© 2007 Plant Management Network.
Alternatives to Pyrethroids for Managing Corn Earworm in Sweet Corn, Seed Corn, Tomatoes and Peppers
Richard Weinzierl, 1102 S. Goodwin Avenue, Department of Crop Sciences, University of Illinois, Urbana 61801
Weinzierl, R. 2007. Alternatives to pyrethroids for managing corn earworm in sweet corn, seed corn, tomatoes and peppers. Online. Plant Health Progress doi:10.1094/PHP-2007-0719-05-RV.
To date, there are no reports of widespread failures of pyrethroid insecticides to control the corn earworm, Helicoverpa zea (Boddie) (Lepidoptera, Noctuidae), in midwestern North America. However, observations of decreased effectiveness of pyrethroids in small-plot insecticide evaluations, lower-than-expected mortality in adult bioassays using diagnostic doses of pyrethroid insecticides, and scattered control failures in commercial sweet corn fields lead to the inference that H. zea resistance to pyrethroids is a potential problem in the midwestern United States and southeastern Canada. Sweet corn, seed corn, tomatoes, and peppers are among the crops most often protected from H. zea by applications of pyrethroids. If resistance compromises pyrethroid effectiveness, adopting or developing alternative management practices will be essential. This paper summarizes the current status of management alternatives for H. zea, specifically in commercial production of sweet corn, seed corn, tomatoes, and peppers.
Cultural Practices that Minimize or Discourage H. zea Infestations
Of the common cultural practices that are feasible in commercial production of the crops addressed in this paper, early planting – and thereby avoidance — is the most effective in the Midwest, the northeastern states, and southeastern Canada. Because H. zea does not overwinter in substantial numbers in most of these regions, early plantings of sweet corn, seed corn, tomatoes, and peppers often mature before migrating moths from southern states arrive and initiate infestations. From mid-northern Illinois and Indiana through south-central Minnesota, Wisconsin, and Michigan, for example, sweet corn, tomatoes, and peppers harvested before mid- to late August usually escape severe infestations (8). However, weather systems may transport H. zea moths to the region as early as mid-June, and overwintered populations may initiate infestations in June or July from mid-Missouri through southern Illinois and southern Indiana, so early planting does not always prevent losses. Additionally, continued harvest over the entire potential season, including all of August and much or all of September, is necessary for the economic viability of many farms, so early crops are not a complete answer to management needs.
Intercropping and trap cropping are additional cultural practices, but the effectiveness and practicality of these approaches are not well established for H. zea. By far the most attractive crop for ovipositing H. zea moths in the Midwest is corn with fresh silks (7), so strips of sweet corn or dent corn (or during mid-summer, millions of acres of dent corn) with fresh silks can divert moths from depositing eggs on tomatoes and peppers. Unfortunately, no truly effective trap crops exist for sweet corn or seed corn that is silking when moths are depositing eggs.
Host Plant Resistance
Host plant resistance is another potential tool for managing H. zea. Breeding for dent corn and sweet corn resistance to corn earworm has been ongoing for decades (3,4,15,20) and has focused on physical characteristics of plants as well as antibiotic compounds in silks. Nonetheless, although current sweet corn hybrids exhibit differential susceptibility to earworm damage based largely on husk coverage of the ear tip and length of the silk channel, no highly resistant sweet corn, tomatoes, or peppers are available except for genetically modified sweet corn hybrids that produce the Bacillus thuringiensis (Bt) Cry1A(b) toxin [e.g., (1,2)].
