Herbivory Improves the Fitness of Predatory Beetles

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 2x2 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 examined.

We analyzed 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 lab-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 P. 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 lab-reared beetles (2,452–145,348 ng per beetle); cholesterol and sitosterol were the dominant sterols in field-collected and lab-reared beetles.

Our findings indicate that herbivory benefits lady beetle fitness across the Coccinellini, and that this was entirely a paternal effect. Our data provides 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 toward omnivory. 17-Jul-2020