Host shift capability of a specialist seed predator of an invasive plant: roles of competition, population genetics and plant chemistry

Bottom-up and top-down pressures mediate competition between two generalist insects

The effects of competition can have far-reaching consequences for individuals, populations, and communities and therefore we should strive toward a deeper understanding of competitive interactions. In some cases, dietary generalists may be predicted to experience weak competition effects because of their ability to use a wide range of host plants. However, competition between insects frequently occurs indirectly, which can hinder insects' abilities to avoid competitive interactions. Therefore, competition may be as strong among dietary generalists as among dietary specialists. Yet competition between insects that are dietary generalists is infrequently studied. We tested for evidence of competitive interactions between two common, temporally separated, generalist insects: the western tent caterpillar (Malacosoma californicum), which feeds early in the season, and the fall webworm (Hyphantria cunea), which feeds later in the season. Both species frequently use a common host plant species (chokecherry) as a preferred host at our field sites. We tested the relative strength of bottom-up effects resulting from competitive interactions between these two generalists with laboratory-rearing trials at the relevant time of year for each insect. We recorded three common fitness measures (development time, pupal mass, and survival) for caterpillars reared on chokecherry with no damage from either of our focal species, with tent caterpillar damage, and with fall webworm damage. To test the strength of top-down pressures on fall webworm larval fitness and any potential interactions with bottom-up effects, we reared larvae in the field either exposed to or protected from predators on host plants that either did or did not have tent caterpillars feeding on them earlier in the season. We found evidence of bottom-up fitness effects on tent caterpillars and top-down and bottom-up fitness effects on fall webworms confirming that tent caterpillars and fall webworms compete indirectly. Tent caterpillars had lower pupal mass when reared on leaves from shrubs damaged by fall webworms. Fall webworms had lower pupal mass and longer development time when reared on leaves from shrubs damaged by tent caterpillars. In field trials, fall webworms reared on shrubs damaged by tent caterpillars had a lower survival and pupal mass. We show evidence of indirect competition in temporally separated generalists through leaf quality (bottom-up effects) and natural enemies (top-down effects).

How the seed coat affects the mother's oviposition preference and larval performance in the bean beetle (Acanthoscelides obtectus, Coleoptera: Chrysomelidae, Bruchinae) in leguminous species

Background

The host specificity and host range of the dry bean beetle, Acanthoscelides obtectus (Coleoptera: Chrysomelidae, Bruchinae), a seed predator of beans, is poorly known. In addition, the female oviposition preference and larval performance relationship is complicated by the respective importance of seed coat and cotyledon, because, paradoxically, females lay eggs on the basis of stimuli of the seed coat alone, without directly being able to assess the quality of the cotyledon’s suitability for larval development. Conversely, the thickness of seed coat may prevent first instar larvae from entering the seeds, even if cotyledons are suitable for development.

Methods

The seeds of 62 leguminous species and 75 cultivars and accessions occurring in Hungary were evaluated for preference-performance relations. The preference of female bean beetles for seeds was measured in no-choice egg-laying tests. The ability of first instar larvae to overcome the seed coat as a physical barrier was tested with intact seed coat, while pre-drilled seed coats allowed the larvae to assess the suitability of cotyledon for development. The number of emerging adults was recorded. The thickness of seed coats and the weight of seeds were measured. Nonparametric tests and logistic regression were used for the statistical analyses of data and effect sizes were also calculated.

Results

Seeds of 18 leguminous species (35% of them Lathyrus) supported larval development to adults if the seed coat was pre-drilled; however, only nine leguminous species supported development to adults if the seed coat was intact. Seed coat thickness beyond a critical threshold of 0.1 mm strongly influenced survival of first instar larvae. There was no overall positive correlation between oviposition preference and larval performance, except for 16 so-called acceptable non-hosts (Kendall’s τ = 0.3088). A. obtectus females also showed an ovipositional hierarchy of legume species even in no-choice tests.

Conclusions

The results suggest that whereas the use of some acceptable non-host species by the A. obtectus is possible in seed stores, the same is unlikely under outdoor conditions, where the recognition of a diverse set of seed pod-related compounds would be necessary to induce egg-laying.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01892-9.

Ability of a Generalist Seed Beetle to Colonize an Exotic Host: Effects of Host Plant Origin and Oviposition Host

The colonization of an exotic species by native herbivores is more likely to occur if that herbivore is a generalist. There is little information on the life-history mechanisms used by native generalist insects to colonize exotic hosts and how these mechanisms are affected by host properties. We examined the ability of the generalist seed beetle Stator limbatus Horn to colonize an exotic species. We compared its host preference, acceptability, performance, and egg size when ovipositing and developing on two native (Pithecellobium dulce (Roxb.) Benth and Senegalia riparia (Kunth)) and one exotic legume species (Leucaena leucocephala (Lam.)). We also analyzed the seed chemistry. We found that females recognize the exotic species as an unfavorable host for larval development and that they delayed oviposition and laid fewer and larger eggs on the exotic species than on the native species. Survivorship on the exotic host was 0%. Additionally, seeds of the native species contain five chemical compounds that are absent in the exotic species, and the exotic species contains three sterols, which are absent in the native legumes. Genetically based differences between beetles adapted to different hosts, plastic responses toward new hosts, and chemical differences among seeds are important in host colonization and recognition of the exotic host. In conclusion, the generalist nature of S. limbatus does not influence its ability to colonize L. leucocephala. Explanations for the colonization of exotic hosts by generalist native species and for the success of invasive species must be complemented with studies measuring local adaptation and plasticity.

PRODUTOS FITOSSANITÁRIOS ALTERNATIVOS SÃO INOFENSIVOS?

A agricultura sustentável tem sido cada vez mais utilizada como uma forma de produzir alimentos com segurança alimentar, em detrimento a um modelo de agricultura insustentável. E para isto são utilizados diversos produtos fitossanitários no combate a pragas e doenças ocorrentes nestes agroecossistemas. Dentre estes compostos os mais utilizados são: a calda bordalesa, a calda viçosa, a calda sulfocálcica e produtos derivados do metabolismo secundário vegetal. Porém tais compostos têm sido alvo de críticas pelo fato de que em seu manejo medidas de segurança devem ser tomadas. Portanto, ainda existem muitas lacunas dentro do conhecimento acerca do uso destes produtos, bem como das medidas de proteção que devem ser tomadas quando se utilizam tais produtos, assim sendo, nota-se a necessidade de muitos estudos exploratórios afim de melhor compreender o uso e os benefícios que tais produtos podem proporcionar ao ser humano em termos econômico e ambiental.