This compilation indicates that parthenogenesis is much more common among insect pests than related non-pest insect species. The same pattern may occur for other groups of organisms. For instance, there are a number of pest mites that reproduce parthenogenetically. These include the earth mites belonging to the Penthaleidae that comprise pasture and crop pests (Umina et al. 2004
), the Bryobia mites that are pests of pasture and crop plants (Weeks & Breeuwer 2001
; Arthur et al. in press
) and false spider mites that are pests of citrus (Kitajima et al. 2007
). Unfortunately, the mode of reproduction of insufficient numbers of mites is known to undertake a general comparison of the incidence of parthenogenetic reproduction in pest and non-pest mites.
Several factors may contribute to the high incidence of parthenogenesis among pests. The most likely reason is that agricultural environments are stable and uniform with an abundance of resources. In these environments, the same genotype may be continuously favoured by selection; this leads to a selective advantage of some parthenogenetic lineages over a sexual population, whereas sexual reproduction is favoured in variable environments with periods of directional selection (Burger 1999
). Empirical data on invertebrate groups such as oribatid mites support the notion that parthenogens are more likely to have an advantage over sexuals in constant than in variable environments (Domes et al. 2007
Another factor is that environmental cues leading to sexual reproduction may be absent in agricultural ecosystems. Cyclical parthenogens such as aphids that reproduce sexually on some occasions, but not others, often require specific hosts to trigger a sexual phase and these hosts might be absent, particularly if the aphids have invaded agricultural areas from other regions. If the environmental cues are missing for long enough, then the sexual reproduction might be lost permanently when there is DNA decay in genes essential for sexual reproduction (Wilson & Sunnucks 2006
). In grape phylloxera, there is also evidence that lineages in viticultural regions away from the native range of phylloxera have become asexual (Corrie et al. 2002
), in contrast to patterns of reproduction where different hosts are available (Downie et al. 2000
Finally, sporadic colonization might favour parthenogenesis in ephemeral agricultural crops. Colonization is unlikely to favour parthenogenesis in pastures, perennial crops, forests and orchards, but may be important in some horticultural and broad acre crops that are re-sown annually in a patchy arrangement. Some genetic studies suggest that colonization of these crops by pests might involve only a small number of invaders (Daly & Gregg 1985
; Vialatte et al. 2007
), and these invaders may have difficulty in finding mates. However, we suspect that this factor is less important than the others already discussed because many of the groups in with high numbers of parthenogens—such as the Coccidae, Curculionidae and Diaspididae—are pests of woody plants rather than annual crops.
Parthenogenetic species are often thought to represent general purpose genotypes capable of having a high fitness across different environments (Lynch 1984
), but it is now recognized that they can also contain high levels of genetic variability and comprise specialist genotypes adapted to different conditions (Vrijenhoek 1984
; Harshman & Futuyma 1985
; Fox et al. 1996
; Weeks & Hoffmann 1998
). Genotypes can predominate on different hosts or at different times of the year. Parthenogenetic lineages appear capable of exploiting specific niches (Vrijenhoek 1984
; Weeks & Hoffmann 1998
), and the agricultural environment potentially provides a stable distribution of such niches.
Pests are nevertheless faced with unique sets of selection pressures in agricultural environments. In particular, the application of agricultural chemicals provides novel selection pressures to which pests have to adapt. Although parthenogenetic lineages can differ in susceptibility to pesticides (Umina & Hoffmann 1999
), the rates of adaptation will be slower than in sexual species. Therefore, insecticide resistance development is likely to be of less concern in parthenogenetic species, particularly if the resistance involves multiple loci as opposed to single mutational steps. Parthenogens may even have higher inherent rates of resistance to pesticides, as in the case of parthenogenetic earth mites when compared with their close sexual relatives (Umina et al. 2004
), but ongoing evolutionary responses are likely to be slower. Given that sexual reproduction is thought to be at least partly selected through parasites and pathogens (West et al. 1999
; Jokela et al. 2003
; Kumpulainen et al. 2004
), parthenogenetic pests might be particularly susceptible to control by Metarhizium and other biological control agents. In natural populations, sexual forms of species appear to be more persistent in environments where there is a high load of pathogens and parasites (Jokela et al. 2003
), and this factor might be exploited in controlling parthenogenetic pests.
In summary, parthenogenesis appears to be relatively more common among pests than among non-pest groups of insects and mites, at least in those taxa where asexual and sexual species are found. This is consistent with the advantages parthenogenetic organisms are likely to have over sexuals in stable and resource-rich environments. Different control strategies might be implemented against parthenogenetic pests than for sexual species, such as through pesticides where resistance mechanisms require responses through multiple genes and/or biopesticides. This might eventually produce shifts in pest complexes away from parthenogenetic pests in crops that are sprayed or that carry multiple gene resistance mechanisms.