Japanese Beetle are Back!! I have begun to notice these past few days that a few Japanese Beetles have begun feeding on the edges of a few fields. Japanese beetles can feed on both corn and soybeans. They are also known to feed on up to 300 different plant species. It is not un-common to notice them feeding in your flower garden, trees, the roadside ditch, and in the case below, a soybean field. They are now in their adult stage of the life cycle and these beetles will continue to feed and thrive in the current hot and dry weather for about another 30 days.
With all vegetative feeding insects, thresholds for tissue and leaf loss will vary dependent on the cost of the treatment for control as well as the intended commodity price for sale of the crop.
IDENTIFICATION AND LIFE CYCLE
Adult Japanese beetles are nearly a ½-inch long, have a metallic green head and neck region, reddish to bronze wing covers, rows of six white bristle bunches along each side of their abdomen, and live 30 to 60 days. Adults emerge from the soil starting in late May and early June, with peak emergence occurring 4 to 5 weeks later. Mating occurs soon after emergence causing the females to burrow 2 to 4 inches into the soil and lay 1 to 4 eggs every 3 to 4 days for several weeks.
The grubs emerge from the eggs in about 10 days and grow quickly to full size, about 1-inch long. The grubs feed on the roots of living plants and then overwinter. When soil temperature climbs above 50 °F in the spring, the grubs begin to move toward the soil surface to feed and pupate prior to emerging as adults.
CORN SCOUTING AND THRESHOLDS
The adults can feed on leaves, tassels, silks, and pollen. Corn leaves may appear lacy or skeletonized, but leaf feeding is rarely of economic importance. Economic damage can occur when beetles clip silks during pollination, which can result in partially pollinated ears. Silk clipping after pollination does not affect yield potential.
When scouting corn for Japanese beetles, a representative portion of the entire field should be evaluated. If sampling is only conducted near field edges, where populations of Japanese beetles are usually clumped together, populations across the field could be overestimated. An insecticidal treatment should be considered during corn silking stage if:
• There are 3 or more Japanese beetle adults/ear, and
• Silks have not been clipped to less than a 1/2-inch, and
• Pollination is less than 50% complete, and
• Japanese beetles are still present and actively feeding.
SOYBEAN SCOUTING AND THRESHOLDS
Although Japanese beetles can cause extensive defoliation, soybean plants have the capability to compensate for the damage, and defoliation seldom affects yield potential (Figure 4). Flowering soybean fields should be scouted for the presence of Japanese beetles and the extent of defoliation. The percent defoliation should be estimated on randomly selected leaves in at least five different areas of the field. Insecticide applications should be considered if:
• 30% defoliation occurs prior to bloom, or
• 20% defoliation occurs after bloom, and
• Japanese beetles are present and actively feeding.
Individual state insecticide recommendations for the control of Japanese beetles can differ and must be followed. Damage from Japanese beetles can add to other stresses the crop is experiencing, and economic thresholds may need to be adjusted if plants are under moisture stress. This, along with commodity prices, should be taken into consideration when using thresholds to determine if insecticide treatment is needed. Insecticides may initially control or knock-down a population; however, poor residual activity and the mobility of the insect could lead to the need for a second application if populations resurge later. With all treatments, consider an insecticide with multiple or different modes of action (MOA).
John W. McNamara
Wiles Bros. Inc.
606 Wiles Road
Plattsmouth NE. 68048