Size and sex of cricket prey predict capture by a sphecid wasp

Abstract

Female‐biased predation is rare in nature; however, sphecid wasps often take more female than male prey, including Isodontia mexicana, which hunt Oecanthus tree crickets. This study tests the hypothesis that wasps prefer females because they are larger than males. This predicts a female sex bias only for sexually size‐dimorphic prey. Prey from artificial I. mexicana nest holes in Central Ontario were compared with surviving crickets sampled from the hunted population. Sex ratios of prey and survivors were examined and compared with the occurrence of female‐biased sexual size dimorphism. Logistic regression was used to determine whether body size, sex, species, and life stage of crickets predicted capture by wasps. As predicted, wasps took a disproportionate number of adult females only of sexually size‐dimorphic prey Oecanthus nigricornis. No sex bias was found in adult prey of Oecanthus quadripunctatus or in nymphal prey of either species. However, female‐biased sexual size dimorphism did not necessitate female‐biased predation: even though female O. nigricornis nymphs were larger than males, female nymphs were not hunted more often. Body size was a significant predictor of predation, but this relationship was non‐linear. There was also evidence of an interaction among sex, life stage, and body size of prey in relation to predation risk. These results support the size‐preference hypothesis, but do not rule out alternative hypotheses. For example, sex differences in behaviour or life‐history traits that develop in adulthood may also contribute to differences in predation risk. Predation that consistently targets large adult females of a population may result in evolutionary changes in the behaviour or life history of the prey species.

Darwinian balancing selection: Predation counters sexual selection in a wild insect

“…Females—lacking any stridulatory modifications—possess narrow tegmina that wrap around the slender body when perched on plant stems. It unlikely that the wasps use calling song to locate male prey because wasps take many silent juvenile crickets, and from adult populations between 69% and 92% of prey are female (O'Neill and O'Neill , ; Ercit ).…”

“…Using digital photos (from an AmScope 5MP microscope digital camera mounted to a Wild Heerbrugg M5A dissecting microscope) and ImageJ software, we measured average femur length and width, and tibia length of both legs, pronotum length, tegmen width, and head width. Leg size may be important in scramble competition (e.g., Kelly et al ) or escaping predation (Schulte et al ); pronotum length is a common proxy for overall body size in orthopterans (Judge and Bonanno ; Robson and Gwynne ), and is strongly correlated with body mass in O. nigricornis (Ercit ); tegmina are used for singing, and are the largest visible structure (Fig. B, C); and head width may be important in intermale contests, as is seen in other orthopterans (Kelly ; Judge and Bonanno ).…”

Temporal variation in selection on male and female traits in wild tree crickets

“…Isodontia mexicana are common solitary wasps, found throughout southern Canada and the United States, and can be an important predator of O. nigricornis (Bohart and Menke ; Iwata ; O'Neill and O'Neill , ; Ercit ). Female I. mexicana sting and paralyze their prey and carry them back to their nest to provision for their offspring (Iwata ).…”

“…I. mexicana are often inhabitants of artificial trap nests (Krombein ), so their provisioning behavior is easy to observe. I. mexicana take significantly more adult female tree crickets than male (O'Neill and O'Neill , ; Ercit ), and this is due partly to female‐biased sexual size dimorphism in prey (Ercit ), as well, adult female crickets with ovaries heavy with eggs may be easier to catch (Ercit et al. ).…”

“…Thus, I compared strength and shape of selection among years and between the sexes. Sex differences in viability selection are particularly of interest in this system because the predator preferentially hunts females (O'Neill and O'Neill , ; Ercit ), and this differential predation may affect male and female phenotypes differently, potentially producing sexual dimorphism.…”