Field Crops News

Do Weeds Improve Black Cutworm Control by Its Parasite?

Black cutworm an important pest of seedling corn is more often a problem in reduced tillage corn fields because the moth prefers to lay eggs on crop residue and weeds. However, several of these weeds could serve as nectar sources for Meteorus rubens, a wasp parasitoid of black cutworm larvae. Parasitized black cutworm larvae damage about 35% fewer corn plants, and eventually die. Laboratory studies by Foster and Ruesink (1984) demonstrated that wasps lived 3 to 5 days longer when caged with flowering weeds (common chickweed, shepardspurse, wild mustard, wild parsnip or lady's thumb smartweed) compared with wasps provided only water. There were no significant differences between weed species in their effects on the wasp's survival or reproduction. Plants without flowers did not increase wasp longevity, suggesting that availability of nectar rather than moderation of microclimate was the factor influencing wasp survival. When wasps were provided black cutworm larvae to parasitize, the flower-fed wasps produced 118-217 offspring, while the number of offspring was signficantly reduced (77) for the water-only wasps. In larger scale greenhouse studies, Foster & Ruesink (1986) examined the effect of common chickweed on M. rubens survival and parasitism. They again found increased wasp survival and cutworm parasitism in the presence of chickweed, but the effect was projected to reduce damage to corn seedlings by only about 5%. The authors concluded that in this case the beneficial effects of weeds were probably not enough to outweigh the detrimental effects of weeds in increasing black cutworm densities.

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Lady Beetles Prefer Greenbugs Over Russian Wheat Aphid

Laboratory studies were conducted to evaluate differences in prey preference and growth and development of seven-spotted and covergent lady beetles when fed either Russian wheat aphids or greenbugs as their sole food source. Both immature (fourth instar larvae) and 2-day old adult lady beetles were tested for preference between the two aphids; both aphid species were equally acceptable to either lady beetle species. Also, survival to adulthood and adult weight of male and female beetles of both lady beetle species were similar regardless of which aphid they fed upon. There was a significant delay in development (egg hatch to adulthood) of both lady beetle species when fed upon Russian wheat aphids, however it was not large (0.4 days for seven-spotted and 1.1 days for convergent lady beetles). Both greenbugs and Russian wheat aphids may occur together in wheat fields. Russian wheat aphid feeding results in leaf curling of wheat, effectively hiding aphids from many larger natural enemies such as lady beetles. Because these two lady beetles had no preference for Russian wheat aphid over greenbugs, where both aphids are present lady beetles will likely feed first on greenbugs, which feed on the underside of leaves and are more easily found than Russian wheat aphids feeding inside curled wheat leaves.

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