
Article 3 
October 1992 
Effects of Neighboring Nectar-Producing Plants on Populations of 
Pest Lepidoptera and Their Parasitoids in Broccoli Plantings 
J. Z. Zhao 
Michigan State University 
G. S. Ayers 
Michigan State University 
E. J. Grafius 
Michigan State University 
F. W. Stehr 
Michigan State University 
Follow this and additional works at: https://scholar.valpo.edu/tgle 
 Part of the Entomology Commons 
Recommended Citation 
Zhao, J. Z.; Ayers, G. S.; Grafius, E. J.; and Stehr, F. W. 1992. "Effects of Neighboring Nectar-Producing 
Plants on Populations of Pest Lepidoptera and Their Parasitoids in Broccoli Plantings," The Great Lakes 
Entomologist, vol 25 (4) 
Available at: https://scholar.valpo.edu/tgle/vol25/iss4/3 
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. 
1992 THE GREAT LAKES ENTOMOLOGIST 253 
EFFECTS OF NEIGHBORING NECTAR-PRODUCING PLANTS ON 

POPULATIONS OF PEST LEPIDOPTERA 
AND 
THEIR PARASITOIDS II\! 

BROCCOLI PLANTINGS 

J. Z. Zhao1,z, G. S. Ayersl, E. J. Grafius1 and F. W. Stehr! 
ABSTRACT 
Eggs and 
larvae of 
the imported cabbageworm, Piens rapae, were much 
more abundant in broccoli interplanted with nectar-producing plants than in 
broccoli monoculture. More diamondback moth larvae, Plutella xylostella, 
occurred in broccoli interplanted with or adjacent to nectar-producing plants 
than 
in broccoli monoculture. Density 
of cabbage looper larvae, Trichoplusia 
ni, was similar among the three types of broccoli plantings. For Cotesia rube­
cula, established in Michigan after introduction from Yugoslavia, pupae were 
more numerous in broccoli interplanted with nectar-producing lants than in 
other 
plots. High 
parasitism rates of diamondback moth, mainly by
Diadegma insulare, were observed in every plot, but there were no differences 
in parasitism of diamondback moth between the treatm nts. Results indicate 
that 
the interactions between pests, parasitoids 
and nectar-producing plants 
are complex and may be different for each species. 
Diversification of agricultural crops often lowers est populations. In a 
summary 
of 198 herbivore species in 150 published studies, 53% of these 
species were less 
abundant in more diversified agroecosystems and only 18% 
were more abundant (Risch et al. 1983). Herbivore movement patt rns related 
to 
host-finding may be more 
important than the activities of natural enemies 
in explaining the reduction of monophagous pest populations in diverse 
annual systems (Risch 1981, Risch et al. 1983). Floral nectar is an important 
source of nutrition o both adult Lepidoptera and Hymenoptera biocontrol 
agents (Kevan and Baker 1984). Understanding how the abundance of pest 
Lepidoptera and their parasitoids in crops are affected by neighboring nectar­
producing plants is valuable for the design and manipulation of pest manage­
ment systems. 
Pest 
Lepidoptera, especially 
the imported cabbageworm, Piens rapae (L.), 
diamondback moth, Plutell  xylostella (L.), and cabbage looper, Trichoplusia 
ni (Hiibner), are key insect pests in crucifers (cabbage, broccoli, etc.) in North 
America (Harcourt 1963) and have wide distribution in other regions. Pre­
vious studies have reported that some herbs neighboring cabbage may be 
attractive 
to imported cabbageworm adults, resulting in higher damage 
to 
cabbage (Latheef and Irwin 1979). Population densities of diamondback moth 
in polycultures are often lower than in monocultures of crucifers (Buranday 
IDepartment of Entomology, Michigan State University, East Lansing, MI 
48824-1115. 
2Institute of 
Vegetables 
and Flowers, Chinese Academy of Agricultural Sciences, 
Beijing 
100081, p, 
R. China. 
1
Zhao et al.: Effects of Neighboring Nectar-Producing Plants on Populations of
Published by ValpoScholar, 1992
254 THE GREAT LAKES ENTOMOLOGIST Vol. 25, No.4 
and Raros 1973, Taleker et al. 1986). In Hawaii, mono specific cabbage plots 
had greater 
densities of diamondback 
moth larvae and lower parasitism com­
pared 
with plots of cabbage interplanted with 
tomato (Bach and Tabashnik 
1990). 
