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© 2006 Plant Management Network.
Accepted for publication 4 November 2005. Published 5 January 2006.


Field Efficacy of Insecticides for Control of Lepidopteran Pests on Collards in Virginia


Roberto J. Cordero, Department of Entomology, Virginia Polytechnic Institute & State University, Blacksburg 24060; Thomas P. Kuhar and John Speese, III, Eastern Shore Agricultural Research and Extension Center, Virginia Polytechnic Institute & State University, Painter 23420; Roger R. Youngman, Department of Entomology, Virginia Polytechnic Institute & State University, Blacksburg 24060; Edwin E. Lewis, Department of Entomology, University of California at Davis 95616; Jeffrey R. Bloomquist and Loke T. Kok, Department of Entomology, Virginia Polytechnic Institute & State University, Blacksburg 24060; and Anthony D. Bratsch, Department of Horticulture, Virginia Polytechnic Institute & State University, Blacksburg 24060


Corresponding author: Tom Kuhar. tkuhar@vt.edu


Cordero, R. J., Kuhar, T. P., Speese, J., III, Youngman, R. R., Lewis, E. E., Bloomquist, J. R., Kok, L. T., and Bratsch, A. D. 2006. Field efficacy of insecticides for control of Lepidopteran pests on collards in Virginia. Online. Plant Health Progress doi:10.1094/PHP-2006-0105-01-RS.


Abstract

Control of lepidopteran pests on collards (Brassica oleracea L. Acephala group) can be challenging because the leaves are the marketable portion of the crop and because of insecticide resistance problems in diamondback moth, Plutella xylostella (L.). In 2003 and 2004 in two locations of Virginia, field efficacy tests were conducted on several conventional standard insecticides as well as several new and more IPM-compatible insecticides (applied at their lowest-labeled rates) for control of lepidopteran pests in collards. Lepidopteran pest species included the following (in relative order of abundance): P. xylostella, Pieris rapae (L.), Trichoplusia ni (Hübner), Spodoptera frugiperda (J. E. Smith), Estigmene acrea (Drury), S. exigua (Hübner), Helicoverpa zea (Boddie), Evergestis rimosalis (Guenée), and Hellula rogatalis (Hulst). The most efficacious insecticides over the five experiments included acephate, emamectin benzoate, esfenvalerate, methomyl, methoxyfenozide, novaluron, indoxacarb, and spinosad. Although acetamiprid, Bacillus thuringiensis subsp. kurstaki, and azadirachtin have shown efficacy against lepidopteran pests in other studies, they were inconsistent in their performance in these experiments. Insecticide options that provide reliable control of the suite of lepidopteran pests that attack collards in Virginia, and that are relatively less toxic to natural enemies and thus can fit well into integrated pest management programs include indoxacarb, spinosad, novaluron, emamectin benzoate, and methoxyfenozide.


Introduction

Collards (Brassica oleracea L. acephala group) are a popular cruciferous vegetable grown in the southeastern U.S., and are highly-attractive to diamondback moth, Plutella xylostella (L.), a major pest of cruciferous crops worldwide (21). Diamondback moth larvae feed on leaves, which for collards are the marketable portion of the crop. Although some minor feeding injury is acceptable for local marketability, the presence of live insects on leaves or defoliation levels exceeding 10% render the crop unmarketable (Fig. 1).



A
 
B

Fig. 1. Examples of marketable (A) and unmarketable (B) collards leaves based on local market acceptability on the eastern shore of Virginia.


