Herbivory improves the fitness of predatory beetles.

ABSTRACT

While many predatory arthropods consume non-prey foods from lower trophic levels, little is known about what drives the shift from predator to omnivore. Predatory lady beetles often consume non-prey foods like plant foliage and pollen. One species, Coccinella septempunctata, eats foliage to redress sterol deficits caused by eating sterol-deficient prey. Here we explore how omnivory benefits lady beetle fitness. We reared seven species of lady beetles-from five genera distributed across the tribe Coccinellini-on pea aphids in the presence or absence of fava bean foliage; pea aphids have very low sterol content. Foliage supplements lengthened the development times of four species and decreased survival to adulthood of two species; it had no effect on adult mass. We mated beetles in a 2 × 2 factorial design (males with or without foliage paired with females with or without foliage). For each species, we observed a profound paternal effect of foliage supplements on fitness. Females mated to foliage-supplemented males laid more eggs and more viable eggs compared to females mated to non-supplemented males. Foliage-supplemented males produced 2.9-4.6 times more sperm compared to non-supplemented males for the three species we examined. We analysed the sterol profile of four beetle species reared on pea aphids-with or without foliage-and compared their sterol profile to field-collected adults. For two laboratory-reared species, sterols were not detected in adult male beetles, and overall levels were generally low (total ng of sterol/beetle range 3-33 ng); the exception being Propylea quatuordecimpunctata females (total ng of sterol/beetle range 50-157 ng). Laboratory-reared lady beetle sterol content was not significantly affected by the presence of foliage. Field-collected beetles had higher levels of sterols compared to laboratory-reared beetles (2,452-145,348 ng per beetle); cholesterol and sitosterol were the dominant sterols in both field-collected and laboratory-reared beetles. Our findings indicate that herbivory benefits lady beetle fitness across the Coccinellini, and that this was entirely a paternal effect. Our data provide a rare example of a nutritional constraint impacting fitness in a sex-specific manner. It also shows, more broadly, how a nutritional constraint can drive predators towards omnivory.