Bee forage plots, including nectar-producing plants in 34 genera, were 
planted 
in 
1989 at the Michigan State University Entomology Research 
Farm, East Lansing, Michigan (Ayers, unpublished data). The objective of our 
study 
was to evaluate 
the effects of some of these nectar-producing plants 
interplanted with or adjacent to broccoli on densities of the major Lepidoptera 
pests 
of broccoli, 
Le. imported cabbageworm, diamondback moth, and cab­
bage looper. A second objective was to measure the effects of nectar­
producing plants on parasitism of imported cabbageworm y Cotesia rubecula 
(Marshall) (Braconidae), and of diamondback moth larvae by Diadegma insul­
are (Cresson) (Ichneumonidae) and Microplitis plutellae Muesebeck (Ich­
neumonidae). C. rubecula is a solitary parasitoid that attacks early instar 
imported cabbageworms. It generally kills the cabbageworm by the third or 
fourth instar and forms a white cocoon. A related species. Cot sia (Apanteles) 
glomeratus, also attacks imported cabbageworms, but does not kill the larvae 
until they 
are mature, after significant amounts of feeding 
and foliar damage 
has occurred. D. insulare is a very common parasitoid of diamondback moth in 
the 
U.S. 
and Canada (Harcourt ). M plutellae is much more rare (Idris and 
Grafius 1992). 
MATERIALS AND METHODS 
Field plot experiment. A plot containing nectar-prodUCing plants in 34 
genera was planted in 1989 (20 rows and 2.4 m between rows). Anise hyssop, 
Agastache foeniculum, was among the most prolific nectar producers in this 
plot. Broccoli seedlings were started in the greenhouse and transplanted to 
the 
field on 
9 July 1991. 
Treatments were: (1) broccoli interplanted between the anise hyssop rows 
in the nectar-producing plant plot; (2) brocco i adjacent to the the nectar­
producing plant plot; (3) broccoli monoculture about 120 m east of the the 
nectar-producing plant plot. Treatment (1) was one row of broccoli (24 plants 
and 15 m long) between two of the nectar-producing plant rows. Treatments (2) 
and (3) 
were 
30 m long and four rows wide. Plant spacing was 0.6 m between 
plants 
within rows and 1 
m between rows. The numbers of pest Lepidoptera 
and their parasitoids were counted weekly on 10-20 broccoli plants in each 
treatment 
from 
30 July to 3 September 1991. Only the middle two rows of 
broccoli plants in treatments (2) and (3) were sampled. 
Diamondback moth larvae release. Because field diamondback moth pop­
ulations were low, 120 s cond and third instar diamondback moth from a 
laboratory culture (reared on broccoli leaves) were released on 15 July 1991 on 
12 marked broccoli plants in treatments (1) and (3) to investigate diamond­
back moth parasitism rates. Larvae, parasitized and non-parasitized pupae 
were collected on 21 August, 1991. The larvae were reared in the laboratory 
until 
pupation. Parasitized pupae were 
kept in petri dishes until emergence of
adult 
parasitoids for identification. Data 
analysis. Since 
the nectar-producing pla ts were originally planted 
in a single block for bee forage research, the tr atm nts were not spatially 
replicated and could not be statistically compared. Parasitism rates of 
diamondback moth were calculated as the number of parasitized pupae 
divided by the total number of parasitized and unparasitized pupae per plant. 
Differences in parasitism rates based on diamondback moth larvae released 
2
The Great Lakes Entomologist, Vol. 25, No. 4 [1992], Art. 3
https://scholar.valpo.edu/tgle/vol25/iss4/3
1992 THE GREAT LAKES ENTOMOLOGIST 255 
were analyzed using a t test (Zar 1974). Precise parasitism rates of imported 
cabbageworm by C. rubecula could not be calculated because C. rubecula 
pupates on the plants but imported cabbageworm larvae prefer to pupate off 
the host plants 
(Harcourt 
1963). A relative parasitism rate of imported cab­
bageworm was calculated as the season-long totals of parasitized pupae 
divided by the season-long totals of large ICW larvae (fourth and fifth instar) 
plus parasitized pupae per plant. The parasitoids of the cabbage looper were 
not 
investigated. 
