Biological control of cactus weeds: implications of hybridization between control agent biotypes.
Results of recent research on Dactylopius opuntiae, a biological control agent for cactus weeds (Opuntia spp.) in South Africa and elsewhere, challenge the maxim that genetic diversity of agents necessarily enhances the chances of success in biological weed control. Two biotypes (ficus and stricta) of D. opuntiae, each specific to a different Opuntia species (O. ficus-indica and O. stricta), interbred freely, at least under insectary conditions. We therefore carried out crossbreeding experiments to determine the viability and host preferences of progeny produced by these crosses. Unlike their parents, F1 hybrids were not species specific, developing equally well on either of the parental hosts, O. ficus-indica and O. stricta. The situation was more complex in F2 backcrosses between hybrids and in crosses between parent strains and hybrids because male cochineal insects contributed only maternally inherited genes to their progeny, due to their unusual haploid-diploid (lecanoid) mechanism of sex determination. Some F2 combinations produced cohorts of progeny that were either entirely true-bred (i.e. host-specific) or entirely hybrid (i.e. not host-specific) genotypes, while other combinations produced groups of siblings with some individuals (theoretically half) that were true-bred genotypes and the balance were hybrid genotypes. The lack of host specificity of hybrids should enhance overall biological control of the target species directly, because hybrids attack both host plants, and indirectly, because hybrid nymphs have greater chances of finding a suitable host during passive dispersal. However, this advantage will be negated when F2 crosses produce host-specific nymphs on host plants that are incompatible for their survival. These findings show that only pure strains of D. opuntiae should be released in monocultures of the target weeds. More generally, they caution that the possible consequences of mixing genotypes of a biological control agent species should be investigated before different provenances are amalgamated to enhance genetic diversity.