This growing season farmers should not only keep an eye out for pest insects, but also the beneficials that prey on them.
“If we are familiar with the natural enemies, we can make a better decision on whether the risk of damage by the pest insects is relatively low,” says Dr. Héctor Cárcamo, Agriculture and Agri-Food Canada research scientist.
There isn’t a tried and true method of working beneficial numbers into economic thresholds yet. But Cárcamo says researchers are starting to get a better idea of how big a bite some beneficials take out of pest populations.
“The number one thing that I think growers can do to protect those beneficial insects that are already there, including the generalists and the specialized parasitoids, is to only spray if you really have to spray. So you should be familiar with the economic thresholds and do enough monitoring and then wait to see if you have actually reached those damaging levels,” says Cárcamo.
Generalists and specialists
The beneficial insects that control pests can be divided into generalists and specialists.
Generalists aren’t picky eaters — any insect is a potential meal. They include spiders, daddy long legs, and ground beetles. Cárcamo says generalists are the most underappreciated natural enemies.
“But I think they are extremely important because having a large number of those is like having a buffer system or reserve. And by having healthy populations of those, we can avoid future pest problems in the long-term,” says Cárcamo. Generalists can’t control large outbreaks, but they reduce the risk of smaller outbreaks.
Cárcamo says field operations affect generalist populations. For example, some canola growers mix insecticides with herbicides as a pre-emptive strike against lygus bugs in the bolting stage. Cárcamo says it is a good idea to spray before flowering if there is a lygus bug problem because it won’t harm pollinators. But he says farmers who spray when there aren’t lygus bug problems are doing themselves a disservice by reducing natural enemies.
“Just because it’s easy and simple and cheap, it’s not necessarily a good investment for your future as far as the environmental stability of your crop and your future management,” says Cárcamo.
Specialist insects attack specific crop pests. For example, there is a small, black wasp that attacks lygus bug nymphs in alfalfa. Cárcamo says the wasp has a short life span, so they’re difficult to scout in the fields. But if farmers open up the lygus bug nymphs, they can see the worms growing inside.
Western Canadian wasps don’t attack lygus bugs in canola, but Cárcamo and his colleagues are working on a project to see if a wasp found in Eastern Canada will venture into canola crops to parasitize lygus bugs. They also need to make sure the Eastern wasp doesn’t affect the Prairie wasp.
Bertha armyworms may be a problem in parts of the Prairies this year. Bertha armyworms and cutworms are susceptible to disease and natural predators, including rodents, birds and insects such as ground beetles, harvestmen and spiders.
If farmers see sea gulls or other birds following the tractor and feasting, that field likely has a cutworm outbreak, says Vincent Hervet, a graduate student at the University of Lethbridge doing research at AAFC’s Lethbridge Research Centre.
Flies and wasps both parasitize cutworms and bertha armyworms, but the flies are more prevalent. The flies resemble blowflies, and are either completely black or dark grey. They often have long, thick bristles that are more pronounced at the abdomen tip. Their abdomens are round or cone-shaped. They are larger than house flies and may have grey and black stripes on the abdomen.
It’s difficult to figure out how many bertha armyworms and cutworms have been parasitized in the field. Hervet and his colleagues collect caterpillars, bring them back to the lab, and track how many die from the fly and wasp larvae.
“Especially for bertha armyworm we know in some cases the percentage of deaths due to parasitism have been very, very high. And mostly fly parasitoids,” says Hervet.
Biological control is the key control method for the cereal leaf beetle, says Cárcamo. The T. julis wasp, which is about two to three millimeters long, lays five to six eggs in each beetle larva. Two wasp generations per season make them even more effective. The wasps have taken out up to 40 per cent of the cereal leaf beetles in some sites around Lethbridge, and can control between to 70 to 90 per cent of the cereal leaf beetles in the B.C. interior.
Cárcamo says the cereal leaf beetles are unlikely to reach economic levels on the Prairies because they prefer moist, humid habitats.
“So by having some effect through the parasitoid, even if it’s only 30 or 40 per cent, that should be enough to keep the pest at low levels,” says Cárcamo.
“We should let the parasitoid do the job and not worry too much about spraying for this pest.” †