PEA APHID–PARASITOID INTERACTIONS: HAVE PARASITOIDSADAPTED TO DIFFERENTIAL RESISTANCE?

Abstract

In insect host–parasitoid systems, successful immune responses of the host kill the parasitoid, and successful development of the parasitoid will kill the host. Thus, hosts and their parasitoids can exert strong selection on each other that may lead to the coevolution of host defense traits and parasitoid counter-defenses. In upstate New York, pea aphids (Acyrthosiphon pisum) specialized on alfalfa are, on average, two times more physiologically resistant to parasitism by Aphidius ervi parasitoids than pea aphids specialized on clover. There is genetic variation within populations of Aphidius ervi in the ability to overcome pea aphid resistance; thus an evolutionary response to the differences in aphid resistance is possible. Here, I document that pea aphid populations that are more resistant to parasitism in laboratory assays exhibit lower rates of parasitism in the field, suggesting that the genetically based differences in aphid resistance may influence rates of successful parasitism. I found that the difference in resistance between pea aphids specialized on alfalfa or on clover in New York is present in additional populations of pea aphids assayed over several years and is present in Maryland as well as New York. However, despite the potentially large difference in selective regimes due to differences in aphid resistance, parasitoids collected from alfalfa fields and clover fields do not differ in their ability to overcome pea aphid resistance. There is no evidence that they are adapted to aphids from their home crop, or locally adapted to aphids from their home field. The mobility of this parasitoid, and its relatively recent introduction to North America as a biological control agent, may contribute to the lack of adaptation.