RESULTS AND DISCUSSION 
Lepidoptera 
pest abundance. The effects of nectar-producing plants on 
pest 
abundance were different for imported cabbbageworm, diamondback 
moth, and cabbage looper. Imported cabbbageworm eggs and larvae were 
much more 
abundant in broccoli interplanted with nectar-producing plants 
than 
in broccoli adjacent 
to nectar-producing plants or broccoli monoculture 
(Fig. 1 a&b, Table 1). Only a few imported cabbbageworm pupae were found 
during the season in the three broccoli plots because the larvae leave the 
broccoli plants to pupate. 
There were generally more diamondback moth larvae in broccoli inter­
planted or adjacent to nectar-producing plants than in broccoli monoculture 
(Fig. Ie). However, the density of diamondback moth pupae in broccoli inter­
planted with nectar-producing pla ts was much lower than in broccoli adja­
cent to nectar-producing pla ts or monoculture (Table 1). 
The season-long total of cabbage looper larvae was similar between the 
three plots (Table 1), although the density was higher on 20-27 August in 
broccoli plants interplanted with nectar-producing plants (Fig. Id). 
Most adult Lepidoptera feed extensively on floral nectar (Kevan and 
Baker 1984), so the attractiveness of nectar-producing plants to imported 
cabbageworm butterflies and diamondback moths may be an important rea­
son for mOre eggs and larvae in broccoli interplanted with nectar-producing 
plants than in 
broccoli monoculture. Imported cabbageworm butterflies were 
seen visiting 
the n ctar-producing plants as well as the brocco i. Diamondback 
moths 
were observed only on the broccoli. Cabbage looper females were appar­
ently 
not attracted to nectar-producing plants and may not rely as much on 
them 
for food. 
Parasitism of imported cabbageworms and diamondback 
moth larvae. No 
unparasitized imported cabbageworm pupae were found in broccoli monocul­
ture 
or interplanted with nectar-producing plants. The relative parasitism rates 
of imported cabbageworm 
by C. rubecula were 39.9%, 22.2%, or 26.2% in 
broccoli interplanted with nec cing plants, adjacent to nectar-
producing plants or broccoli mon e, respectively. The number of C. rube­
cula pupae was much higher in broccoli interplanted with nectar-producing 
plants than in 
other plots (Table 
1). It is evident that C. rubecula has been 
established in Michigan after introduction from Yugoslavia in 1985. This is 
the 
only known establishment site of 
C. rubecula in the eastern U.S. (initially 
provided by Dr. B. Puttler, U.S.D.A. Biocontrol Laboratory, Columbia MO; 
current address, University of Missouri, Columbia, MO). 
Diadegma insulare is the major parasitoid of diamondback moth in Michi­
gan 
(Idris and Grafius 
1992). Only D. insulare was found on 21 August in this 
study. However, in the broccoli monoculture on 7 August, the parasitism of 
diamondback moth was 68.2% by D. insulare and 31.8% by M. plutellae, 
indicating that the latter could also be a important parasitoid of diamondback 
moth 
in early 
August in Michigan. High parasitism rates of diamondback 
3
Zhao et al.: Effects of Neighboring Nectar-Producing Plants on Populations of
Published by ValpoScholar, 1992
256 
• 
THE GREAT LAKES ENTOMOLOGIST Vol. 25/ No.4 
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ro 
0.. 
.... 
0 
0.. 
.... 
0 
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a 
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c:: 
c:: 
ro 
0 
~ 
15 
10 
5 
o 
15 
10 
5 
0 
3 
2 
0 
4 D. Cabbage loopers 
3 
2 
0 } t; 8 ~ ::I ~ ]~ eo eo eo 
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C. Di amolldback moth larvae 
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--
broccoli interplanted 
broccoli adjacent 
--<>-­
brocco
Ii monocul ture 
Figure 1. Population abundance of imported cabbageworm, diamondback moth and 
cabbage looper in brocc li as affected by proximity to nectar-producing plants. 
4
The Great Lakes Entomologist, Vol. 25, No. 4 [1992], Art. 3
https://scholar.valpo.edu/tgle/vol25/iss4/3
1992 THE GREAT LAKES ENTOMOLOGIST 257 
Table 1. -Numbers of imported cabbageworm eggs, larvae, pupae and parasitized pupae; 
diamondback moth 
larvae, 
pupae and parasitized pupae; and cabbage looper larvae in broccoli 
as 
affected 
by nectar-producing plants. 
