Model for Implementing a Genetically Improved Strain of a Parasitoid

Abstract

Computer simulation was used to estimate the effects of biological and operational parameters on introduction and establishment of a laboratory-selected strain of the walnut aphid parasite, Trioxys pallidus , resistant to azinphosmethyl. Regional establishment was promoted by migration of the parasite between orchards, low survivorship of the susceptible strain when treated with insecticide relative to the resistant strain, and minimal refuge from insecticide. Splitting available resistant parasitoids into smaller groups and releasing the groups in different orchards increased the speed of establishment. Increasing the frequency of insecticide application or increasing the relative frequency of use of azinphosmethyl over alternative pesticides also accelerated establishment. The model suggests that under current cultural practices in California walnut orchards, establishment of the resistant strain of Trioxys pallidus so that it makes up 50% of the individuals in at least 90% of the orchards, will require 5–7 yr. Possible genetic improvement projects should be evaluated not only on the potential for improving desirable traits, but also on the potential for implementing the new strain. Our results suggest that replacement of one natural enemy biotype with another may not always occur in <5 yr, despite relatively strong selection for the new biotype.