CHEMICALLY based Cannibalism (1989-1996)

Chemical Ecology

Chemical Ecology on the specific example of Utetheisa
With the heading “Insects as caring parents” Eisner entitled the natural history of the moth Utetheisa in 1988. The larvae prefer Crotalaria –plants as food plants. Not only they tolerate the toxic Pyrrolizidin alkaloids but also store them by themselves. This acquired chemical freight functions as a chemical defence against potential predators. Additionally, these alkaloids serve as essentially precursor of the male pheromone Hydroxydanaidal for the adults because alkaloid-free animals can not synthesise it (Eisener & Meinwald, 1987): Without Hydroxydanaidal, Utetheisa males are widely chanceless to mate (Conner et al., 1990). Because during the mating process enormous amounts of the alkaloid are transferred, this male pheromone amount provides specific information for the mating female about a male’s alkaloid-donor. Cannibalism in Utetheisa
a) Biohazard of the eggs: Utethesia larvae eat eggs of their own species, especially when these eggs contain the alkaloid and when they own just few amounts. If the larvae had accessed already this alkaloid, either through natural feeding or through a special diet in a lab experiment, they will eat definitely less but they will still prefer eggs which contain alkaloids. Concerning the food enemies, outdoor experiments show that only alkaloid-containing clutches are widly save. Utethesia eggs contain 1 - 2 ng alkaloid (on average) as parental investment which guarantees a protection against predators until they produce it by themselves after hatching (that is why Eisner [1988] talked about caring parents). For newly hatched larvae (that means very small larvae) the amounts within the eggs are definitely substantial, particularly, when many clutches with different alkaloid content (cause of different parents) are on one and the same forage crop. However, for fully grown larvae the alkaloid uptake through egg cannabilism is of no consequence, because they are able to gain particularly access to seed filled with alkaloid (Bogner & Eisner 1991).
b) Cannibalism of the larvae: Whereas for Utheisa-eggs the parental transfer provides the only alkaloid source, in the course of a larvae life alkaloids will be up taken through eating food plants and stored afterwards. Thereby, variations definitely exist, especially if the food plants have only few seeds, the greatest alkaloid amounts on a plant. Especially at these seeds, single outdoor observations of cannibalism are possible. Thereby, one can see larvae, which already have been eating inside a Crotalaria, were eaten by another one. Lab experiments with alkaloid-free and alkaloid-containig breeded larvae show that cannibalism takes place especially when big alkaloid-free larvae (5. moult stadium) meet small alkaloid-containing ones.
c) Biohazard of the pupae: Very effective in the view of alkaloid-absorption is the eating of pupae from the own species. Pupae from a lab alkaloid diet breeding contain about 630ng alkaloid. This means that just one cannibalism act will almost last to compare the limited access to this substance during larval life: Alkaloid-free breeded larvae have saved ca 530ng alkaloid in their own body tissue after eating a alkaloid-containing pupa which shows a very effective transfer. Lab experiments showed clearly the alcoid-driven cannibalism tendency despite the significant pupal cuticula barrier. The danger of cannibalism could be the reason that Utetheisa-larvals usually pupate apart from their forage crops although they are chemically safed from food enemies (Details in Bogner & Eisner 1992). Considering the meaning of the alkaloid in the life of Utetheisa, a plenty of alkaloid always should be an advantage. Captured wild animals show considerable fluctuations in the systemic alkaloid content which traces back to different accesses to herbal alkaloid resources. However, the optimization of the acquired protection is not always an advantage because the danger of cannibalism can grow with increasing alkaloid content in the own body. The winners of the alkaloid accumulation can become quickly the loosers during an inter-species competition by recognizing through the primary loosers which were previously described. Obviously, there is a balance between the chemical protection and upcoming danger of cannibalism. There will be situations where a maximum uptake of herbal alkaloids is an advantage for the best protection of robbers and advanced chances of propagation (high robber density, many forage crops with a huge alkaloid amount) and others, where inter-species eatening the biggest danger is (alkaloid-need, high population density of Utetheisa).

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