MY RESEARCH ON PARASITOIDS

Behavioural Ecology of Insect Parasitoids

Anisopteromalus calandraeWhen several animals exploit a limited resource, they could compete to access this supply. There are different kinds of competition: when the exploitation of an individual diminishes the quantity of the resource for the competitors and when two or more individuals compete directly at the same time for the same resource. Individuals may then modify their behaviour with competition and choose a strategy in order to optimize the benefits/ costs ratio.

Parasitoids accomplish their development at the expense of a host organism. In this case, the host may be considered as a resource which availability influences directly the fitness of each female competing for laying an egg on these hosts. Most of parasitoid species belong to the hymenoptera order.

They reproduce by arrhenotokous parthenogenesis: males provide from non-fertilized egg whereas females provide from a fertilized egg. In this case, females have the choice between producing male or female and could thus adjust her offspring sex ratio. One of the most important factors explaining sex ratio adjustment is the competition level. In his Local Mate Competition theory (LMC), Hamilton predicts that, when a female forages alone, the sex ratio must be female biased, whereas when they forage in group, sex ratio would evolve to the equilibrium. This strategy allows individuals to disperse as many genes as possible: in the situation where a female forages alone, a female biased sex ratio permits brothers to mate only with their sisters with a diminishing competition between them. But when many females are present in the same reproductive area, the enhancement of the number of males permits them to mate with their sisters but also daughters of other females.

In solitary species, only one adult can emerge from a given host, disregarding the number of eggs initially laid, because of mortal larval combats. Whereas it seems to be disadvantageous to lay several eggs on the same host or on an already parasitized host, this behaviour, called superparasitism, is often observed in many solitary parasitoids. Recently, it has been shown that under certain conditions (especially when unparasitized hosts are very scarce or when travel between reproductive patches are too long), superparasitism could be adaptative. When females perform superparasitism, it is possible that the probability that their offspring win the competition and emerge from that host vary from one host to another depending on different factors.

Among these factors, the time elapsed since the first oviposition and the sex of eggs present on the host are the most important. In fact, the older the first egg is at the moment the second egg is laid, the lower the chances the second egg win the competition. This is due to the earlier hatching of the first egg. The earlier the first egg hatches, the longer the time the larva originally from this first egg has to destroy the second egg prior to its own hatching.
Moreover, when several eggs manage to hatch, the fighting abilities could also vary from one larva to another, especially according to their sex. 

An aspect of my study was to determined whether females are able to discriminate between different categories of parasitized hosts (hosts recently parasitized vs hosts parasitized by an egg being about to hatch or hosts parasitized by male eggs vs hosts parasitized by female eggs). For this purpose, I analyse oviposition strategies of parasitoid females (host selection, sex ratio adjustment…) when they are confronted to these different kinds of hosts. 


feeding tubeAnother aspect of my research is to analyse the trade-off between current and future reproduction. In parasitoids, when females encounter a host, they could lay an egg on and thus invest in current reproduction, or puncture the host haemolymph and thus invest in future reproduction. In fact, this behaviour called “host feeding” gives to the females additional nutritive reserves which enhance their lifespan and/or their egg maturation. The strategy employed by the females is strongly dependent on their physiological state. For example, we can imagine that old females will favor egg laying whereas females with low nutritive reserves or low egg load will favor host feeding.

This trade-off could also influence host selection. Whereas we have seen that already parasitized hosts are of less quality for immature development, these hosts could eventually be profitable for host feeding. In fact, when a female parasite a host, she injects a venom into its haemolymph which induces both the paralysis of the host and modifications in its haemolymph composition. If a host is already parasitized, the female should invest less energy to feed on and haemolymph modifications could induce a higher nutritive value for this host or at the opposite, could alter this nutritive resource. At all events, all these features should influence host selection when parasitoid females decide to lay an egg or to feed on a host.



Chemical Ecology of Insect Parasitoids

wasp My research on the oviposition strategies of parasitoid females leads me up to take an interest in the field of chemical ecology. Chemical ecology is the study of the chemicals involved in the interactions between living organisms. I demonstrated that females are able to distinguish a host recently parasitized from a host parasitized by an egg being about to hatch. They adjust their oviposition strategies by laying more eggs on host freshly parasitized, i.e. on hosts which confer to their offspring the best survival rate.

But, what can give the information to the females? The parasitoid wasps I study lay their eggs on Coleopteran larvae which develop inside seeds. Hosts and eggs are thus not directly accessible to female wasps. When a female encounter a seed containing a host, she taps the surface of the seed with her antennae to precisely locate the host, and then, she insert her ovipositor into the seed in order to probe the host.

The cue the female perceives could be involved at different levels. In fact, when a first female lays an egg, she can leave some cuticular hydrocarbons on the seed. In some species, females also deposit a chemical marker on the host after oviposit. These two cues could inform other females of the presence of a parasitoid egg, but they could also evolve with time which could give information about the time elapsed since the first oviposition. Another possible origin is the egg itself. During its development, the egg could produce some chemicals allowing females to assess its development stage. The last possible cue comes from the host. After venom injection by the first female, the host could suffer physiological modifications. The host could also produce some volatiles in response to the stress of the first sting which could quantitatively or qualitatively evolve with time. All these factors could give the information about the time elapsed since the first oviposition. By using behavioural analysis as well as chemical approaches (Gas Chromatography, Mass Spectrometry…), I showed that a modification of the chemical profil of the host after parasitism is at the origin of this discrimination.




CONCERNING BIOLOGICAL CONTROL

ovipositionI’m sure you are wondering “a study on parasitoid oviposition strategies, well, but why?” I could answer you that my research is completely fundamental and that my only goal is to increase our scientific knowledge. This would be correct. But even if my studies are fundamental, the study of parasitoid biology is essential in biological control.


Biological Control

pesticideThe fight against insect pest in agriculture often leads to the use of pesticides and insecticides. These chemical products often induce a pollution in the environment by reaching some non-target species and sometimes are danger to human health. Biological control is an alternative to the use of pesticides. It consists on the use of natural enemies of insect pest like predators or parasitoids. For example, ladybirds, the natural predator of aphids is often employed to control their population on rose-bushes. 


The good, the bad and the ugly

ovipositionParasitoids are dependant of other insects to survive. In fact, to develop until adult stage, they need another organism. Following the example of Alien, the creature of the movie of the same name, which has to lay an egg into a human, parasitoids have to lay their eggs on or in other insects called hosts. Then, after hatching, the larva feeds on the host and grows at the expense on it until they reach the adult stage. At that time, the adult can emerge from the host like an Alien from the abdomen of men.

For these reasons, parasitoids are good agent against pests in cultures. Their survival directly depends on their capacities to locate and to parasite these pests. In nature, parasitoids regulate the populations of their hosts. By releasing parasitoids in infested cultures, one can regulate their density or even eradicate it. The species I study (Anisopteromalus calandrae) is a little wasp, parasitoid of the bruchid larvae and pupae. Bruchids are pests of cowpea seed stocks and represent a real scourge, notably for African people. A. calandrae is naturally widespread in subequatorial areas in Africa and in the South-East Asia and used throughout the world in biological control.



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