Biological control of cactus weeds: implications of hybridization between control agent biotypes
Article first published online: 11 DEC 2002
Journal of Applied Ecology
Volume 39, Issue 6, pages 900–908, December 2002
How to Cite
Hoffmann, J. H., Impson, F. A. C. and Volchansky, C. R. (2002), Biological control of cactus weeds: implications of hybridization between control agent biotypes. Journal of Applied Ecology, 39: 900–908. doi: 10.1046/j.1365-2664.2002.00766.x
- Issue published online: 11 DEC 2002
- Article first published online: 11 DEC 2002
- Received 10 October 2001; final copy received 26 July 2002
- Dactylopius opuntiae;
- genetic diversity;
- 1Results 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.
- 2Two biotypes of D. opuntiae, each specific to a different Opuntia species, interbred freely, at least under insectary conditions. We therefore carried out cross-breeding experiments to determine the viability and host-preferences of progeny produced by these crosses.
- 3Unlike their parents, F1 hybrids were not species-specific, developing equally well on either of the parental hosts, Opuntia ficus-indica and Opuntia stricta. The situation was more complex in F2 back-crosses 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.
- 4The 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.
- 5These 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.