WESTERN PRODUCER — Unwelcome pests are eating your crop, but in three weeks they’ll morph into beneficial insects that could pollinate or devour other bad pests.
The dilemma for farmers is to spray or not to spray?
It’s a difficult question to answer without a good identification of the pests and predators involved. But identifying the bugs and knowing their lifecycles can be a big challenge. How are farmers to know if the potential economic benefits of keeping the bugs outweigh the potential crop damage?
This real-life situation is posed to Tyler Wist, entomologist with Agriculture Canada in Saskatoon. Wist was one of 20 scientists who worked with the Western Grains Research Foundation to create the Pests and Predators Field Guide to insects.
The 100-page booklet and app released last year include descriptions and high-resolution photos of the common insects that cause damage and those that do good in prairie crops.
According to the authors, the field guide has three main purposes.
- Quickly identify and manage harmful insect pests
- Quickly identify and protect beneficial predator insects
- Link the pests that damage your crops to the predators that help control those pests
In a phone interview, Wist said that some insects are beneficial at one stage of life but harmful at another stage. He said the common cabbage white butterfly is a good example.
“In the larval stage, they’re eating mustard and they’re eating canola. But in the adult stage, they’re pollinating. You see them flying around and you think, ‘Hey, look at them doing all that cool pollinating service for free.’ But then they lay their eggs on the plants and the caterpillars go ahead and eat the plants.
“Another good versus bad is the painted lady butterfly, also known as the thistle caterpillar. It’s a pollinator as an adult, but a potential pest of sunflower and soybeans as larva. They get the name thistle caterpillar from their preference towards feeding on Canada thistle as larva.
“It’s a pretty butterfly too. We see them flying into the field, but then they attack your soybeans and sunflower.
“Yes, they love to ravage Canada thistle, but if you don’t have Canada thistle in your soybean field, we know what plants they’ll go after next. They lay the eggs on your crop and the larvae eat the rest of the crop. If you have a lot of Canada thistle, you might like this insect.”
Discussing beneficials, Wist said one insect that hasn’t received a lot of attention is the bee fly. Juvenile bee flies are actually grasshopper egg predators. Kill the bee flies and you’ll be sure to have more grasshoppers. In their adult stages of life, bee flies are excellent pollinators. He said they’re beneficial in all stages.
“I studied bee flies in my masters. They’re like flying bottle brushes. They spread pollen all over the place. But they’re lazy. As soon as the sun goes behind a cloud, they just settle down on the ground to rest.”
Researchers have done a lot of pea aphid work in recent years, looking at threshold levels in crops like lentils and fababeans. He said the theses are currently being written up. During the field work, Wist and his team found some beneficial relationships between insects and crops that had not been previously recognized.
“There’s something called a soft wing flower beetle that’s an aphid predator. It has the same colouration as the cereal leaf beetle. But we’re in a pulse crop. I’m not likely to find a cereal leaf beetle in a field of lentils or fababeans or peas. So, if you see a cereal leaf beetle in your pulse field, check your Pest and Predators Field Guide again to see if it’s really a soft wing flower beetle.”
Meghan Vankosky, research scientist with Agriculture Canada in Saskatoon, offers insight into the relationship between predator and potential crop-damaging insects.
“Beneficial insects include parasitoids and predators. We call them field heroes. Parasitoids lay eggs inside or onto their insect hosts. Developing parasitoid larvae eventually kill their hosts. In short, the parasitoid kills the pest that’s eating your crop. Predators tend to be generalists that eat anything they catch. Common predators in field crops include carabid ground beetles. The relationship between predator and prey is not as tight as the relationship between a parasitoid and its host.
“An example of a parasitoid would be Bracon cephi. This is a parasitoid of wheat stem sawfly. Wheat stem sawfly is native to Canada. They were here before we began growing wheat. When we started growing wheat, we gave them an excellent host. Bracon cephi is the beneficial that helps control wheat stem sawfly.
