I never intended to write a second part for ladybird cannibalism, but walking back from work yesterday I found a wall where many Harlequin ladybird larvae had congregated to pupate. I noticed that some of the fully developed, stage 4 larvae were eating either other larvae their size or pre-pupae, a very vulnerable stage as they are soft, and attached to the substrate in preparation for pupation. The cannibals had their entire heads inside their unfortunate victims. If some of you had read the previous post and thought that ladybirds were not really cannibals for eating trophic eggs (and their late developing siblings), then think again. Cannibalism is a common survival strategy in predatory insects, and many ladybirds are cannibalistic to some extent, with the tendency varying with species. This strategy may intensify in response to several factors, such as population density, food deprivation or poor food quality. In addition, the larger the difference in larval size, the more likely cannibalism is, that is, larger larvae will tend to eat smaller ones. Harlequin ladybirds in particular are highly cannibalistic, and cannibalism is an important source of larval mortality. Larval cannibalism is expected to be selected when the benefits of canibalism outweigh the costs. Benefits include the added survival when consuming a conspecific and lowered intraspecific competition, costs include the danger of succumbing to the potential victim, injury or acquiring parasites. In addition, indirect fitness costs can be incurred if cannibals feed on their siblings. It is expected that cannibalistic species would avoid eating their siblings, and this is what S.B. Joseph and co-workers found in Harlequins. In laboratory trials in which third instar larvae (potential cannibal) where housed with a first instar larvae, either a sibling or an unrelated individual, ladybirds were almost two times more likely to prey upon an unrelated than a sibling (almost 80 of the non-siblings were cannibalised as opposed to about 45% of siblings). Siblings that ended up being eaten had to be encountered more times. Unrelated individuals were attacked much faster than relatives. All this indicated that Harlequins have kin recognition mechanisms. In a species that can attain large population densities, and therefore, where potential prey is common, it might make sense to avoid siblings. In scarcer species, a sibling might be the only chance for survival, and that might explain why sibling recognition hasn't been found on other ladybird species.
Michaud, J. (2003). A comparative study of larval cannibalism in three species of ladybird. Ecological Entomology, 28 (1), 92-101 DOI: 10.1046/j.1365-2311.2002.00481.x
Joseph, S.B., Snyder, W.E., & Moore, A.J. (1999). Cannibalizing Harmonia axyridis (Coleoptera: Coccinellidae) larvae use endogenous cues to avoid eating relatives. Journal of Evolutionary Biology, 12 (4), 792-797 DOI: 10.1046/j.1420-9101.1999.00077.x
Ware, R., & Majerus, M. (2007). Intraguild predation of immature stages of British and Japanese coccinellids by the invasive ladybird Harmonia axyridis. BioControl, 53 (1), 169-188 DOI: 10.1007/s10526-007-9135-8