Natural enemies, particularly hymenopteran parasitoids, can play an important role in reducing lepidopteran pest populations (1,7,11,14), but insecticide applications are typically needed for economic pest control. The efficacy of many conventional insecticides such as organophosphates, carbamates, pyrethroids, and even Bacillus thuringiensis (Bt) may vary considerably from region to region due to resistance levels in diamondback moth (3,13,15,19,20). Moreover, many integrated pest management (IPM)-compatible insecticides that have been registered on vegetable crops in the U.S. in recent years such as indoxacarb, methoxyfenozide, spinosad, emamectin benzoate, azadirachtin, novaluron, and acetamiprid have not been thoroughly tested for efficacy in collards. In addition, in order to be truly effective in collards, an insecticide must control not only diamondback moth, but other pests that may damage the leaves. Based on previous studies (1,8,9,10,11), the following lepidopteran species may attack collards in Virginia: imported cabbageworm, Pieris rapae (L.) (Pieridae); cabbage looper, Trichoplusia ni (Hübner) (Noctuidae); fall armyworm, Spodoptera frugiperda (J. E. Smith) (Noctuidae); corn earworm, Helicoverpa zea (Boddie) (Noctuidae); cabbage webworm, Hellula rogatalis (Hulst) (Pyralidae); beet armyworm, Spodoptera exigua (Hübner) (Noctuidae); cross-striped cabbageworm, Evergestis rimosalis (Guenée) (Pyralidae); and saltmarsh caterpillar, Estigmene acrea (Drury) (Arctiidae) (Fig. 2). The rank of species importance generally varies with latitude (7). The objective of this study was to assess the current field efficacy of several conventional broad-spectrum insecticidal products as well as several newer IPM-compatible products for lepidopteran pest control in collards in Virginia.


 
Plutella xylostella (L.) (Plutellidae)   Pieris rapae (L.) (Pieridae)

 
Trichoplusia ni (Hübner) (Noctuidae)   Spodoptera frugiperda (J. E. Smith) (Noctuidae)

 
Estigmene acrea (Drury) (Arctiidae)   Spodoptera exigua (Hübner) (Noctuidae)

 
Helicoverpa zea (Boddie) (Noctuidae)   Evergestis rimosalis (Guenée) (Pyralidae)

Fig. 2. Lepidopteran pests on collards in Virginia.


Insecticide Efficacy Trials

In 2003, a field experiment was conducted at two locations, Virginia Tech Kentland Research Farm (80°25’W, 37°14’N; elevation ≈640 m), near Blacksburg, VA and the Virginia Tech Eastern Shore Agricultural Research and Extension Center (ESAREC) (75°49’W, 37°35’N; elevation ≈12 m) near Painter, VA. Collards (variety ‘Vates’) were direct seeded at a rate of 10 plants per m on 0.9-m row centers on 23 July at Kentland and on 22 August at ESAREC. The experiment consisted of 11 insecticide treatments each representing a different insecticide class (Table 1) plus an untreated check arranged in a randomized complete block and replicated 5 times. Each individual plot consisted of a single 6-m row flanked on both sides by an untreated guard row. Treatments were applied using a gas-pressurized sprayer that delivered spray at 355 liters/ha at a pressure of 2.81 kg/cm2 through a boom, with one hollow cone nozzle oriented over the center of the row and two hollow cone drop nozzles oriented to the sides of the row. Latron B-1956 spreader sticker was added to each treatment and the untreated check at 0.25% v/v of spray. Plants were inspected weekly for the presence of insect pests. When ~50% of the plants had at least one lepidopteran larva, insecticide treatments were applied. Two foliar applications (17 and 30 September) were made at Kentland and a single foliar application (24 October) was made at the ESAREC.


Table 1. Insecticides tested in efficacy trials on collards in Virginia, 2003-2004.

Insecticide (ai) Product name
(manufacturer)
Insecticide class
Acephate* Orthene 97
(Valent)
Organophosphate
Acetamiprid Assail 70WP
(Cerexagri)
Neonicotinoid
Azadirachtin Neemix 4.5EC
(Certis USA)
Botanical, Neem extract
Bacillus thuringiensis subsp.
kurstaki strain HD-1
DiPel DF
(Valent)
Microbial
Emamectin benzoate Proclaim 5WDG
(Syngenta)
Avermectin
Esfenvalerate Asana XL
(DuPont)
Pyrethroid
Indoxacarb* Avaunt 30WG
(DuPont)
Pyrazoline
Methomyl Lannate LV
(DuPont)
Carbamate
Methoxyfenozide Intrepid 2F
(Dow AgroSciences)
Insect growth regulator
Novaluron Rimon 0.83EC
(Crompton)
Insect growth regulator
Spinosad SpinTor 2SC
(Dow AgroSciences)
Spinosyns

 * As of 2005, this insecticide is not currently registered for use on collards in Virginia, but some formulations of the active ingredient are labeled on other cruciferous crops.


