Pests & Predators, Ep 5: Killer wasp has only one target — wheat stem sawfly

Wheat stem sawfly can cause a lot of damage to a wheat yield, and there aren’t a lot of options to control it. Enter: Bracon sephi, an orange wasp that is less than a quarter inch long.

The way that this small wasp attacks wheat stem sawfly sounds like something out of science fiction, but in this episode of the Pests & Predators podcast, Scott Meers from Mayland Consulting, is here to say that the science is very real.

“Very high populations can result in 25 to 30 per cent losses just from the feeding inside the stem, then you lose more when the heads drop to the ground,” says Meers of wheat stem sawfly. Sawfly populations increase in dry season and decrease in wet seasons. “The interesting thing is the cycle’s actually being driven by its natural enemy, but it manifests itself as dry weather,” says Meers. (Story continues below video)

Wheat stem sawfly arrives in late June or early July and lays eggs inside the wheat stem. Once hatched, the larvae mine up and down inside the wheat stem, eventually cutting the stem at the end of the season to get out.

Bracon sephi females come along and search out the sawfly larvae inside the wheat stem. The stressed wheat gives off a chemical signal which acts as the initial cue for the female wasps. Then, using vibrational soundings, the wasp finds the sawfly larvae, jabs her ovipositor through the stem into the larvae, paralyzes it, and lays an egg into the sawfly larvae. The wasp larvae then consumes the sawfly larvae alive and pupates in the stem.

The wasp is a specialist in attacking wheat stem sawfly; it has no other host. If populations of this beneficial wasp build up enough, sawfly aren’t a problem anymore.

Bracon sephi goes through two life cycles per year, whereas wheat stem sawfly goes through one. The first life cycle of the wasp occurs in mid-July and the second one starts in late August and goes into early September, which tends to coincide with dry land wheat harvest. If it’s a dry year, and wheat harvesting happens earlier, and the second generation of the wasp is rendered useless. But, if harvest is delayed and happens later than early September, then that second generation of the wasp gets to do its work. With a few years in a row of delayed harvesting, the wheat stem sawfly population crashes.

Most farmers aren’t going to delay harvest if conditions are right, but there’s still a way to encourage beneficial insect numbers. Meers says that cutting taller and leaving the bottom third of the plant while harvesting can help populations, as that’s where the wasp overwinters.

Want more info on other beneficial insects? Check out other Pests & Predators Podcast episodes here.

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