Weevil contraceptive: parasitic castration to fight agricultural pests

The latest Paper of the Month for Parasitology is ‘Prevalence of a nematode castrator of the carrot weevil and impact on fecundity and survival‘ by Annie-Ève Gagnon, Guy Boivin, Guy Bélair

Not a day goes by without pesticides making news headlines. Risks to human health and to the environment on the one hand, agricultural productivity and food security on the other. Whether for or against these products, two things are clear, pests can destroy crops and there are more and more people to feed. A very good example of compromise among these divided opinions used to take place in carrot fields.

LIstronotus oregonensis (carrot weevil)

A tiny beetle, known as the carrot weevil, can devastate a carrot field, leaving almost nothing behind. Fortunately, growers have access to many options to control this pest. Natural predators and crop rotations usually do a good job and when the number of pests is too high, monitoring traps and models can tell when to use pesticides. This combination of different practices is known as integrated pest management or IPM. It ensures productivity while limiting the negative impacts – the best of both worlds. Unfortunately, because of the rising temperature, the carrot weevil has started to reproduce a second time during the summer in many regions. Worst, the usual insecticide seems to have lost its effectiveness against some populations of the weevil. As a consequence, the current IPM program has become ineffective and more pesticides are being applied.

In this paper, we described the potential of an indigenous parasite to reduce the reproduction of the carrot weevil. Bradynema listronoti is a species of parasitic nematode that was discovered on an experimental farm following several years of carrot monoculture. We have shown that the fecundity of infected female weevils was greatly reduced. It was initially thought that the nematode fed on the female gonads. However, our study showed that the decrease in fecundity is the result of an inhibition of the maturation of the reproductive system. Several genes involved in this process are highly repressed when the parasite is present. It is still not clear if this is induced by the weevil as a consequence of infection or if it is directly triggered by the nematode.

These results have opened the way for the development of a new biological control strategy for the carrot weevil. Following 16 years of data, it was possible to evaluate the persistence of the parasite over the years and to determine the best timing for field applications. A first trial will be conducted during summer 2019 in commercial plots. On a more fundamental perspective, this system is currently being used to better understand parasitic castration and how gene expression is modulated. Some castrators have been shown to secrete hormones directly into the host or to interfere with the host endocrine organs. This will reduce fertility but may also result in juvenilization of the host which could have other impacts on insect behavior such as a reduction in mate searching. This information will be of great interest for the development of the next generation of IPM strategies in different crop-pest systems.

The paper ‘Prevalence of a nematode castrator of the carrot weevil and impact on fecundity and survival‘ is available free for a month.

Annie-Ève Gagnon, Guy Boivin, Guy Bélair and Benjamin Mimee are research scientists at the Saint-Jean-sur-Richelieu Research and Development Centre of Agriculture and Agri-Food Canada.

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