Species/stage Season-long totals/pl nt in broccolil 
interplanted with 
adj
acent to nectar 
nectar plants 
monoculture Imported cabbageworm 
Eggs 31.6 3:2 1.0 
Larvae 
25.1 4.4 3.9 Pupae 
0.0 0.2 0.0 Cotesia rubecula pupae 5.8 0.6 0.8 
Diamondback moth 
Larvae 5.2 5.2 2.8 
Pupae 
0.1 0.6 0.5 Parasitized pupae2 0.8 0.6 0.5 
% parasitism 88.9 84.3 78.6 
Cabbage looper 
Larvae 6.2 4.3 6.1 
lSix 
sample 
dates. 
2Diadegma insulare and Microplitis plutellae. 
moth, mainly by D. insulare, were observed in every treatment (88.9%,84.3%, 
and 
78.6% in 
the plots of broccoli interplanted with nectar-producing plants, 
adjacent to nectar-producing plants, and broccoli monoculture, respectively, 
Table 1). Diadegma insulare were observed on broccoli plants in all plots and 
visiting the nectar-producing plants. 
Diamondback moth larvae release. In the diamondback moth larvae 
release experiment, there was no significant difference in parasitism rates of 
diamondback oth between broccoli interplanted with nectar-producing 
plants 
and monoculture (Table 
2). 
Nectar-producing plants attract and serve as food source for the adults of 
both 
biological control 
agents (Hymenoptera) and the pest Lepidoptera of 
cruciferous vegetables. Interactions between nectar-produci ants, pests 
and 
biocontrol 
agents are therefore complex and ay be di erent for each 
pest 
species. 
Further research is needed to investigate the effects of nectar­
producing plants o  specific pest Lepidoptera and pa sitoids related with the 
interplanting patterns and the ratio between nectar-producing plants and cru­
ciferous plants. 
Table 2. -Parasitism of diamondback moth after larvae were released in broccoli inter­
planted with 
nectar-producing 
plants or as a monoculture. 
Broccoli plot types Number of diamondback moth larvae 
released recaptured parasitized % parasitism1 
Interplanted 60 17 8 47.1 
Monoculture 60 14 5 35.7 
iNo 
significant difference 
in % parasitism between broccoli plot types (P > 0.05, t test). 
5
Zhao et al.: Effects of Neighboring Nectar-Producing Plants on Populations of
Published by ValpoScholar, 1992
258 THE GREAT LAKES ENTOMOLOGIST Vol. 25, No.4 
ACKNOWLEDGMENTS 
We wish to thank Beth Bishop, Walter Pett and Abd-Ghani Idris for their 
help. This work was supported in part by UNDP/FAO Project CPR/88/056, 
the 
Michigan Vegetable Council 
and the Michigan Agricultural Experiment 
Station. 
LITERATURE CITED 
Bach, C. E. and B. E. Tabashnik. 1990. Effects of nonhost plant neighbors on population 
densities and parasitism rate of the diamondback moth. Environ. Entomol. 
19:987-994. 
Buranday, R. P. and R. S. Raros. 1975. Effects of cabbage tomato intercropping on the 
incidence and oviposition of the diamondback moth, Plutella xylostella (L.). Philipp. 
Entomol. 2:369-374. 
Harcourt, D. G. 1960. Biology of the diamondback moth Plutella maculipennis (Curt.) in 
eastern 
Ontario. 
III. natural enemies. Can. Entomol. 92:419-428. 
_~
1963. Biology of cabbage caterpillars 
in eastern Ontario. Proc. Entomol. Soc. 
Ont.93:61-75. 
Idris, A. B. 
and E. J. Grafius. 1992. Field studies on the impact of insecticides on the
diamondback moth and parasitism by Diadegma insulare. J. Econ. Entomol. 
(submitted). 
Kevan, P. G. and H. G. Baker. 1984. Insects on flowers. In:Huffaker, C. B. and R. L. 

Rabb (eds.). Ecological Entomology. John Wiley & Sons, New York. pp. 607-631. 

Latheef, M. A. and R.D. Irwin. 1979. The effect of companionate planting on lepidopte­

ran pests 
of cabbage. Can. Entomol. 111:863-864. 
Risch, S. 
J. 1981. Insect herbivore abundance in tropical monocultures and polyculture­
s:an experimental test of two hypotheses. Ecology 62:1325-1340. 
Risch, S. J., M. Altieri and D. Andow. 1983. Agroecosystem diversity and pest con­
trol:data, tentative conclusion, and new research direction. Environ. Entomol. 
12:625-629. 
Taleker, N. S., S. T. Lee and S. W. Huang. 1986. Intercropping and modification of 
irrigation method for the cont ol of diamondback moth. In:Taleker, N.S. and T.D. 