To evaluate efficacy, any live species of lepidopteran larvae were counted on 20 leaves per plot approximately 5 to 8 days after each application. Also, on 11 November (ESAREC) and 31 October (Kentland), 20 arbitrarily-chosen leaves per plot were picked from the top of the plant and rated by eye as marketable (no insects present and <10% injury) or unmarketable (≥ 10% defoliation and/or the presence of a larva) (Fig. 1).

In 2004, the experiment was repeated three more times at the same two locations described previously. The experimental design and methods were the same as in 2003, with the exception of the following details: collards were planted on 4 June at ESAREC-1, on 26 July at Kentland, and on 10 August at ESAREC-2; insecticides were applied on 14 and 29 July (ESAREC-1), 14 September and 5 October (Kentland), and 14 November (ESAREC-2); lepidopteran larvae were sampled on 19 July and 4 August at ESAREC-1, on 22 September and 13 October at Kentland, and on 19 and 21 November at ESAREC-2; plots were harvested and rated for damage on 4 August at ESAREC-1, 13 October at Kentland, and 21 November at ESAREC-2.

Data including lepidopteran larval density and proportion of marketable collard leaves at harvest were analyzed using ANOVA and means separated by Fisher’s protected LSD at P ≤ 0.05 (17). To stabilize variances, proportion data were transformed [arcsin sqrt (x + 0.001)] before analysis; however, actual percentages are presented in the results.


Effects of Insecticide Treatments Applied to Collards in the 2003 Growing Season

At Kentland, total lepidopteran pest pressure on collards was high and averaged more than 8 larvae per 20 leaves in the untreated control plots (Table 2). Pieris rapae was the dominant species observed (67% of the total larvae) with P. xylostella (31%) and a mix of other lepidopteran species comprising the rest. At five days after treatment, there was a significant treatment effect on densities of P. rapae (F = 3.59; df = 4, 40; P < 0.0018) and total lepidopteran larvae (F = 5.82; df = 4, 40; P < 0.0001), but not P. xylostella (F = 1.44; df = 4, 40; P = 0.1996). The untreated control had the highest overall density of lepidopteran larvae, but was not significantly different than azadirachtin. Methomyl, acephate, and esfenvalerate had the fewest total lepidopteran larvae. Insecticide treatment also had a significant effect on the percentage of marketable leaves at harvest (F = 3.61; df = 4, 40; P < 0.0031). All treatments had a higher percentage of marketable leaves than the untreated control except azadirachtin and Bt kurstaki (Table 2).


Table 2. Density of lepidopteran larvae and percentage marketable leaves (5 days after treatment) on collards after lowest-labeled rate applications of insecticides in field efficacy trials conducted at Kentland Farm near Blacksburg, VA in 2003.

Treatment Rate
(kg[ai]
/ha)
Mean no. live larvae
per 20 leaves
% of leaves
marketable

(Rating 1 or 2)
at harvest
Plutella
xylostella
Pieris
rapae
Total
lepidoptera
Acephate 1.087 0.2   0.8 bcd     1.6 d       88.0 ab
Acetamiprid 0.084 1.0   1.4 bcd     3.2 bc       79.0 bc
Azadirachtin 0.011 1.8   3.2 a     5.4 ab       68.0 cd
Bt subsp. kurstaki
strain HD-1
0.605 0.8   1.0 bcd     2.0 cd       70.0 bcd
Emamectin benzoate 0.008 1.0   0.8 cd     2.0 cd       78.0 bc
Esfenvalerate 0.032 0.8   0.8 d     1.8 d       84.0 ab
Indoxacarb 0.072 0.8   2.0 abc     3.0 cd       83.0 ab
Methomyl 0.504 0.2   0.8 cd     1.2 d       94.0 a
Methoxyfenozide 0.112 0.8   2.2 ab     3.6 bc       76.0 bc
Spinosad 0.026 0.8   1.4 bcd     2.4 cd       88.0 ab
UTC  -- 2.4   3.4 a     8.2 a       55.0 d

Means in a column with a letter in common are not significant (P > 0.05, Fisher’s protected LSD).