A wide range of predators and parasites attack the corn earworm in corn, peppers, and tomatoes, but none are used commonly for insect management in commercial production of these crops. King and Coleman (9) reviewed the potential for biological control of Heliothis (and Helicoverpa) species, and they noted that in the United States, Trichogramma spp. are the most common egg parasites; Cardiochiles nigriceps, Microplitis croceipes, and Cotesia marginiventris are the most common larval parasites. Predominant predators include coccinellid beetles (Hippodamia convergens, Coleomegilla maculata, and Scymnus spp.), heteropterans (Geocoris spp., Orius spp., Nabis spp., and Podisus spp.), and chrysopids (Chrysopa spp.) (9). Knutson (10) summarized the use of Trichogramma spp. in biological control of H. zea and Heliothis virescens in cotton, but no similar recommendations are available for earworm control in commercial production of corn, tomatoes, or peppers. Where nearly total prevention of infestation of sweet corn ears and tomato and pepper fruits is required to meet market demands, biological control efforts using natural enemies seem infeasible following heavy migratory flights of H. zea moths and the resulting concentration of egg-laying that occurs on these crops in August and September.
For the management of pyrethroid-resistant H. zea in commercial vegetable crops and seed corn, insecticides are essential. Evidence suggests that resistance to the pyrethroids will be class-wide, so the following discussion of alternatives focuses on insecticides with structures and modes of action that differ from those of the pyrethroids. A summary of the effectiveness against H. zea of insecticides currently labeled on sweet corn, seed corn, tomatoes, and peppers is presented in Table 1. This summary is based on product labels, listings in the Midwest Vegetable Production Guide for Commercial Growers, 2006 (6), the 2006 Illinois Agricultural Pest Management Handbook (16), and numerous published and unpublished reports of insecticide evaluations in the Midwest.
Sweet corn. In sweet corn, the need for control of H. zea can be determined from counts of male moths in pheromone traps. When captures exceed thresholds and fresh silks are present, applications of insecticides – usually pyrethroids — are recommended on 2- to 5-day intervals, with the specific recommendation dependent on numbers of moths captured, temperature, and the market for the crop (fresh-market versus processing) (6,8,16).
If pyrethroid insecticides fail to control H. zea because of resistance, currently available non-pyrethroids are at best partially effective as replacements (Table 1). Those that offer only poor to fair control of H. zea include Bt applied as a spray, indoxacarb, methoxyfenozide, methomyl, and carbaryl. Foliar applications of Bt are ineffective for H. zea control in sweet corn because larvae do not feed on treated tissues before entering the silk channel. Carbaryl (Sevin) is rated only fair, but it is a General-Use pesticide used widely by small-scale growers who do not possess a Pesticide Applicator license and cannot purchase or apply Restricted-Use pesticides.
Thiodicarb (Larvin), a carbamate, currently is not used widely in the Midwest, but its effectiveness and residual activity against the fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), in Florida (14), coupled with limited observations of effectiveness against H. zea (19), suggest an efficacy rating of Good (or better) for earworm control. Per-acre costs for applications of thiodicarb are not expected to differ substantially from those of pyrethroids (11).
Trials from throughout the Midwest indicate that spinosad is less effective than the pyrethroids against pyrethroid-susceptible H. zea when applied at the same intervals, but spinosad formulations have performed better than most other nonpyrethroids. In other crops, including cole crops and apples, spinosad’s effectiveness seems to be dependent on more frequent applications than needed for organophosphates or pyrethroids. In 2006 in Illinois, spinosad applied at 2- to 3-day intervals resulted in approximately 80 to 90 percent control in comparison with untreated checks. Although such frequent applications and the high cost of spinosad would likely be too expensive for sweet corn grown for processing, these costs could be recouped in fresh-market sweet corn, especially organic sweet corn, if treatment provides high levels of control.
Bt sweet corn. Considered here to be a form of insecticide treatment, Bt sweet corn controls H. zea adequately for many fresh-market growers, with one or two insecticide applications sometimes needed to suppress other pests (1,2,12,13,17,18). Currently no sweet corn grown for processing includes Bt hybrids because of international marketing concerns. Neither spinosad nor Bt hybrids control western corn rootworm beetles, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), or Japanese beetles, Popillia japonica Newman (Coleoptera: Scarabeidae), two pests of increasing importance in the Midwest, both of which clip the silks of corn, sometimes preventing pollination.