Griggs (eds.). Diamondback Moth Management:Proc. of the First International Work­
shop at AVRDC, Shanhua, Taiwan. pp.145-152. 
Zar, J. H. 1974. Biostatistical Analysis. Prentice-Hall, Tnc., Englewood Cliffs, N.J. 
6
The Great Lakes Entomologist, Vol. 25, No. 4 [1992], Art. 3
https://scholar.valpo.edu/tgle/vol25/iss4/3


Effects of Neighboring Nectar-Producing Plants on Populations of Pest Lepidoptera and Their Parasitoids in Broccoli Plantings

Valparaiso University

The Great Lakes Entomologist Volume 25 Number 4 - Winter 1992 Number 4 - Winter 1992 Article 3 October 1992 Effects of Neighboring Nectar-Producing Plants on Populations of Pest Lepidoptera and Their Parasitoids in Broccoli Plantings J. Z. Zhao Michigan State University G. S. Ayers Michigan State University E. J. Grafius Michigan State University F. W. Stehr Michigan State University Follow this and additional works at: https://scholar.valpo.edu/tgle  Part of the Entomology Commons Recommended Citation Zhao, J. Z.; Ayers, G. S.; Grafius, E. J.; and Stehr, F. W. 1992. "Effects of Neighboring Nectar-Producing Plants on Populations of Pest Lepidoptera and Their Parasitoids in Broccoli Plantings," The Great Lakes Entomologist, vol 25 (4) Available at: https://scholar.valpo.edu/tgle/vol25/iss4/3 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. 1992 THE GREAT LAKES ENTOMOLOGIST 253 EFFECTS OF NEIGHBORING NECTAR-PRODUCING PLANTS ON POPULATIONS OF PEST LEPIDOPTERA AND THEIR PARASITOIDS II\! BROCCOLI PLANTINGS J. Z. Zhao1,z, G. S. Ayersl, E. J. Grafius1 and F. W. Stehr! ABSTRACT Eggs and larvae of the imported cabbageworm, Piens rapae, were much more abundant in broccoli interplanted with nectar-producing plants than in broccoli monoculture. More diamondback moth larvae, Plutella xylostella, occurred in broccoli interplanted with or adjacent to nectar-producing plants than in broccoli monoculture. Density of cabbage looper larvae, Trichoplusia ni, was similar among the three types of broccoli plantings. For Cotesia rube­cula, established in Michigan after introduction from Yugoslavia, pupae were more numerous in broccoli interplanted with nectar-producing lants than in other plots. High parasitism rates of diamondback moth, mainly byDiadegma insulare, were observed in every plot, but there were no differences in parasitism of diamondback moth between the treatm nts. Results indicate that the interactions between pests, parasitoids and nectar-producing plants are complex and may be different for each species. Diversification of agricultural crops often lowers est populations. In a summary of 198 herbivore species in 150 published studies, 53% of these species were less abundant in more diversified agroecosystems and only 18% were more abundant (Risch et al. 1983). Herbivore movement patt rns related to host-finding may be more important than the activities of natural enemies in explaining the reduction of monophagous pest populations in diverse annual systems (Risch 1981, Risch et al. 1983). Floral nectar is an important source of nutrition o both adult Lepidoptera and Hymenoptera biocontrol agents (Kevan and Baker 1984). Understanding how the abundance of pest Lepidoptera and their parasitoids in crops are affected by neighboring nectar­producing plants is valuable for the design and manipulation of pest manage­ment systems. Pest Lepidoptera, especially the imported cabbageworm, Piens rapae (L.), diamondback moth, Plutell  xylostella (L.), and cabbage looper, Trichoplusia ni (Hiibner), are key insect pests in crucifers (cabbage, broccoli, etc.) in North America (Harcourt 1963) and have wide distribution in other regions. Pre­vious studies have reported that some herbs neighboring cabbage may be attractive to imported cabbageworm adults, resulting in higher damage to cabbage (Latheef and Irwin 1979). Population densities of diamondback moth in polycultures are often lower than in monocultures of crucifers (Buranday IDepartment of Entomology, Michigan State University, East Lansing, MI 48824-1115. 2Institute of Vegetables and Flowers, Chinese Academy of Agricultural Sciences, Beijing 100081, p, R. China. 