Sugar and pollen supply enhances aphid control by hoverflies in strawberry
Larvae of many hoverfly species (Diptera: Syrphidae) are efficient aphid predators, while the adults feed on pollen and nectar. Strawberry crops can be infested by several aphid species, such as the strawberry aphid Chaetosiphon fragaefolii (Cockerell) and the potato aphid Macrosiphum euphorbiae (Thomas), both of which can cause economic damage. Protected cultivation of strawberry seems to be ideal for introducing biological control agents that depend on flower resources, but the nectar and pollen provided by the crop may not be the most suitable for all flower-dependent insects. In this study, we evaluated the biocontrol potential of hoverflies released preventively or curatively against the potato aphid in strawberry in two separate greenhouse experiments and the effect of adding sugars and pollen on their control efficacy. In the curative experiment, Eupeodes corollae (Fabricius) and Sphaerophoria rueppellii (Wiedemann) was released and supported with flowering plants of Fagopyrum esculentum Moench (buckwheat). Both species reduced the aphid numbers significantly compared to the control treatment. In the preventive experiment, we tested three syrphid species, Episyrphus balteatus (De Geer), S. rueppellii, and E. corollae, in the presence and absence of additional food sources (sugar solution and pollen). All three syrphid species reduced the aphid population to a similar extent, but only in the presence of additional food sources. This study shows that hoverflies perform better in strawberry crops when the adults have access to different supplementary food sources.
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All data from the greenhouse experiments were analyzed by fitting Generalized Linear Mixed Models (GLMM). In the curative experiment, 3 plants per cage were assessed for the number of aphids and hoverfly eggs. Thus time, cage, and plant within each cage were included as random effects, while predator species (including no predator) were included as the fixed effect. When analyzing the total number of hoverfly adults per cage (i.e. for a total of 6 plants), only time and cage were included as random effects. In the preventive experiment, one plant per cage was assessed for the number of aphids and hoverfly eggs, while hoverfly adults were assessed per cage. First, to analyze the population build-up of aphids, a one-way GLMM including treatment (with 7 levels) as the explanatory variable was fitted. Then, to analyze the number of syrphid eggs and adults, two-way GLMMs were fitted, including predator species, the presence/absence of the feeder, and their interaction as the fixed explanatory variables. In all models, time and cage were included as random effects. Error distributions for all models were initially set to Poisson for count data and their fit was assessed through residual diagnostics for hierarchical regression models. When overdispersion was found, the distribution was changed to negative binomial and reassessed. Post-hoc tests using Tukey’s HSD to separate means were performed on significant main effects and when the interaction between main effects was not significant. All statistical analyses were performed using the statistical software R 4.0.2 (R Core Team, 2021).