Control of corn flea beetle and Stewart's wilt in sweet corn with imidacloprid and thiamethoxam seed treatments

Abstract

Two field-plot experiments were conducted in western and central New York to evaluate imidacloprid and thiamethoxam seed treatments for control of corn flea beetle, Chaetocnema pulicaria Melsheimer, and Stewart's bacterial wilt in three varieties of sweet corn. Seed treatments significantly reduced flea beetle feeding injury to leaves in all varieties and reduced disease incidence 37–83% in the susceptible variety ‘Sprint’. Seed treatments did not control Stewart's wilt as effectively as genetic resistance. Disease incidence in the resistant varieties ‘Dynamo’ and ‘Bonus’ was relatively low (⩽5%) with or without a seed treatment. Additional on-farm evaluations conducted in western New York confirmed the results obtained from our field plots with incidence of Stewart's wilt ≈88% lower in imidacloprid-treated fields compared with non-treated fields. Laboratory germination tests indicated that seed treatments with imidacloprid may have some phytotoxic effects, depending on sweet corn variety and vigor of the seed lot. A general recommendation is that only high quality seed lots should be treated with imidacloprid and that carry-over seed should not be used.

Introduction

Stewart's bacterial wilt is an important disease of sweet corn in the central and eastern United States. In 1999, losses in sweet corn yield attributed to Stewart's wilt exceeded $1 million in the state of New York alone (Hoffmann et al., 2000). The disease is caused by the bacterial pathogen, Erwinia stewartii, (Smith) Dye, and is vectored almost exclusively by the corn flea beetle, Chaetocnema pulicaria Melsheimer (Coleoptera: Chrysomelidae) (Pepper, 1967). The bacteria overwinter in the gut of adult C. pulicaria (Poos, 1936). In the spring, beetles infest early plantings of corn and transmit the pathogen to the plant by feeding and defecating on leaves (Dill, 1979). Secondary infection and spread of the disease occurs as beetles feed on infected plants and disperse throughout the field.

Once a plant is infected, bacteria multiply in the vascular tissue, restricting the flow of nutrients and water (Pepper, 1967). Symptoms of infection include light yellow to brown stripes or streaks with wavy or irregular margins on the leaves and stalks. Stems clogged with the multiplying bacteria show a typical discoloration when cut in cross section. Seedlings that survive early infections remain stunted, tassel prematurely, and frequently produce unmarketable ears. Severe infections of Stewart's wilt may result in death to the plant (Ivanoff, 1933; Suparyono and Pataky, 1986). Disease severity depends on growth stage of the corn plant at the time of infection, resistance or susceptibility of the hybrid, and the abundance of inocula (Pataky et al., 1990). Climatic conditions after inoculation can affect disease severity as well. Warm temperatures encourage faster symptom development and movement of the bacteria through the plant and arid conditions impact plant growth and health, which in turn, can affect severity.

Stewart's wilt resistance occurs in a number of sweet corn hybrids (Ivanoff and Riker, 1936; Pataky et al., 1988) and new varieties are continually being screened and rated for Stewart's wilt resistance (Pataky et al (1998), Pataky et al (2000a)). Planting resistant varieties is the best means of disease control. However, resistant varieties are not available for all planting and market windows for processing and fresh market corn production (Hoffmann et al., 1995). Also, plant resistance is not always effective if plants encounter severe flea beetle infestations or if plants are infected too early (Suparyono and Pataky, 1989).

Another strategy for managing Stewart's wilt is to control corn flea beetles before they can transmit the pathogen to corn. Applications of the systematic insecticide carbofuran at planting were shown to provide adequate control of flea beetles and significantly reduced Stewart's wilt incidence on susceptible varieties (Heichel et al., 1977; Ayers et al., 1979; Hoffmann et al., 1993). However, the granular formulation of carbofuran is no larger registered on corn in the USA and liquid formulations of the insecticide have been identified for possible regulatory action under the US Food Quality Protection Act and the Environmental Protection Agency's pesticide re-registration program (Anonymous, 1999). Foliar applications of esfenvalerate, permethrin, or lambda-cyhalothrin are also used to control corn flea beetles in the USA, but are often not effective because beetles colonize fields rapidly and re-colonize fields after spraying, which makes timing of sprays difficult to optimize (Munkvold et al., 1996).

Planting seed treated with a systemic insecticide is a new option currently being researched for managing corn flea beetle populations and concomitant Stewart's wilt incidence in sweet corn. Seed application results in less chemical exposure to humans and the environment. Imidacloprid (Gaucho^®, Bayer AG, Germany), a chloronicotinyl insecticide has shown promise as a seed treatment for control of corn flea beetle. Munkvold et al. (1996) showed that seed treated with imidacloprid at 3 or 6 g a.i./kg of seed killed flea beetles and reduced the number of feeding wounds and Stewart's wilt symptoms in greenhouse studies. Pataky et al. (2000b) recently published one of the first field evaluations of imidacloprid seed treatments on sweet corn. They showed that imidacloprid and another neonicotinoid insecticide, thiamethoxam, reduced the incidence of Stewart's wilt by ≈50–85% relative to non-treated controls. The objectives of our study were to evaluate further the ability of seed-treatment insecticides to control corn flea beetle feeding injury and concomitant Stewart's wilt disease in processing sweet corn in New York, and to assess any potential phytotoxicity associated with the seed treatment application.