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Soybeans
Volume 59 Number 16 Date 08/21/2014 SOYBEAN APHID - Densities have continued to increase this month and reports indicate scattered soybean fields have been treated for aphid control. All soybeans in the seed-filling stages should be evaluated one last time before the end of August to determine if populations have reached or surpassed the economic threshold of 250 aphids per plant on 80% of the plants. Final treatments should be applied before the R5.5 (mid-seed) growth stage since treating at R6 (full seed) or later has not produced a consistent yield benefit. GREEN CLOVERWORM - Larvae ranging from mid- to late-instar are common but not abundant in the southern and western counties. Defoliation levels in surveyed fields are minor at less than 5-10%. This sporadic soybean pest is highly susceptible to parasitism and disease, and is normally controlled biologically without insecticide use. Control is probably unwarranted this year. NORTHERN CORN ROOTWORM - A DATCP surveyor reports that this rootworm species was unusually abundant in a Rock County soybean field sampled earlier this week, with nearly every plant showing some degree of defoliation on the upper leaves. The presence of large numbers of northern corn rootworm beetles in soybeans suggests that the beetles are feeding on the leaves and pollen, and not that egg deposition is occurring. An Iowa State study conducted from 2005-2007 confirmed that the northern corn rootworm is capable of laying very low numbers of eggs in soybeans, but the behavior is probably rare and does not pose a threat to rotated corn. Insecticide use is not recommended even for significant populations of these beetles in soybeans unless combined defoliation levels of this and other leaf feeding insects exceeds the 20% threshold for beans in the seed-filling stages. -- Krista Hamilton, DATCP Entomologist 
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