1Zhao et al.: Effects of Neighboring Nectar-Producing Plants on Populations ofPublished by ValpoScholar, 1992254 THE GREAT LAKES ENTOMOLOGIST Vol. 25, No.4 and Raros 1973, Taleker et al. 1986). In Hawaii, mono specific cabbage plots had greater densities of diamondback moth larvae and lower parasitism com­pared with plots of cabbage interplanted with tomato (Bach and Tabashnik 1990). Bee forage plots, including nectar-producing plants in 34 genera, were planted in 1989 at the Michigan State University Entomology Research Farm, East Lansing, Michigan (Ayers, unpublished data). The objective of our study was to evaluate the effects of some of these nectar-producing plants interplanted with or adjacent to broccoli on densities of the major Lepidoptera pests of broccoli, Le. imported cabbageworm, diamondback moth, and cab­bage looper. A second objective was to measure the effects of nectar­producing plants on parasitism of imported cabbageworm y Cotesia rubecula (Marshall) (Braconidae), and of diamondback moth larvae by Diadegma insul­are (Cresson) (Ichneumonidae) and Microplitis plutellae Muesebeck (Ich­neumonidae). C. rubecula is a solitary parasitoid that attacks early instar imported cabbageworms. It generally kills the cabbageworm by the third or fourth instar and forms a white cocoon. A related species. Cot sia (Apanteles) glomeratus, also attacks imported cabbageworms, but does not kill the larvae until they are mature, after significant amounts of feeding and foliar damage has occurred. D. insulare is a very common parasitoid of diamondback moth in the U.S. and Canada (Harcourt ). M plutellae is much more rare (Idris and Grafius 1992). MATERIALS AND METHODS Field plot experiment. A plot containing nectar-prodUCing plants in 34 genera was planted in 1989 (20 rows and 2.4 m between rows). Anise hyssop, Agastache foeniculum, was among the most prolific nectar producers in this plot. Broccoli seedlings were started in the greenhouse and transplanted to the field on 9 July 1991. Treatments were: (1) broccoli interplanted between the anise hyssop rows in the nectar-producing plant plot; (2) brocco i adjacent to the the nectar­producing plant plot; (3) broccoli monoculture about 120 m east of the the nectar-producing plant plot. Treatment (1) was one row of broccoli (24 plants and 15 m long) between two of the nectar-producing plant rows. Treatments (2) and (3) were 30 m long and four rows wide. Plant spacing was 0.6 m between plants within rows and 1 m between rows. The numbers of pest Lepidoptera and their parasitoids were counted weekly on 10-20 broccoli plants in each treatment from 30 July to 3 September 1991. Only the middle two rows of broccoli plants in treatments (2) and (3) were sampled. Diamondback moth larvae release. Because field diamondback moth pop­ulations were low, 120 s cond and third instar diamondback moth from a laboratory culture (reared on broccoli leaves) were released on 15 July 1991 on 12 marked broccoli plants in treatments (1) and (3) to investigate diamond­back moth parasitism rates. Larvae, parasitized and non-parasitized pupae were collected on 21 August, 1991. The larvae were reared in the laboratory until pupation. Parasitized pupae were kept in petri dishes until emergence ofadult parasitoids for identification. Data analysis. Since the nectar-producing pla ts were originally planted in a single block for bee forage research, the tr atm nts were not spatially replicated and could not be statistically compared. Parasitism rates of diamondback moth were calculated as the number of parasitized pupae divided by the total number of parasitized and unparasitized pupae per plant. Differences in parasitism rates based on diamondback moth larvae released 2The Great Lakes Entomologist, Vol. 25, No. 4 [1992], Art. 3https://scholar.valpo.edu/tgle/vol25/iss4/31992 THE GREAT LAKES ENTOMOLOGIST 255 were analyzed using a t test (Zar 1974). Precise parasitism rates of imported cabbageworm by C. rubecula could not be calculated because C. rubecula pupates on the plants but imported cabbageworm larvae prefer to pupate off the host plants (Harcourt 1963). A relative parasitism rate of imported cab­bageworm was calculated as the season-long totals of parasitized pupae divided by the season-long totals of large ICW larvae (fourth and fifth instar) plus parasitized pupae per plant. The parasitoids of the cabbage looper were not investigated. RESULTS