Early season corn flea beetle management

Corn flea beetles (Chaetocnema pulicaria) are an early-season pest that can be especially active after mild winters. The Stewart’s Wilt bacterium is vectored by corn flea beetles. Although most commercial hybrids have tolerance to Stewart’s wilt bacterium, seed corn inbreds can be susceptible to the disease. Mild winters can lead to a planting season with greater beetle activity, and consequently, a greater risk of early Stewart’s wilt infection.

Description and Biology

Corn flea beetles are shiny, black, and are approximately 1.8 mm long with elongated hind legs. They overwinter as adults in grassy areas around corn fields, and mated females lay their eggs close to the base of corn plants in the spring. Eggs hatch into larvae after six days. Larvae feed on corn roots and pupate. Within two weeks beetles emerge from the soil.¹ The generations during corn emergence can be a potential problem.

Corn flea beetles jump like fleas on a dog and feeding on corn leaves creates white streaks on leaves as tissue is scraped away. If populations are extreme, their direct feeding can kill leaves and seedlings. Through their feeding, which can begin when corn seedlings emerge, bacteria that cause Stewart’s Wilt can be passed along to the corn plant. Corn fields that are most likely to sustain injury are those planted in cool, wet springs because they can grow slowly and are exposed to feeding for a longer period of time.

Beetle Overwintering

Beetle survival overwinter is dependent on temperature.  A winter temperature index can be used to help forecast flea beetle survival (Table 1). The following index is derived from Celsius temperatures over the winter months December, January, and February and uses the formula: ∑ [-17.7°C – mean monthly temperature °C) x -1.8].²

Scouting

Fields should be scouted every four to five days from early May to late June. Five sets of 20 seedling plants per field should be examined to determine corn flea beetle densities.¹

Threshold

Certain inbreds used for seed corn production and some hybrid corn lines may be susceptible to the Stewart’s wilt bacterium. Fields planted with susceptible corn products have a threshold of six beetles per 100 plants prior to the fifth-leaf stage.¹ For hybrids tolerant of the disease, thresholds of an average of five or more beetles per plant prior to the fourth-leaf stage may justify treatment.

Treatment

Stewart’s Wilt causes more severe symptoms when infection occurs early, and seed treatments may be used to help protect seedlings from early infection. Seed treatments such as clothianidin (Poncho^® 250), imidacloprid (Gaucho^® 480 FL), and thiamethoxam (Cruiser^® 5 FS) can be used to help protect against corn flea beetle damage to emerging seedlings.³ Eastern Canada research from 2000 demonstrated corn flea beetle feeding could be reduced by 60% using seed treatments.⁴ Growers using fungicide seed treatment products alone should scout for flea beetles and determine if their numbers would warrant a foliar insecticide application. It is typically uneconomical to make foliar insecticide applications unless the hybrid is highly susceptible to Stewart’s Wilt.³

Stewart’s Wilt

Beetles may harbour the Stewart’s Wilt bacteria throughout the winter or they may acquire the bacteria by feeding on infected plants in the spring.⁵ Beetles can then transmit the bacteria through the plant wounds created from feeding on corn plants. The disease has two phases: seedling phase and the leaf blight phase which occurs after tasseling.⁵ The first phase is generally more severe with the possibility of infected seedlings becoming wilted and dying if the base of the stalk becomes rotted. To observe for the rot, dig seedlings and slice them lengthwise. A yellow ooze may flow from the vascular bundles. Observe the stem base for a rotted cavity. The leaf blight phase is characterized by long water-soaked lesions extending the length of the leaf. Lesions eventually turn necrotic and can resemble those of Goss’s Wilt; therefore, a laboratory confirmation may be required.

Summary

Fields with corn products susceptible to Stewart’s wilt may need protection against corn flea beetles if winter temperatures are mild. Consult your DEKALB^® brand seed representative for information on corn products with tolerance to Stewart’s Wilt and other diseases.