Searching Activities by Coccinellids on Rolled Wheat Leaves Infested by the Russian Wheat Aphid

Because leaves of many Gramineae infested by the Russian wheat aphid, Diuraphis noxia, do not unfurl, some predators (and other natural enemies) do not reach this aphid. We quantified the search strategies of five exotic aphidophagous coccinellids to compare their abilities to exploit the curled leaf. We manually curled leaves of wheat seedlings to form tubular leaf structures. Activities of coccinellid adults and larvae on rolled leaves and open leaves, and on D. noxia-infested and D. noxia-uninfested leaves, were studied. Adults and larvae were on plants greater-than-or-equal-to 65% of the time. Larvae of all species and adults of Scymnus frontalis and Propylea quatuordecimpunctata searched more for prey on rolled leaves than on open leaves. Searching time inside rolled leaves was greater by S. frontalis adults than by other adults and greater by S. frontalis and P. quatuordecimpunctata fourth instars than by Coccinella septempunctata, Hippodamia variegata, and Hippodamia tredecimpunctata. There were no significant differences in searching times among second instars inside rolled leaves. The large size of adults and fourth instars limited access to leafrolls by all species compared to S. frontalis. Openings <2.6 mm in diameter in a divided petri dish inhibited entry by S. frontalis adults. The presence of D. noxia did not elicit increased time within rolled leaves by coccinellids of any lifestage. We conclude that S. frontalis is the best adapted of these five coccinellids for finding D. noxia in rolled leaves.