Seed corn. The alternatives for corn earworm control are fewer in seed corn; thiodicarb is not labeled, and not all seed production involves Bt-transformed inbreds. Additionally, the increased costs that would be associated with frequent applications of spinosad may not be recouped with the current price structure for seed corn. Although control in seed corn need not be as complete as in sweet corn, adequate management may be very difficult if spinosads, carbaryl, and methomyl are the only registered products that provide fair or better control of H. zea.
Tomatoes. Foliar applications of Bt or spinosad offer better control in tomatoes than they do in corn because H. zea larvae feed on foliage and the surface of fruits as well as internally in fruits. Ingestion of plant surface tissue results in ingestion of Bt spores and toxins sprayed onto plants, making control possible. Even so, these alternatives are far less cost-effective than the pyrethroids because (1) they are more expensive per application; (2) more frequent applications are required; and (3) unlike pyrethroids, they are not effective against most other key pests, particularly stink bugs, requiring additional control practices and costs. Insecticides that provide only fair control of H. zea in tomatoes (perhaps adequate when pest pressure is low) include carbaryl, endosulfan, methomyl, and methoxyfenozide.
Peppers. H. zea has not often been a severe problem in peppers in the Midwest, in part because of pyrethroid applications targeting the European corn borer. However, when late season peppers are fruiting at the same time that moth flights are heavy and most alternative hosts have senesced, extensive fruit damage can occur in untreated fields. Feeding on the fruit surface of most varieties is limited, and instead larvae tunnel into fruit and feed within plant tissues, protected from insecticide applications. As a result, foliar applications of Bt are much less effective than they are in tomatoes, and spinosad is slightly less effective in peppers as well. Methomyl (Lannate) gives fair control but requires frequent applications.
Summary and Conclusions
In summary, current alternatives for the control of H. zea in sweet corn, seed corn, tomatoes, and peppers are limited. The older carbamates (carbaryl, methomyl, and thiodicarb), organophosphates (acephate, dimethoate, and methyl parathion), and organochlorine (endosulfan) that remain registered on these crops generally are less effective than the pyrethroids have been. Additionally, they are considered by the United States Environmental Protection Agency to pose greater risks to human health, and they are the targets of continuing evaluations to assure their uses meet the standards of the 1996 Food Quality Protection Act. Of the newer insecticides that generally target lepidopteran larvae and are registered on one or more of these crops, neither indoxacarb nor methoxyfenozide has proven to be adequately effective against this insect. Formulations of Bt used in foliar applications offer a partially effective substitute for pyrethroids in tomatoes but not in the other crops covered here, and the cost of using them is increased by the need for frequent applications. In addition, other controls are needed against non-lepidopteran pests. They are less effective against H. zea in corn and peppers because of the nature of the insect’s feeding behavior in these crops. Transgenic Bt sweet corn offers a partial solution in some situations as described above, but it is not yet accepted widely enough internationally to allow processors to risk market access by growing it. Spinosad is moderately effective against H. zea if applied frequently, but its cost and its ineffectiveness against a number of other key pests challenge the profitability of producing sweet corn for processing, as well as seed corn. Thiodicarb may be important in the short term as an effective alternative to the pyrethroids for use in sweet corn.
The potential loss of effectiveness of pyrethroids against H. zea as a result of resistance would severely disrupt pest management programs in sweet corn, seed corn, and tomatoes. As a result, efforts to slow the evolution of resistance in the southern United States – the annual source of migrant populations – are especially important, not only in southern source regions but also in the Midwest, the northeastern United States, and southeastern Canada. Equally important is the identification, development, and registration of new insecticides with different chemical structures and modes of action. Among new lepidopteran-active insecticides not yet labeled on these crops, rynaxypyr and pyridalyl initially appear to be very effective (5, 19); novaluron and metaflumizone are far less effective [(19), personal communication, W. Hutchison, University of Minnesota].
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