AND DISCUSSION Lepidoptera pest abundance. The effects of nectar-producing plants on pest abundance were different for imported cabbbageworm, diamondback moth, and cabbage looper. Imported cabbbageworm eggs and larvae were much more abundant in broccoli interplanted with nectar-producing plants than in broccoli adjacent to nectar-producing plants or broccoli monoculture (Fig. 1 a&b, Table 1). Only a few imported cabbbageworm pupae were found during the season in the three broccoli plots because the larvae leave the broccoli plants to pupate. There were generally more diamondback moth larvae in broccoli inter­planted or adjacent to nectar-producing plants than in broccoli monoculture (Fig. Ie). However, the density of diamondback moth pupae in broccoli inter­planted with nectar-producing pla ts was much lower than in broccoli adja­cent to nectar-producing pla ts or monoculture (Table 1). The season-long total of cabbage looper larvae was similar between the three plots (Table 1), although the density was higher on 20-27 August in broccoli plants interplanted with nectar-producing plants (Fig. Id). Most adult Lepidoptera feed extensively on floral nectar (Kevan and Baker 1984), so the attractiveness of nectar-producing plants to imported cabbageworm butterflies and diamondback moths may be an important rea­son for mOre eggs and larvae in broccoli interplanted with nectar-producing plants than in broccoli monoculture. Imported cabbageworm butterflies were seen visiting the n ctar-producing plants as well as the brocco i. Diamondback moths were observed only on the broccoli. Cabbage looper females were appar­ently not attracted to nectar-producing plants and may not rely as much on them for food. Parasitism of imported cabbageworms and diamondback moth larvae. No unparasitized imported cabbageworm pupae were found in broccoli monocul­ture or interplanted with nectar-producing plants. The relative parasitism rates of imported cabbageworm by C. rubecula were 39.9%, 22.2%, or 26.2% in broccoli interplanted with nec cing plants, adjacent to nectar-producing plants or broccoli mon e, respectively. The number of C. rube­cula pupae was much higher in broccoli interplanted with nectar-producing plants than in other plots (Table 1). It is evident that C. rubecula has been established in Michigan after introduction from Yugoslavia in 1985. This is the only known establishment site of C. rubecula in the eastern U.S. (initially provided by Dr. B. Puttler, U.S.D.A. Biocontrol Laboratory, Columbia MO; current address, University of Missouri, Columbia, MO). Diadegma insulare is the major parasitoid of diamondback moth in Michi­gan (Idris and Grafius 1992). Only D. insulare was found on 21 August in this study. However, in the broccoli monoculture on 7 August, the parasitism of diamondback moth was 68.2% by D. insulare and 31.8% by M. plutellae, indicating that the latter could also be a important parasitoid of diamondback moth in early August in Michigan. High parasitism rates of diamondback 3Zhao et al.: Effects of Neighboring Nectar-Producing Plants on Populations ofPublished by ValpoScholar, 1992256 • THE GREAT LAKES ENTOMOLOGIST Vol. 25/ No.4 ....c:: ro 0.. .... 0 0.. .... 0 ..D a ::lc:: c:: ro 0 ~ 15 10 5 o 15 10 5 0 3 2 0 4 D. Cabbage loopers 3 2 0 } t; 8 ~ ::I ~ ]~ eo eo eo ..:" «( " ..:::I E'" 0 r- :::: N 0 N r- e­<'l '"A. Imported cabbageworm eggs p... / ~ ---",," /7 B. Imported cabbageworm larvae .0 /--~.-.-.-" -"" C. Di amolldback moth larvae ,... -0­--broccoli interplanted broccoli adjacent --<>-­broccoIi monocul ture Figure 1. Population abundance of imported cabbageworm, diamondback moth and cabbage looper in brocc li as affected by proximity to nectar-producing plants. 4The Great Lakes Entomologist, Vol. 25, No. 4 [1992], Art. 3https://scholar.valpo.edu/tgle/vol25/iss4/31992 THE GREAT LAKES ENTOMOLOGIST 257 Table 1. -Numbers of imported cabbageworm eggs, larvae, pupae and parasitized pupae; diamondback moth larvae, pupae and parasitized pupae; and cabbage looper larvae in broccoli as affected by nectar-producing plants. Species/stage Season-long totals/pl nt in broccolil interplanted with adjacent to nectar nectar plants monoculture Imported cabbageworm Eggs 31.6 3:2 1.0 Larvae 25.1 4.4 3.9 Pupae 0.0 0.2 0.0 Cotesia rubecula pupae 5.8 0.6 0.8 Diamondback moth Larvae 5.2 5.2 2.8 Pupae 0.1 0.6 0.5 Parasitized pupae2 0.8 0.6 0.5 % parasitism 88.9 84.3 78.6 Cabbage looper Larvae 6.2 4.3 6.1 lSix sample dates. 