Nitrogen and water inputs to tomato plant do not trigger bottom-up effects on a leafminer parasitoid through host and non-host exposures

BACKGROUND

Bottom-up and top-down forces are major components of biological control against pests in an agro-ecosystem. Understanding the multi-trophic interactions between plants and secondary consumers would help optimize pest control strategies. We manipulated nitrogen and/or water inputs to tomato plants (Solanum lycopersicum) to test whether these manipulations could trigger bottom-up effects on the parasitoid Necremnus tutae via host (Tuta absoluta) and/or non-host (Bemisia tabaci) exposures, and compared the control efficacy of N. tutae on T. absoluta in the presence and absence of B. tabaci.

RESULTS

The results showed no cascading effects of plant nitrogen and/or water inputs on N. tutae via either host or non-host exposure. The bottom-up force was mitigated by chewing or sap-feeding insect consumers at the second energy level. By contrast, the top-down force on T. absoluta from parasitoids was enhanced by an additionally provided non-host, which could produce alternative food sources extending N. tutae longevity and enhancing the fitness of its offspring.

CONCLUSION

Our results provided evidence for the combination of bottom-up and top-down approaches in tomato integrated pest management programs. © 2017 Society of Chemical Industry.

The body size dependence of trophic cascades

Trophic cascades are indirect positive effects of predators on resources via control of intermediate consumers. Larger-bodied predators appear to induce stronger trophic cascades (a greater rebound of resource density toward carrying capacity), but how this happens is unknown because we lack a clear depiction of how the strength of trophic cascades is determined. Using consumer resource models, we first show that the strength of a trophic cascade has an upper limit set by the interaction strength between the basal trophic group and its consumer and that this limit is approached as the interaction strength between the consumer and its predator increases. We then express the strength of a trophic cascade explicitly in terms of predator body size and use two independent parameter sets to calculate how the strength of a trophic cascade depends on predator size. Both parameter sets predict a positive effect of predator size on the strength of a trophic cascade, driven mostly by the body size dependence of the interaction strength between the first two trophic levels. Our results support previous empirical findings and suggest that the loss of larger predators will have greater consequences on trophic control and biomass structure in food webs than the loss of smaller predators.

Interaction webs in agroecosystems: beyond who eats whom

Studies of insect communities in agroecosystems have historically focused on a food web approach: who eats whom. Recent research has highlighted the importance of indirect effects in determining the abundance and distribution of insects and their effects on other insects and plants. These indirect interactions include apparent competition where an herbivore indirectly suppresses another herbivore by attracting shared predators, cases where predators or even other herbivores alter the behavior and/or physiology of herbivores in ways that result in decreased plant consumption, and mutualisms that can generate a network of indirect effects that alter the abundance of many species within a community. The consequences of these indirect interactions have been modeled and proof-of-concept studies have demonstrated their potential importance, but studies of the consequences of these interactions on crop yield are sorely needed. Documenting the prevalence and consequences of these indirect effects in multiple crops will allow researchers to compare and contrast responses across systems and identify key species or characteristics of agroecosystems that dictate when and where these effects are important. This research will ultimately allow growers to manipulate these interactions to increase ecosystem services provided by insects and increase crop yield.