Farmers’ arsenal against insect pests in crops can easily include weapons of mass destruction that are free and require little maintenance.
They come in all shapes and minute sizes and their presence contributes to ecosystem stability.
These weapons are otherwise known as nature’s array of beneficial insects, those that battle pest species that reduce crop yield and profitability.
John Gavloski, Manitoba’s provincial entomologist, told those in an Alberta Agriculture webinar that removal of beneficial insects makes pest problems more likely. The keys to preserving insects that can work with crop production include:
- using insecticides only when economic thresholds are exceeded
- using selective insecticides where possible
- spraying only patches, field edges or strips where practical
- rotating crops
- maintaining habitat for beneficial insects
The last item requires farmers to leave cover in field edges and shelter belts and try to have flowering species, with a variety of blossom shapes and sizes, throughout the growing season.
Don’t tank mix insecticides and spray “just in case” of a problem, said Gavloski.
“You can inadvertently do more harm than good.”
As for the good that bugs do, Gavloski said that depending on type, beneficial insects pollinate, eat other insects, eat weeds and weed seeds, help decompose stubble and dung and improve the soil.
He identified 10 orders of insects and arachnids that help manage crop pests:
- Ordonata: dragonflies, damselflies
- Orthoptera: crickets
- Dermaptera: earwigs
- Thysanopterta: predacious thrips
- Hemiptera: minute pirate bugs, damsel bugs, stink bugs
- Neuroptera: green lacewings
- Coleoptera: ground beetles, rove beetles, lady beetles
- Diptera: tachinid flies, hover flies, stiletto flies
- Hymenoptera: parasitic wasps, ants
- Araneida: spiders
Insects in this category are “true bugs” and a few families within this order are good predators of pest species, said Gavloski. Damsel bugs are one example. There are 12 species in Canada and they kill by injecting digestive enzymes into prey. In field conditions, they’ve been known to attack diamondback moths, and will sometimes kill more than they actually eat, which is a bonus.
“They’re very quick. I’ve even seen them catch things like lygus bugs and feed on them. They will feed on a variety of things,” Gavloski said.
Minute pirate bugs, also “true bugs,” are other beneficial insects.
“They can be quite a valuable early season predator of things like aphids.”
Minute pirate bugs are often used in greenhouses to battle mites and thrips but that’s not economical in a field situation.
The stink bug group is large but some are strictly predators of other insects, said Gavloski. The two-spotted stink bug is often found in potato fields, where it attacks Colorado potato beetle as well as other insects.
The larval form of green lacewing looks like a small brown alligator. These larvae are very quick and able to catch and eat aphids, caterpillars and anything their size or smaller, including their siblings.
They kill by injecting digestive juices into their prey.
Gavloski said years with large aphid infestations will also yield large lacewing numbers. These beneficial insects will eat diamondback moth larvae and cocoons and have also been seen to eat flea beetles.
Manitoba homeowners may have seen the multicoloured Asian lady beetle in recent years. Gavloski said they frequently get into homes and were plentiful in that province last year, which was also a big aphid year.
There are about 930 species of ground beetle in Canada and about 390 in Alberta. They will eat any insect that they are able to overpower, but are often overlooked because they are largely nocturnal.
Ground beetles can be brown, black or metallic green and are of many different sizes. Their heads are narrower than their thorax and abdomen and they usually have large mandibles and hardened wings, often with lines on those wings.
Ground beetles eat the eggs, larvae and pupae of Colorado potato beetles, as well as cutworms, root maggots, diamondback moth larvae, sunflower beetle larvae and wheat midge larvae.
Rove beetles make up another large group of beneficials, with more than 1,100 species in Canada and 265 of those in Alberta. They will feed on various fly maggots and pupae and are good predators of root maggot. One adult can eat 23 eggs or more than two pupae per day.
“They do have big appetites,” said Gavloski.
That is also characteristic of lady beetles. The seven-spotted lady beetle female can eat 115 soybean aphids on one day. Other species are also voracious eaters.
There are 500 species of hover flies in Canada and they are often mistaken for wasps or bees, said Gavloski. They are pollinators but their larvae prey on aphids, thrips and other insects.
Hoverfly larvae have a slug-like appearance, but are able to hook aphids and extract their fluids “like somebody sucking on a freezie.”
There are thousands of species of parasitic hymenoptera and probably more that haven’t yet been identified, said Gavloski. Some of these wasps sting but a much larger number can’t sting or are so small that a sting on a person would not be noticed.
Parasitic wasps lay eggs on caterpillars and the resulting larvae eat the host from the outside, eventually killing it. Some types feed on diamondback moth, others on bertha armyworm or cereal leaf beetle.