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Parasitoids help control sawfly

Researchers are evaluating the value of wheat stem sawfly’s natural predators

Wheat stem sawfly can cause major yield losses. Scientists are investigating parasitoids, small wasps and nematodes, as well as fungi that can be used to control the pest.

Héctor Cárcamo, research scientist at Agriculture and Agri-Food Canada has been conducting research on beneficial insects that attack wheat stem sawfly larvae. Sawfly, said Cárcamo, has a very interesting biology. Adults do not live long, about 10 days in total, and they start coming out of the wheat stubble at the end of June and peak around early July. “They don’t even eat,” said Cárcamo. “They just come out of the stubble where they overwinter at the base of the plant, and they mate. The female lays eggs and she comes out with a set number of eggs.”

The female, interestingly, has the option to lay female or male eggs, meaning she has the ability to control the sex ratio. “This allows it to increase the population very rapidly,” said Cárcamo.

The larvae mine inside the stem, which interferes with the transport of nutrients and water inside the stem. By the end of the summer when the plant starts to dry, the larvae will migrate to the base of the stem where they hibernate for the winter. In doing this, they cut the stem, causing it to fall over. In bad infestations, entire fields will topple.

The enemy: A parasitoid

An important aspect of this story is the presence of a natural enemy, a small, orange parasitic wasp called Bracon cephi. “This parasitoid is very important for biological control and management of the wheat stem sawfly,” said Cárcamo. “It’s reasonably compatible with the solid stem option of pest management.”

The use of a solid stem cultivar in combination with the helped lower wheat stem sawfly populations. “But I think the parasitoid deserves lot of credit because we found very high levels of parasitism near the end of the outbreak in 2009,” explained Cárcamo. “We were finding 70 per cent parasitism by the wasp.”

Wet summers, he explained, interact with the sawfly and the parasitoid in the following way. The parasitoid has two generations per year. The adults come out and attack the larvae in the stem around the end of July once the larvae are big enough. The adult wasp will put her stinger inside the stem and inject a poison on the larvae that paralyzes the larva, but does not kill it. Then she lays an egg on the body of the sawfly larva. Once hatched, the parasitoid larva will attach itself to the wheat stem sawfly larva and suck the juices out of the larva until it is shrunken.

Following that, the parasitoid larva will pupate inside the stem, later exiting to feed on more larvae. “In years that are wet and a little bit cooler, the wheat will stay greener longer so the sawfly larvae will be able to feed longer, and then it will provide a host or food larva for the new generation of wasps that come out,” explained Cárcamo. “If the weather is hot and dry then the stem dries quickly and then the larva of the sawfly will migrate to the base and it won’t be available for the wasp anymore. They have a harder time finding them once they are in the base.”

This information led Cárcamo and his colleagues to offer new recommendations for managing sawfly, which is to leave more stubble in the field at harvest time. The more stem you leave at harvest the more chance the parasitoid has of surviving.

In the U.S., entomologist and insect ecologist Gadi Reddy of Montana State University’s Western Triangle Research Center has conducted research with insect-killing nematodes, small roundworms that naturally live in the soil. They enter the insect body and release enzymes with bacteria that kills insects. The predatory nematodes feed on the dead tissues of the insect body.

Recently, Reddy and his colleagues learned that spraying adjuvants help to make wheat stubble softer so that the nematodes can penetrate it more easily and attack the larvae. While some are highly efficient at moving into the stubble, others are not. His work is still in progress, but he hopes to have final results in one to two years.

About the author

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Melanie Epp is a freelance farm writer based in Guelph, Ont.

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