Experimentally induced host-shift changes life-history strategy in a seed beetle

Potential for Acanthoscelides obtectus to Adapt to New Hosts Seen in Laboratory Selection Experiments

Effective pest management strategies for a targeted pest species must rely on accurate, reliable and reproducible estimates of population dynamics. Importance of such approaches is even more conspicuous when assessing pest’s potential to utilize other stored products. Using an experimental evolution approach, we have focused our attention on a common bean pest, the seed beetle (Acanthoscelides obtectus). We looked into the potential to invade and sustain population growth on two suboptimal host plants (chickpeas and mung beans). Such an approach simulates steps of the host-shift process in storages. By analyzing population dynamics during initial encountering with a new host plant, we detected a population drop for both novel hosts. However, transgenerational development in a novel environment resulted in a constant population growth in chickpeas, but not in mung bean populations. Reversal of chickpea selected populations to original host plant has led to a severe decrease in population parameters due to low viability of immatures, while the opposite trend was detected in mung bean populations. This paper highlights the importance of good practice in estimating population dynamics for economically important species. With special emphasis on storage pest species, we discuss how this approach can be useful for estimating invading potential of pest insects.

Host shift induces changes in mate choice of the seed predator Acanthoscelides obtectus via altered chemical signalling

The mechanisms of host shift in phytophagous insects are poorly understood. Among the many proposed processes involved, sexual selection via semiochemicals has recently been suggested. This hypothesizes that sexual communication using pheromones is modified as a result of development on a new host, and such plant-induced phenotypic divergence in mate recognition cues can lead to reproductive isolation between host lines. We tested this hypothesis on Acanthoscelides obtectus, an oligophagous bruchid of Phaseolus vulgaris beans worldwide, which also develops in acceptable non-hosts, such as chickpea (Cicer arietinum L.). Male sex pheromone blends of the bean, chickpea and chickpea/bean host lines during artificially induced host shifts showed different composition. Bean-reared females did not distinguish between blends, whereas chickpea and chickpea/bean females preferred the chickpea male pheromone. However, electrophysiological (EAG) responses to male odour of antennae of the three female host lines were similar, all preferring bean-reared males. Egg-laying choice tests revealed a uniform preference for bean seeds across female host lines, even after multiple generations, whereas larvae did not distinguish between bean and chickpea seeds. We conclude that the development of divergent chemical signalling systems during host shifts does not facilitate the evolution of host races in A. obtectus, because oviposition preferences remain unaffected.

Evolution of larval competitiveness and associated life-history traits in response to host shifts in a seed beetle

Resource competition is frequently strong among parasites that feed within small discrete resource patches, such as seeds or fruits. The properties of a host can influence the behavioural, morphological and life-history traits of associated parasites, including traits that mediate competition within the host. For seed parasites, host size may be an especially important determinant of competitive ability. Using the seed beetle, Callosobruchus maculatus, we performed replicated, reciprocal host shifts to examine the role of seed size in determining larval competitiveness and associated traits. Populations ancestrally associated with either a small host (mung bean) or a large one (cowpea) were switched to each other's host for 36 generations. Compared to control lines (those remaining on the ancestral host), lines switched from the small host to the large host evolved greater tolerance of co-occurring larvae within seeds (indicated by an increase in the frequency of small seeds yielding two adults), smaller egg size and higher fecundity. Each change occurred in the direction predicted by the traits of populations already adapted to cowpea. However, we did not observe the expected decline in adult mass following the shift to the larger host. Moreover, lines switched from the large host (cowpea) to the small host (mung bean) did not evolve the predicted increase in larval competitiveness or egg size, but did exhibit the predicted increase in body mass. Our results thus provide mixed support for the hypothesis that host size determines the evolution of competition-related traits of seed beetles. Evolutionary responses to the two host shifts were consistent among replicate lines, but the evolution of larval competition was asymmetric, with larval competitiveness evolving as predicted in one direction of host shift, but not the reverse. Nevertheless, our results indicate that switching hosts is sufficient to produce repeatable and rapid changes in the competition strategy and fitness-related traits of insect populations.