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What we found when we tested a botanical pesticide to combat locust invasions

A community volunteer uses a motorised spray to apply pesticide on February 25, 2020 at a locust hatch site in eastern Kenya. Photo by TONY KARUMBA/AFP via Getty Images

Swarming locusts are famous for their destructive potential. Currently, locust outbreaks in East Africa are threatening the livelihoods of hundreds of farmers and their families. For instance, a recent swarm in Kenya consumed more than 2,000 tons of food.

Despite the fact that pest management authorities are faced with a well-known, ancient plague, it seems that current measures are failing. The outbreak in East Africa started months ago and has, so far, spread into eight countries. How is this possible, considering all the technical advances that have been made over many years?

As a biologist who has specialised in insect research for the past 20 years, I believe current pest management regulations need to be revised. Currently, outbreaks are managed using chemical pesticides or an insect fungus (Metarhizium sp.). Neither is a good option.

Chemical (synthetic) pesticides may be harmful to the environment and humans because of their neurotoxic effects. They target an animal’s nervous system and, at higher concentrations, the nervous system of people handling the substance.

Insecticide fungi are applied as spores to kill the locusts. This can take a long period of time and requires certain climate conditions, so it often won’t work as expected. This fungus also has unwanted side-effects on species that aren’t being targeted, such as scarab beetles and termites.

My colleague and I found that linseed oil, combined with some essential oils, is highly effective in killing two of the most problematic locust species: Schistocerca gregaria and Locusta migratoria. Both species are very destructive and are responsible for most outbreaks in Africa. This botanical pesticide can be sprayed with conventional spraying devices and kills adult locusts within 24 hours.

Botanical pesticides have already proved successful against several insect pest species, but this is one of the first to target locust swarms effectively.

In our study we tested the toxicity of a linseed oil and bicarbonate emulsion against swarming locusts and looked for plant essential oils that would enhance its toxicity. The tests we did were encouraging. Our study shows how a botanical pesticide can be effective against locusts, and has properties that are less harmful to people and the environment. Mixing the oil components of this botanical pesticide is simple and the agricultural industry already produces these oils at large scale for various purposes.

Botanical solution

The emulsion we tested was made up of botanical pesticides known to be harmless to humans because of their historical use as cosmetics, spices or healing aids.

We combined the linseed emulsion with low concentrations of caraway, wintergreen and orange peel oils to develop a new formula that is effective against desert and migratory locust species after a single spray treatment. Within 24 hours, the formula killed between 80% of desert locusts and 100% of migratory locusts.

It worked because the combination of linseed oil and bicarbonate led to a quick hardening of this oil on the surface of insects. This suffocated them. We also believe that it affects the central nervous system and sensory information processing of locusts.

We tested it on other insects to see its impact on non-target species. We found that it was toxic to ladybird adults and is highly toxic to their larvae. It didn’t, however, affect the vitality of the mealworm beetle adults that we also tested it on. We aren’t sure why, but the mealworm beetles behaved normally, even eight days after spray treatment.

Because this botanical pesticide can have a negative effect on some non-target species, we propose that it should be sprayed on crowded locust colonies at their evening resting places and should not sprayed over huge areas. This botanical insecticide emulsion can also be mixed in the field, unlike other pesticides, so no laboratory equipment or protective clothing are needed.

Problems of extensive pesticide use

The agent of choice that’s being used in the current outbreak countries is an insecticide that relies on fungus spores to kill the locusts. The Food and Agriculture Organisation of the United Nations ordered four tons, for US$76 million, to combat the current outbreak in East Africa.

The fungus is called Metarhizium sp. and it forms spores inside locusts, with fatal consequences. It was created and tested during a 13-year French research programme called LUBILOSA. It is found to infest locusts between about seven and 14 days after single treatment and it needs high humidity and moderate temperatures to work effectively. To my knowledge, few field studies are available that demonstrate the effect of this fungus on adult locusts. Only one was performed in Nigeria in the LUBILOSA field studies.

Three major problems are associated with this fungus-based pesticide:

First of all, the LUBILOSA user handbook from 1999 instructs users to dissolve the spores in diesel or kerosene – also called “mineral oils”. But spraying large areas of land with this would have serious environmental consequences. Mineral oils are known to contain toxins and carcinogens that can affect human health. Another problem is that they don’t degrade easily and stay in the environment for a long period of time.

The second problem is that infestation is not fast enough to control large swarms and it needs specific environmental conditions to work. The fungus-based insecticide takes about 14 days to affect adult locusts. To produce spores, it needs high humidity and moderate temperatures, more than 20°C at night and less than 38°C during the day. In areas where locusts cause massive problems, these climate conditions are rarely found.

The third problem is that current Food and Agriculture Organisation regulations favour a fungus over other pesticides. This regulation indirectly prevents the development and registration of novel, more effective botanical pesticides.

Regulatory change is therefore necessary. The regulation should be quickly revised to prevent further problems caused by massive locust outbreaks.

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