“Entomologists believe a number of pests have shifted host plants. They’ve migrated to new crops where they have no enemies. Many parasitoids co-evolved with their insect hosts and as a result they tend to be highly specific. They can only parasitize one species of insect host.”
Vankosky said when she attends events, she finds that many people ask what they can do to increase populations of beneficials. Farmers who subscribe to the concept have described how they protect their beneficial insects. And they’re seeing more predators and parasitoids in their fields.
In terms of saving on crop protection products, as well as environmental protection, the concept of working with beneficial insects breaks down unless the farmer can skip one spray operation. If the beneficials are working hard to save the crop but threshold numbers of harmful pests remain high enough to justify spraying, the practice of relying on beneficial insects doesn’t hold water.
“It all depends on how quickly your population of predators and parasitoids develops and how effective they are at killing hosts. Compare that to how quickly your pest population develops.
“There certainly are cases where we have very effective natural enemies of host insects. The beneficials hold that pest population below the threshold. We’re working to develop dynamic action thresholds to account for the impact of beneficials when making decisions on whether or not to spray.
“One of the examples is cereal aphids. Predators and parasitoids consume these aphids. If there are enough beneficials to keep the aphids in check, you may be able to skip one spray application. That’s significant in terms of reducing your input costs and also the environment.”
Vankosky said fields must maintain enough bad bugs to sustain a population of beneficial insects. Once the host insects have been wiped out, the beneficials have no food supply, so they also perish.
Biological control by releasing populations of beneficial insects to control targeted insects is being carried out in orchards, horticultural crops and greenhouses. However, Vankosy said it’s unlikely that fostering and propagating large volumes of designer beneficials will ever become feasible on broad-acre crops. The best these farmers can do is preserve beneficials with prudent insecticide management.
“Biological control of flea beetles in Western Canada is just a dream. I don’t see it happening. However, it’s still important to preserve these field hero beneficials because they all help maintain an ecological balance.
“The awareness of beneficial insects has really come up in the past 10 years. Farmers now realize that not all insects are bad. They do themselves a great disservice by spraying all insects. Spraying according to the calendar just isn’t the same as scouting your fields.”
Until recently, applying insecticide was an all or nothing decision; spraying meant killing the good bugs along with the bad.
Wist said new products are available today that selectively target specific groups of insects. Products such as Beleaf and Coragen are designed to kill insects actively chewing on the crop.
He said both new products from FMC are aimed at specific insects, rather than specific crops. Both are registered for a wide range of crops, making them good integrated pest management tools.
According to the FMC website, Coragen provides broad spectrum control of cutworm, bertha armyworm, grasshoppers, moths and midges. The novel mode of action attacks muscle function and stops insects from feeding in as little as seven minutes, at which point the insect becomes lethargic. Death might take several days, so don’t worry if you find live insects when scouting. They no longer pose a risk to the crop.
Coragen can be used on a variety of crops, it has a 12-hour re-entry period, and has minimal impact on beneficial insects and pollinators.
Wist said Beleaf is another new chemistry that targets piercing and sucking insects like lygus and aphids. It disrupts the insect’s peripheral nervous system, causing rapid feeding cessation. The unique mode of action is effective on key pests with little negative impact on beneficial natural enemies. The active ingredient is flonicamide.
Wist said the Insect Quick Guide section of the booklet features photos of pests most commonly found in cereals, pulses and oilseeds. It includes images of predators and parasitoids that help control these pests.
John Gavloski, a contributing entomologist with Manitoba Agriculture said beneficial insects are categorized by order to enable similar looking insects to be easily compared.
“To farm economically and efficiently, viewing and reacting to insects is important. In any given field there are many insects contributing to farm profits through free services of predation, parasitism, decomposition, pollination and consuming weed seeds.
“Getting to know beneficial insects will enable wise management decisions. Increasing the number of insects you can identify is the first step.”
The Pests and Predators Field Guide is a pocket-sized free booklet and an interactive digital resource.
The web version is available at https://fieldheroes.ca/fieldguide
