Title: Sterol-limitation in a Pollen-fed Omnivorous Lady Beetle (Coleoptera: Coccinellidae) Authors
|Pilorget, Lucia -|
Submitted to: Journal of Insect Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 15, 2009
Publication Date: December 9, 2009
Citation: Pilorget, L., Buckner, J.S., Lundgren, J.G. 2009. Sterol-limitation in a Pollen-fed Omnivorous Lady Beetle (Coleoptera: Coccinellidae). Journal of Insect Physiology. 56:81-87. Interpretive Summary: Lady beetles are best appreciated for their ability to eat insect pests, but most also eat non-prey foods like pollen. In fact this pollen is an important part of their diet- it influences when and where they occur within farmland, and when insecticides are present in the pollen they potentially can harm beneficial species. But little is known about the nutrition of pollen from a lady beetle’s perspective. We fed lady beetles pollens from one of five corn hybrids to see whether or not these pollens were equally suitable for the native lady beetle, Coleomegilla maculata. We found that C. maculata did not do equally well on all of the pollens, and that their performance was well correlated with sterol contents of the pollens. Insects can’t synthesize sterols (chemicals like cholesterol), and must get them from their diet for routine physiological processes like molting. But when we added sterols to a pollen only diet, it didn’t consistently improve their performance. Thus, sterols are a limiting nutrient for pollen-fed lady beetles, but this isn’t the only limiting nutrient. By better understanding the nutrition of lady beetles, we hope to be able to better predict when and where they can be used to manage pests in farmland.
Technical Abstract: Nutritional constraints of non-prey foods for entomophagous arthropods are seldom investigated, yet are crucial to understanding the nutritional ecology of these insects and their functioning within natural and managed environments. In two experiments, I) it was established that pollen from five maize hybrids is of variable quality for the omnivorous lady beetle, Coleomegilla maculata, and the suitability of these pollens was related with their sterol profiles, and II) the effects of augmenting sterols (ß-sitosterol, cholesterol, or ergosterol) in two of the pollens on the fitness and performance of C. maculata were determined. Specific fitness parameters measured in each treatment were preimaginal survival, the duration of each stadia and stage, the duration of the pre-oviposition period, post-mortem adult dry weight, adult hind tibial length, sex ratio, fecundity, cohort generation time (T), net replacement rate (R0) and intrinsic rate of increase (r). Each type of sterol found in the pollens was quantified using GC-MS. Pollens were of variable suitability for C. maculata in experiment I; the preimaginal development rate was positively correlated with the amount of 24-methylene-cholesterol and r was positively correlated with episterol and 24-methylene-lophenol found in the pollens. In experiment II, adding ß-sitosterol to a pollen-only diet improved preimaginal development rates for one of the pollens tested, but did not make it as suitable as pollen + prey (eggs of Ephestia kuehniella). Performance of C. maculata fed the other pollen was entirely unaffected by sterols. This research shows that pollens clearly vary in their sterol contents intraspecifically, which affects their suitability for omnivores that rely on pollen. However, sterols appear to be only one of the limiting nutrients in pollens.