2Diadegma insulare and Microplitis plutellae. moth, mainly by D. insulare, were observed in every treatment (88.9%,84.3%, and 78.6% in the plots of broccoli interplanted with nectar-producing plants, adjacent to nectar-producing plants, and broccoli monoculture, respectively, Table 1). Diadegma insulare were observed on broccoli plants in all plots and visiting the nectar-producing plants. Diamondback moth larvae release. In the diamondback moth larvae release experiment, there was no significant difference in parasitism rates of diamondback oth between broccoli interplanted with nectar-producing plants and monoculture (Table 2). Nectar-producing plants attract and serve as food source for the adults of both biological control agents (Hymenoptera) and the pest Lepidoptera of cruciferous vegetables. Interactions between nectar-produci ants, pests and biocontrol agents are therefore complex and ay be di erent for each pest species. Further research is needed to investigate the effects of nectar­producing plants o  specific pest Lepidoptera and pa sitoids related with the interplanting patterns and the ratio between nectar-producing plants and cru­ciferous plants. Table 2. -Parasitism of diamondback moth after larvae were released in broccoli inter­planted with nectar-producing plants or as a monoculture. Broccoli plot types Number of diamondback moth larvae released recaptured parasitized % parasitism1 Interplanted 60 17 8 47.1 Monoculture 60 14 5 35.7 iNo significant difference in % parasitism between broccoli plot types (P > 0.05, t test). 5Zhao et al.: Effects of Neighboring Nectar-Producing Plants on Populations ofPublished by ValpoScholar, 1992258 THE GREAT LAKES ENTOMOLOGIST Vol. 25, No.4 ACKNOWLEDGMENTS We wish to thank Beth Bishop, Walter Pett and Abd-Ghani Idris for their help. This work was supported in part by UNDP/FAO Project CPR/88/056, the Michigan Vegetable Council and the Michigan Agricultural Experiment Station. LITERATURE CITED Bach, C. E. and B. E. Tabashnik. 1990. Effects of nonhost plant neighbors on population densities and parasitism rate of the diamondback moth. Environ. Entomol. 19:987-994. Buranday, R. P. and R. S. Raros. 1975. Effects of cabbage tomato intercropping on the incidence and oviposition of the diamondback moth, Plutella xylostella (L.). Philipp. Entomol. 2:369-374. Harcourt, D. G. 1960. Biology of the diamondback moth Plutella maculipennis (Curt.) in eastern Ontario. III. natural enemies. Can. Entomol. 92:419-428. _~1963. Biology of cabbage caterpillars in eastern Ontario. Proc. Entomol. Soc. Ont.93:61-75. Idris, A. B. and E. J. Grafius. 1992. Field studies on the impact of insecticides on thediamondback moth and parasitism by Diadegma insulare. J. Econ. Entomol. (submitted). Kevan, P. G. and H. G. Baker. 1984. Insects on flowers. In:Huffaker, C. B. and R. L. Rabb (eds.). Ecological Entomology. John Wiley & Sons, New York. pp. 607-631. Latheef, M. A. and R.D. Irwin. 1979. The effect of companionate planting on lepidopte­ran pests of cabbage. Can. Entomol. 111:863-864. Risch, S. J. 1981. Insect herbivore abundance in tropical monocultures and polyculture­s:an experimental test of two hypotheses. Ecology 62:1325-1340. Risch, S. J., M. Altieri and D. Andow. 1983. Agroecosystem diversity and pest con­trol:data, tentative conclusion, and new research direction. Environ. Entomol. 12:625-629. Taleker, N. S., S. T. Lee and S. W. Huang. 1986. Intercropping and modification of irrigation method for the cont ol of diamondback moth. In:Taleker, N.S. and T.D. Griggs (eds.). Diamondback Moth Management:Proc. of the First International Work­shop at AVRDC, Shanhua, Taiwan. pp.145-152. Zar, J. H. 1974. Biostatistical Analysis. Prentice-Hall, Tnc., Englewood Cliffs, N.J. 6The Great Lakes Entomologist, Vol. 25, No. 4 [1992], Art. 3https://scholar.valpo.edu/tgle/vol25/iss4/3

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Effects of Neighboring Nectar-Producing Plants on Populations of Pest Lepidoptera and Their Parasitoids in Broccoli Plantings

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