Foraging byHippodamia convergens2for Cereal Aphids1on Wheat Plants in the Laboratory3

Abstract

We investigated predation by adult convergent lady beetle, Hippodamia convergeas Guerin-Meneville, on the English grain aphid, Sitobion avenae L., on wheat, Triticum aestivum L., plants in a laboratory arena. A model relating beetle hunger to starvation time was developed and used to calculate initial hunger for beetles used in predation observations. Hunger level was significantly correlated with the time spent searching a wheat plant and the number of aphids eaten by both male and female convergent lady beetles during a plant visit. Time since contacting (either touching or eating) an aphid was slightly negatively correlated with time the lady beetle spent searching a plant. Time since contact also was slightly negatively related to the number of aphids eaten. The mean time required to eat an aphid increased with aphid instar as was expected because of the corresponding increase in size. Female convergent lady beetles took an average of 27.0 seconds to consume a first-instar English grain aphid, with time increasing to 141.2 seconds for an apterous adult. The mean proportion of English grain aphids leaving a plant in response to foraging by lady beetles increased with aphid instar from 0.09 for first instars to 0.49 for adult alatae. The proportion of aphids leaving a plant was significantly correlated with the time a lady beetle spent searching a plant. Although we observed increased residence time and increased time adult convergent lady beetles spent searching when aphids were abundant, the predation rate was independent of the number of aphids on a plant.