Meta-analysis of tree diversity effects on the abundance, diversity and activity of herbivores' enemies

The natural enemies hypothesis predicts that the abundance and diversity of antagonists such as predators and parasitoids of herbivores increases with the diversity of plants, which can lead to more effective top-down control of insect herbivores. However, although the hypothesis has received large support in agricultural systems, fewer studies have been conducted in forest ecosystems and a comprehensive synthesis of previous research is still lacking. We conducted a meta-analysis of 65 publications comparing the diversity, abundance or activity of various groups of natural enemies (including birds, bats, spiders and insect parasitoids) in pure vs. mixed forest stands. We tested the effects of forest biome, natural enemy taxon and type of study (managed vs experimental forest). We found a significant positive effect of forest tree diversity on natural enemy abundance and diversity but not on their activity. The effect of tree diversity on natural enemies was stronger towards lower latitudes but was not contingent on the natural enemy taxon. Overall, our study contributes substantially toward a better understanding of the natural enemies hypothesis in forest systems and provides new insights about the mechanisms involved. Furthermore, we outline potential avenues for strengthening forest resistance to the growing threat of herbivorous insects.

Contribution of hardwood trees to budworm – parasitoid food web dynamics

A major pest of Atlantic forests is the spruce budworm caterpillar which outbreaks every 35 years and causes large scale tree mortality. Historically, budworm management has largely ignored other species in the food web. Broadening the focus could reduce budworm outbreaks while balancing the multiple demands on our forests. However, the food web surrounding budworm including other caterpillar species that are attacked by budworm parasitoids has been relatively undersampled and under-researched. Therefore, we tested two hypotheses: the alternating hardwood-softwood parasitoids hypothesis where parasitoids attack other caterpillars on hardwoods when budworm are rare and attack budworm on balsam fir or other softwoods when budworm are plentiful, and the mixed stands natural enemies hypothesis where stands with a mixture of softwood and hardwood trees harbour greater abundances and diversity of budworm parasitoids. We tested these hypotheses using stable isotope analysis of budworm parasitoids and through community analyses of parasitoids sampled along a hardwood gradient. We found indications that parasitoids do attack caterpillars on hardwoods and budworm on balsam fir, but found mixed results for the natural enemies hypothesis. Our study highlights the importance for budworm management of understanding the dynamics of the food web surrounding budworm.

The effects of intercropping and mixed varieties of predators and parasitoids of cassava whiteflies (Hemiptera: Aleyrodidae) in Colombia

Cassava intercropped with cowpea in Colombia had lower numbers of Aleurotrachelus socialis Bondar and Trialeurodes variabilis (Quaintance) per leaf and per plant than did monoculture cassava. These differences persisted for up to six months after harvest of the cowpea. These results are examined in light of the natural enemies hypothesis, which suggests that natural enemies may be favoured in diversified systems, thereby reducting herbivore load. In this regard, the effects of different cropping systems on the whitefly predator Delphastus pusillus (Le Conte) and on the combined action of the parasitoids Amitus aleurodinus Haldeman and Eretmocerus aleyrodiphaga (Risbec) are discussed. D. pusillus displayed a functional responce and was more abundant in monocultures than in intercrops. Predator:prey ratios were similar between treatments and so low that predation appeared to have little impact on whitefly numbers. Parasitism levels of Aleurotrachelus socialis were not affected by crop combinations. The data suggest that the activity of the natural enemies does not explain cropping system effects on cassava whitefly populations.