Falling Victim to Wasps in the Air: A Fate Driven by Prey Flight Morphology?

Abstract

In prey-predator systems where the interacting individuals are both fliers, the flight performance of both participants heavily influences the probability of success of the predator (the prey is captured) and of the prey (the predator is avoided). While the flight morphology (an estimate of flight performance) of predatory wasps has rarely been addressed as a factor that may contribute to explain prey use, how the flight morphology of potential prey influences the output of predator-prey encounters has not been studied. Here, we hypothesized that flight morphology associated with flight ability (flight muscle mass to body mass ratio (FMR) and body mass to wing area ratio (wing loading, WL)) of Diptera affect their probability of being captured by specialized Diptera-hunting wasps (Bembix merceti and B. zonata), predicting a better manoeuvrability and acceleration capacity achieved by higher FMR and lower WL, and flight speed achieved by higher WL. In addition, wasp species with better flight morphology should be less limited by an advantageous Diptera flight morphology. Overall, the abundance of dipterans in the environment explained an important part of the observed variance in prey capture rate. However, it was not the only factor shaping prey capture. First, higher prey abundance was associated with greater capture rate for one species (B. merceti), although not for the other one. Second, the interaction observed between the environmental dipteran availability and dipteran WL for B. zonata suggests that greater dipteran WL (this probably meaning high cruising speed) decreased the probability of being captured, as long as fly abundance was high in the environment. Third, greater dipteran FMR (which likely means high manoeuvrability and acceleration capacity) helped to reduce predation by B. merceti if, again, dipterans were abundant in the environment. Wasp WL only varied with body mass but not between species, thereby hardly accounting for inter-specific differences in the wasps' predatory patterns. However, the greater FMR of B. zonata, which implies better flight performance and greater load-lifting capacity, may explain why the capture rate in the two wasp species is affected by different factor interactions. In conclusion, although prey availability remains the primary factor shaping prey use, prey flight morphology seems to gain an additional role under conditions of abundant prey, when wasps can avoid flies with better flight ability.

Fig 1. Effects of the interaction between prey availability and flight morphology on the predation by wasps.

For Bembix merceti (A), the three lines show the relationship between flight muscle ratio (FMR) and the number of captured prey at three prey availability levels (low, medium and high) (2010 data). For Bembix zonata (B), the three lines show the relationship between wing loading (WL) and the number of captured prey at three prey availability levels (low, medium and high) (2010 data). Both (A) and (B) highlight that the effect of flight morphology on capture rate is stronger when prey are abundant in the environment. In both (A) and (B) the smaller inner box shows the lines for the highest level of prey abundance in both 2009 an 2010, together with their linear trend lines, in order to remark the greater influence of dipteran availability in 2009, the year of highest prey abundance (note the steeper slope). The same slope differences occurred also at other prey abundances (not shown). In (C), a representation of the wasps’ hunting behaviour in a situation of high (above the oblique line) or low (below the oblique line) prey availability is shown. When availability is high, wasps more often catch flies with lower FMR (white circles) (i.e. they are probably easier to hunt) and WL (white circles) (i.e. easier to hunt if reduced cruising speed is important) (large grey arrow) than those with great FMR and WL (black circles) (small grey arrow). When availability is low, resource scarcity limits avoidance of dipterans with better flight ability and wasps have to include more flies with greater FMR (i.e. which are likely more difficult to hunt) and WL (i.e. which are likely more difficult to hunt if improved cruising speed is important), so that both dipterans with low or high FMR and WL are equally hunted (identical medium-sized grey arrows). In (C), the picture in the bottom left corner shows a female B. zonata carrying its dipteran prey into the nest.