At the ESAREC, the overall density of lepidopteran larvae on collards was lower than Kentland, and P. xylostella accounted for the majority (70%) of lepidopteran larvae observed (Table 3). Other species included: P. rapae, S. frugiperda, and S. exigua. There was a significant treatment effect on densities of P. xylostella (F = 5.63; df = 4, 40; P < 0.00001) and total lepidopteran larvae (F = 6.38; df = 4, 40; P < 0.00001). The untreated control averaged 3.8 larvae per 20 leaves, which was significantly more than all insecticide treatments (Table 3). Several of the treatments including emamectin benzoate, esfenvalerate, indoxacarb, methoxyfenozide, and spinosad had no live larvae present on leaves. Insecticide treatment also had a significant effect on the percentage of marketable leaves at harvest (F = 3.55; df = 4, 40; P < 0.0022). All treatments had a higher percentage of marketable leaves than the untreated control, and esfenvalerate had a higher percentage than azadirachtin.


Table 3. Density of lepidopteran larvae and percentage marketable leaves (~7 days after treatment) on collards after lowest-labeled rate applications of insecticides in field efficacy trials conducted in Painter, VA in 2003.

Treatment Rate
(kg[ai]
/ha)
Mean no. live larvae
per 20 leaves
% of leaves
marketable

(Rating 1 or 2)
at harvest
Plutella
xylostella
Total
lepidoptera
Acephate 1.087 0.2 b        0.2 c        97.0 ab
Acetamiprid 0.084 0.4 b        0.4 bc        93.0 ab
Azadirachtin 0.011 0.4 b        0.4 bc        83.0 b
Bt subsp. kurstaki
strain HD-1
0.605 0.4 b        1.2 b        94.0 ab
Emamectin benzoate 0.008 0.0 b        0.0 c        97.0 ab
Esfenvalerate 0.032 0.0 b        0.0 c        99.0 a
Indoxacarb 0.072 0.0 b        0.0 c        98.0 ab
Methomyl 0.504 0.4 b        0.6 bc        96.0 ab
Methoxyfenozide 0.112 0.0 b        0.0 c        93.0 ab
Spinosad 0.026 0.0 b        0.0 c        94.0 ab
UTC  -- 3.2 a        3.8 a        76.0 c

Means in a column with a letter in common are not significant (P > 0.05, Fisher’s protected LSD).


Effects of Insecticide Treatments Applied to Collards in the 2004 Growing Season

In 2004 at Kentland, a low density of lepidopteran pests was found on collards, averaging only 1.4 total lepidopteran larvae per 20 leaves. Approximately 42% of larvae were P. xylostella, and the rest consisted of a broad mix of species including T. ni, P. rapae, S. frugiperda, H. zea, and E. rimosalis (Table 4). Despite the low density, there was a significant treatment effect on densities of T. ni (F = 6.0; df = 4, 44; P < 0.0001) and total number of live lepidopteran larvae (F = 2.51; df = 4, 44; P < 0.0151). Though densities were relatively low, the untreated control had significantly more T. ni larvae than all of the insecticide treatments, and significantly more total lepidopteran larvae than all treatments except azadirachtin, esfenvalerate, acetamiprid, and methomyl (Table 4). Insecticide treatment did not have a significant effect on the percentage of marketable leaves at harvest (F = 1.41; df = 4, 44; P = 0.2024).


Table 4. Density of lepidopteran larvae and percentage marketable leaves (8 days after treatment) on collards after lowest-labeled rate applications of insecticides in field efficacy trials conducted at Kentland Farm near Blacksburg, VA in 2004.

Treatment Rate
(kg
[ai]
/ha)
Mean no. live larvae
per 20 leaves
% of
leaves
marketable

(Rating
1 or 2)
at harvest
Plutella
xylostella
Trichoplusia
ni
Pieris
rapae
Total
lepid-
optera
Acephate 1.087 0.0 0.0 b 0.0  0.0 d 100.0 a      
Acetamiprid 0.084 0.4 0.0 b 0.0  0.6 abcd 88.0 a      
Azadirachtin 0.011 0.6 0.0 b 0.0  1.0 abc 89.0 a      
Bt subsp.
kurstaki
strain HD-1
0.605 0.0 0.0 b 0.0  0.0 d 97.0 a      
Emamectin
benzoate
0.008 0.2 0.0 b 0.0  0.2 cd 99.0 a      
Esfenvalerate 0.032 1.0 0.0 b 0.2  1.2 abc 84.0 a      
Indoxacarb 0.072 0.0 0.0 b 0.0  0.0 d 100.0 a      
Methomyl 0.504 0.4 0.0 b 0.2  0.6 abcd 97.0 a      
Methoxy-
fenozide
0.112 0.4 0.0 b 0.0  0.4 bcd 98.0 a      
Novaluron 0.087 0.0 0.0 b 0.4  0.4 bcd 97.0 a      
Spinosad 0.026 0.0 0.0 b 0.0  0.0 d 99.0 a      
UTC  - 0.6 0.6 a 0.0  1.4 a 88.0 a      

