Hyposoter
horticola.
Everywhere
she
looked, the level of parasitism observed in the caterpillars was consistently around one third of the population. In a standard parasitoid population, density dependent variation occurs so that levels of parasitism vary through time and space. However, in the case of H. horticola, the levels always remained the same, even at high and low host density.
Perfect timing Investigating this intriguing fact further, van Nouhuys quickly established that the wasp was parasitising M. cinxia at the point just before the caterpillars were hatched from their eggs. “If anything, this just made the whole story even more fascinating,” she regales. “That period of time
in which the caterpillars are
available for parasitism is absolutely tiny – a matter of hours. And yet, practically every cluster of eggs we looked at had this similar ratio of parasitised eggs, meaning that H. horticola had somehow managed to locate every single one of these clusters within that fleeting space of time.” Working on the hypothesis that the
wasps were somehow able to know the locations of the egg clusters before they had reached this critical moment of development, van Nouhuys and her group were able to confirm this theory experimentally by tracking the movements of individuals. The implication of this was that, much like fellow hymenopterans ants and bees, H. horticola was able to store information about resource locations and then return to them at a later date. “Previous thinking on parasitoids was that they were very much the sort of organism that would find a resource, utilise it, and then move on,” explains van Nouhuys. “But what we observed was that they were actually carrying out anticipatory foraging; locating a resource before it was ready to be used with no immediate reward. This gives the wasps more time to find host egg clusters.” The group then started to investigate the
method by which the wasps were keeping track of the egg clusters. Taking inspiration from classic experiments performed
on bees, they
Gregarious Melitaea cinxia caterpillars basking in the sun (Credit: S. van Nouhuys)
experiments. “Ants and bees use what is called path integration; they can find their way from one resource to the next along a specific path, but if you displace them from this path, they become disoriented,” states van Nouhuys. “We wonder if our wasps are much the same.”
levels of competition between individuals is evident – every cluster observed by the researchers had been parasitised – the wasps rarely breach the one third ratio rule, despite the fact that one would assume they would want to exhaust such a valuable resource then and there.
“Ants and bees use what is called path integration; they can find their way from one resource to the next along a specific path, but if you displace them from this path, they become disoriented”
installed
landmarks around the eggs, waited for the wasps to become accustomed to them, and then moved the landmarks. The behaviour observed showed that H. horticola was using these visual cues in much the same way as the bees in the original
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Parasitic patterns From an evolutionary perspective, one of the most intriguing aspects of H. horticola’s
ecology is the Using molecular markers sensitive seeming
restraint that they show when finding an egg cluster. In a situation in which high
enough to differentiate between individual wasps, the researchers were able to clarify that each egg cluster was parasitised primarily by a single female, with an odd egg here or there parasitised
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