Foraging by Hippodamia Convergens1 for the Aphid Sitobion Avenae2 on Wheat Plants Growing in Greenhouse Plots

We investigated predation by adult convergent lady beetle, Hippodamia convergens Guerin-Meneville, on English grain aphid, Sitobion avenae L., on wheat, Triticum aestivum L., growing in 1.8 × 1.8 m plantings in a greenhouse with a soil floor. The wheat was planted to simulate a typical production field, with one seed per 2.5 cm in each row, and rows planted 15.2 cm apart. The plot was artificially infested with aphids. Beetles in one group (designated “light-fed”) were provided with 20 English grain aphids of mixed instars per day, whereas beetles in a second group (designated “heavy-fed”) were provided with 80 English grain aphids per day for 4–6 days before observation. Beetles were observed foraging in the plot for a period of 12 minutes. For each visit to plants, and for movements of beetles on the soil floor of the plot, the amount of time spent searching and eating were recorded, as were incidents of beetle flight from one location to another in the plot or out of the plot. Air temperature, intensity of solar radiation, height of the wheat plants, and density of the aphids were measured concomitant with foraging observations. The number of aphids eaten in the wheat plots averaged 0.16 per minute. The number of aphids eaten per minute increased as the number of aphids per tiller increased (t = 3.80; df = 1, 46; P = 0.0004). Analysis of covariance applied to predation data from the aphid-infested plots indicated that light-fed beetles ate a significantly greater number of aphids per minute than did heavy-fed beetles (F = 5.35; df = 1, 212; P = 0.02). However, the number of aphids per tiller, the covariable in the analysis, was not significant (F = 1.64; df = 1, 211; P = 0.20), indicating that predation rate was not related to aphid density. This observation suggested that the physiological state of the beetles (poorly fed or well fed) was dominant over aphid density in influencing predation rate.