Fine young cannibals. The sequel

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.

References
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

How the ladybird got its spots

Ladybird adults emerge spotless from their pupal stage. To be precise, bright yellow and spotless. In a few hours, the black pigment forming their spots becomes more and more visible. This ladybird pupated in my conservatory over a week ago. Yesterday, I noticed it had just emerged, clinging to its pupal case. The top photo was taken at around 5:00 pm. After a day, this Harlequin ladybird has got all its spots, but still needs to blush a bit.

7:00 pm, yesterday

8:30 this morning

4:15 pm today

Fine young cannibal

Have you ever come across ladybird eggs? Before today, I hadn't. They are really tiny, and the ones I found were cryptically laid on the yellow backgroun of a rose petal. I would easily have miss them but for the young larvae next to them. Look closely, and you will notice that the young ladybird larvae is eating an egg, likely its sibling. Far from odd, this is very common in ladybirds. Why would such behaviour get selected? surely ladybirds which do not canibalised their own siblings should be more successful. The key aspect is that newborn ladybirds can face substantial starvation risks. They are very small and do not move fast. Egg canibalism ensures easy access to a nutritious first meal and is likely to increases their survival. The second aspect is that this sibling cannibalism is actually arranged by their mother: they lay trophic eggs: unfertilised eggs produced for the purpose of being consumed by hatchlings. Right after hatching, ladybird larvae are soft, but after two hours or so they have hardened, and they start feeding on unhatched eggs. These include trophic eggs, but also other eggs that for some reason did not develop, or did not hatch in time: they do not discriminate between different egg types. Normally each newborn ladybird will have just one egg to feed on: the rest of the unhatched eggs will have probably been eaten by her siblings. Jennifer Perry and Bernard Roitberg showed that trophic egg production is an evolved maternal strategy in response to food availability, to minimise starvation risk of their young offspring, a long held hypothesis. In laboratory experiments, they showed that ladybird females adjust the proportion of trophic eggs they lay depending on the food conditions they have experienced. Ladybird females fed few aphids (low food conditions) produced 56% more trophic eggs than ladybirds fed on a plentiful supply of aphids. This way, ladybird mothers, despite abandoning their egg clutch are able to increase their offspring survival using information on the environment they themselves grew on.

Reference
Perry, J. & Roitberg, B. (2005). Ladybird mothers mitigate offspring starvation risk by laying trophic eggs. Behavioral Ecology and Sociobiology, 58 (6), 578-586 DOI: 10.1007/s00265-005-0947-1

Green Shieldbugs

This Green Shieldbug, Palomena prasina, got into our conservatory a few days ago. There are two common green shieldbug species. This one is easy to identify as its wing membranes are dark. The other one, Nezara viridula, a recent invader of the U.K., has transparent wing membranes. The Green Shieldbug is a large species, bright green, with yellowish-red legs and antennae. It flies readily and this specimen flew away from the breakfast bowl after I took its photo. Shield bugs develop from nymphs in several stages that lack wings and might resemble little to the adult. The Green Shield bug has five such instars. The adult overwinters, and before doing so, it acquires a darker, purplish colour. After overwintering, they turn bright green again. Egg laying proceeds at around June and the successive instars develop during the summer, before the new generation of adults emerges in September.

An early instar of Green Shieldbug (30 July 2010)

A fifth instar (4 Oct 2010) Wing buds are visible.

For a great website to identify British bugs, including instars, see British Bugs.

An aphid wolf tale

A few sprigs of a Feverfew plant in the garden are infested with thick clusters of black aphids. Garden ants are their shepherds: they milk their sweet honeydew and defend them in return from predators and parasites. In the same plant, a couple of 14 spot ladybirds lurk around, coveting the aphids, which they feed upon. They better not get too close as the ants will fiercely defend their herd of aphids. I found this 14 spot ladybird, Propylaea quatuordecimpunctata, on the Feverfew. As soon as I returned it to the plant after the photo shoot, the ladybird tried to get to the aphids, but an ant immediately attacked it and the ladybird first climbed onto a flowerhead and, after further attack, dropped onto the ground. The 14 spot is one of the commonest garden ladybirds, it is one out of three yellow ladybirds, and the only one with rectangular, as opposed to rounded, spots. The adult feeds on aphids, supplemented with nectar and pollen.

An early 14 spot on Euphorbia, a source of nectar used by ladybirds (9 April 2011) 

Mating 14 spot ladybirds (14 May 2007)