EVIDENCE FOR NONADAPTIVE EVOLUTION IN PARASITOID VIRULENCE FOLLOWING A BIOLOGICAL CONTROL INTRODUCTION

Abstract

Biological control systems can evolve; founder effects, drift, inbreeding, and adaptation to new environments can occur during the introduction and establishment of exotic pests and their natural enemies. It has been hypothesized that successful biological control agents are those that become locally adapted to their new environment or populations of hosts. However, there are no explicit comparisons of native and introduced populations of biological control agents testing for local adaptation. The pea aphid, Acyrthosiphon pisum, is an introduced pest of legumes. Aphidius ervi, a parasitoid wasp, was introduced from France to control pea aphids. Using reciprocal transplant experiments, I compared introduced populations of pea aphids and A. ervi from New York State with native populations from France. I documented patterns of aphid resistance to parasitism and A. ervi ability to overcome aphid resistance (virulence) in the two localities and explored whether the introduced parasitoids are locally adapted to the introduced pea aphids. I found that parasitoids from native French populations have high rates of parasitism on pea aphids specialized on clover and on pea aphids specialized on alfalfa, regardless of whether the aphids are collected from France or New York. Introduced parasitoids from New York have high rates of parasitism on pea aphids from clover, but low rates of parasitism on pea aphids from either New York or French alfalfa fields. Thus, there is no evidence that the introduced wasps have become adapted to their local populations of pea aphid hosts. On the contrary, the ability of the introduced parasitoids to develop successfully in pea aphids from alfalfa may have been compromised by the biological control introduction. This apparently nonadaptive evolution was cryptic until examined experimentally. A better understanding of the effects of biological control introductions on natural enemies will come through additional comparisons of native and introduced populations of biocontrol agents. Such comparisons may provide valuable insights into the role of microevolutionary change in biological control and suggest useful avenues to enhance the success of biological control introductions.