Means in a column with a letter in common are not significant (P > 0.05, Fisher’s protected LSD).


At the ESAREC, a moderately high density of lepidopteran larvae was found on collards in the early-planted experiment (ESAREC-1). Species complex was comprised of T. ni (52.3%) and P. xylostella (31.3%), with a few P. rapae (8.4%) and other species. There was no significant treatment effect on densities of P. xylostella, T. ni, or total lepidopteran larvae; however, numeric differences were apparent (Table 5). The azadirachtin, Bt kurstaki, acetamiprid, and esfenvalerate treatments had similar numbers of total lepidopteran larvae as the untreated control. Insecticide treatment did have a significant effect on the percentage of marketable leaves at harvest (F = 2.66; df = 4, 44; P < 0.0106). Indoxacarb averaged 97% marketable leaves and was the only treatment that was higher than the untreated control, which averaged 78% marketable leaves. The same aforementioned insecticide treatments that had similar larval numbers as the untreated control also had similar percentages of marketable leaves (59 to 80%).


Table 5. Density of lepidopteran larvae and percentage marketable leaves (6 days after treatment) on collards after lowest-labeled rate applications of insecticides in field efficacy trials conducted at ESAREC-1 in Painter, VA in July 2004.

Treatment Rate
(kg[ai]
/ha)
Mean no. live larvae
per 20 leaves
% of leaves
marketable

(Rating 1 or 2)
at harvest
Plutella
xylostella
Trichoplusia
ni
Total
lepid-
optera
Acephate 1.087 0.4 0.6 1.0        86.0 abc
Acetamiprid 0.084 1.0 2.0 4.4        59.0 d
Azadirachtin 0.011 0.6 3.2 4.4        72.0 bcd
Bt subsp. kurstaki
strain HD-1
0.605 1.6 1.2 4.8        70.0 cd
Emamectin benzoate 0.008 1.0 1.2 2.2        84.0 abc
Esfenvalerate 0.032 0.6 2.2 3.0        80.0 abc
Indoxacarb 0.072 0.2 0.8 1.4        97.0 a
Methomyl 0.504 1.0 1.6 2.6        75.0 bcd
Methoxyfenozide 0.112 0.0 0.4 1.4        84.0 abc
Novaluron 0.087 0.0 0.8 1.6        89.0 ab
Spinosad 0.026 1.0 1.0 2.2        84.0 abc
UTC  -- 1.6 2.0 4.2        78.0 bc

Means in a column with a letter in common are not significant (P > 0.05, Fisher’s protected LSD).


At the ESAREC-2, lepidopteran pest pressure was moderately high and was comprised almost exclusively (97%) of P. xylostella (Table 6). There was a significant treatment effect on densities of P. xylostella (F = 2.98; df = 4, 44; P < 0.0075) and total lepidopteran larvae (F = 3.06; df = 4, 44; P < 0.0063). The untreated control had the highest larval density (5.8 larvae per 20 leaves), but only methomyl, emamectin benzoate, and esfenvalerate had significantly fewer live larvae than the untreated control (Table 6). Insecticide treatment did not have a significant effect on the percentage of marketable leaves at harvest (F = 1.07; df = 4, 44; P = 0.4065).


Table 6. Density of lepidopteran larvae and percentage marketable leaves (~7 days after treatment) on collards after lowest-labeled rate applications of insecticides in field efficacy trials conducted at ESAREC-2 in Painter, VA in November 2004.

