Cassava mealybug control : parasitoid wasps hold the kairomone key

The mealybug Phenacoccus herreni feeds on cassava plant sap, inducing shrivelling. It causes extensive damage in cassava growing areas in South America. However, it can be parasitized by two wasps, Acerophagus coccois and Aenasius vexans which act out a ritual to recognize and select the individuals they are going to parasitize. A wasp moves from one side to the other of a potential victim, investigating it by palpation with their antennae. Once this “drumming and turning” procedure completed, the wasps carry out their oviposition. And when the parasites emerge, the mealybugs die.

The IRD researchers, working jointly with the International Center for Tropical Agriculture (CIAT) in Colombia, have been focusing on chemical signals which enable the wasps to recognize their victims. They showed that a kairomone, an ester called O-caffeoylserine, acts as a cue in this recognition mechanism. It is secreted by the mealybug and is present on its body surface and can be recognized when the wasp comes into contact with it. The ester makes up 0.03 nmol/mg of the mealybug body weight.

O-caffeoylserine was isolated from a sample of mashed adult female mealybugs, the development stage at which the wasps prefer to lay their eggs. This is a new result for science. Although known in an artificially-synthesized form, this ester had never been obtained before from natural biological source material.

The researchers subsequently demonstrated that the O-caffeoylserine was indeed recognized by the wasps. This they did by studying their behaviour in response to small balls of cotton made to look like mealybug bodies, some soaked in an ordinary solvent, others imbibed with O-caffeoylserine. Neither Acerophagus coccois nor Aenasius vexans “recognized” the former, but they investigated those impregnated with the substance and sometimes tried to insert their ovipositor, the egg-laying organ.

These “decoys” also enabled the researchers to determine the concentration at which the ester is most attractive. Cotton balls were soaked with different concentrations of the product then presented to the wasps. The recognition ritual was observed only with concentrations of between 0.015 and 0.03 nmol/mg. Oviposition rarely took place and only when concentration was 0.03 nmol/mg.

What benefit then does O-caffeoylserine offer the mealybug? It appears to have a dual action. It could take part in cuticle sclerotization and tanning. Or it could have a protective role against bacteria and viruses. In any case, it would have a role which is essential.

Yet, perversely, the compound is paramount in attracting the parasitoid wasps to their mealybug victims. Many scientific studies have shown that a parasite is more effective if it has already been in contact with its host or a substance which this synthesizes. Thus, if parasitoid wasps were put artificially into contact with this kairomone, in insect collections for example, they would then be able to parasitize the mealybug more rapidly. The discovery of this substance is therefore an important step towards developing a more effective biological control method against this serious pest.

(1) a chemical compound beneficial for the insect which receives the “message” (in this case the parasitoid wasp) and harmful for the one which emits it (the mealybug).

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