Biological control of cactus weeds: implications of hybridization between control agent biotypes

Abstract

Summary 1.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. 2. Two 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. 3. Unlike their parents, F 1 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 F 2 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 F 2 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. 4. 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 F 2 crosses produce host-specific nymphs on host-plants that are incompatible for their survival. 5. 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.

Biological Control of Cactaceae in South Africa

“…This potential advantage is negated because the F2 and subsequent generations produce progeny which revert to being host specific and at least half of the crawlers that are produced occur on plants on which they cannot develop (Hoffmann et al 2002;Hoffmann 2004).…”

“…In using the two biotypes, consideration needs to be given as to what might happen when the host plants occur sympatrically and the insects are free to interbreed. Laboratory studies showed that the two biotypes interbreed freely and produce viable progeny that develop equally well on either of their parental host plant species (Hoffmann et al 2002).…”

Hybrid vigor in the biological control agent, Longitarsus jacobaeae

“…Further tests on those species could assess the effects of hybridization, thereby avoiding possible negative outcomes as observed by Hoffmann et al. () for D. opuntiae . However, we are not aware of any cases where specificity of hybrids between two agent biotypes, both specific to the same host, would change upon hybridization.…”

“…For example, two biotypes of Dactylopius opuntiae (Cockerell) were found to be specific to two different invasive Opuntia species (Hoffmann et al. ). Upon hybridization, the F1 generation was able to attack either host, while both host‐specific and non‐host‐specific genotypes were produced in the F2 generation potentially reducing effectiveness of biological control in monocultures of either weed (Hoffmann et al.…”

The implications of rapid eco‐evolutionary processes for biological control ‐ a review

“…In the past decade, regulations have become much stricter, partly due to an increasing number of studies demonstrating that different populations of a species can differ in several biological traits but most importantly also in host specificity (Hopper et al, 1993;Hoffmann et al, 2002;Mathenge et al, 2010). Given that, it is now expected that each genetically and/or geographically distinct population of an agent be tested separately for host specificity (Barratt et al, 2010), which has led to the general practice of screening agents only from a single or very few populations to keep cost down.…”