Treatment Rate
(kg [ai]/ha)
Mean no. live larvae
per 20 leaves
% of leaves
marketable

(Rating 1 or 2)
at harvest
Plutella
xylostella
Total
lepidoptera
Acephate 1.087     3.0 ab     3.0 abc 97.5 a
Acetamiprid 0.084     5.3 a     5.5 a 90.0 a
Azadirachtin 0.011     5.0 a     5.3 ab 81.0 a
Bt subsp. kurstaki
strain HD-1
0.605     4.5 a     4.5 abc 82.5 a
Emamectin benzoate 0.008     2.5 ab     2.5 bc 80.0 a
Esfenvalerate 0.032     1.8 bc     2.0 cd 92.5 a
Indoxacarb 0.072     3.0 ab     3.3 abc 90.0 a
Methomyl 0.504     0.3 c     0.3 d 92.5 a
Methoxyfenozide 0.112     5.0 a     5.0 ab 81.4 a
Novaluron 0.087     3.8 ab     3.8 abc 95.0 a
Spinosad 0.026     2.5 ab     2.5 abc 90.0 a
UTC  --     5.8 a     5.8 a 80.0 a

Means in a column with a letter in common are not significant (P > 0.05, Fisher’s protected LSD).


Summary

A wide range of insecticides is registered for use on cruciferous vegetables in the U.S. Our experiments in Virginia showed that not all insecticides (applied at their lowest-labeled rates) provided the same efficacy against lepidopteran larvae on collards. The most efficacious insecticides over the five experiments included acephate, emamectin benzoate, esfenvalerate, methomyl, methoxyfenozide, novaluron, indoxacarb, and spinosad. These insecticides were followed in relative efficacy by Bt kurstaki, acetamiprid, and azadirachtin, which provided relatively inconsistent control of lepidopteran larvae over the experiments.

Interestingly, two of the oldest insecticides used by growers in Virginia, the organophosphate acephate, and the carbamate methomyl, are currently still two of the most efficacious for lepidopteran pest control in crucifer crops. Acephate and methomyl are broad-spectrum toxicants that also control other insect pests such as harlequin bugs, flea beetles, and cabbage aphids. However, these insecticides also kill natural enemies such as insect predators and parasitoids, which can play an important role in regulating certain lepidopteran pest and aphid populations (1,7,11,14). Also, as of 2005, acephate is not registered for use on collards in Virginia, but some formulations of the active ingredient are labeled on other crucifer crops.

A sound integrated pest management program in collards should strive to minimize insecticide applications whenever possible, and if chemical control is necessary, include the use of reduced risk or IPM-compatible products. Our study showed that several insecticides including spinosad, indoxacarb, novaluron, emamectin benzoate, and methoxyfenozide, which are classified as reduced-risk or IPM-compatible products, are efficacious against the lepidopteran pests that attack collards in Virginia. The IPM-compatible insecticides Bt subsp. kurstaki, acetamiprid, and azadirachtin have shown efficacy against lepidopteran pests of crucifers in other studies (12,13), but were inconsistent in their performance in our experiments.

It should be noted that as of 2005, indoxacarb was not registered for use on collards, but was labeled on other cruciferous crops. Also, although spinosad, indoxacarb, and emamectin benzoate have been shown to be relatively safe on predacious hemipterans, mites, coccinellids, lacewings, and some parasitoids (2,16,18,22), they have been shown to be quite toxic to a wide range of parasitic hymenoptera, which are very important to the management of P. xylostella and other pest populations (4,5,6,16,23). However, relatively rapid degradation of surface residues in the field probably improves the compatibility potential of these insecticides with natural enemies. Although more research is needed in this area, there is no doubt that the aforementioned insecticides along with Bt and the insect growth regulators methoxyfenozide and novaluron, are more IPM compatible than traditional broad-spectrum insecticide classes such as organophosphates, carbamates, and pyrethroids, and thus would be better choices for chemical control in a crop such as collards.


Acknowledgments

The authors wish to thank T. Custis (ESAREC) and J. Wooge (Kentalnd Research Farm) for assisting us with field plot establishment and maintenance, and J. Warren, M. Krogh, S. Krogh, J. Young, J. Aigner, M. Rew, J. Speese, and A. Windsor for technical assistance with data collection and plot maintenance. Also, we would like to thank the two anonymous reviewers and Plant Health Progress Senior Editor J. Funderburk for reviewing an earlier draft of this manuscript and providing excellent suggestions.


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