When a crop pest isn’t a crop pest

Insect pests aren’t always true pests. Hard as it may be for farmers to imagine, sometimes the insects do more good than harm.

That was a key message from an April online seminar on insect control in canola organized by the University of Manitoba’s Faculty of Agricultural and Food Sciences.

The three panelists — Alejandro Costamagna, an entomology professor at the U of M; Hector Carcamo of Agriculture and Agri-Food Canada; and Keith Gabert of the Canola Council of Canada — talked primarily about flea beetles and lygus bugs, two of the most common canola pests in Western Canada.

Carcamo, an entomologist based at Lethbridge, said when lygus bugs are spotted in canola fields, they shouldn’t necessarily be viewed as a threat. In fact, the opposite could be true.

“Just because an insect is there, it does not mean that it’s a pest. In some cases, it can be a beneficial insect.”

Carcamo said low levels of insect feeding can stimulate crops like canola to produce better yields.

“We have to remember that insects and plants have had this very long history of a kind of love/hate relationship, where the insects have been attacking the plants and the plants have evolved defences and so on,” he said.

“We need to understand the biology of the insects and we need to appreciate that insect/plant interaction. At low levels, lygus bugs are not a pest. If you have a few lygus bugs attacking the crop at the early flower stage, the plants can actually become more robust, and they can actually over-yield and overcompensate.”

Carcamo said lygus bugs and flea beetles typically pose a threat only at certain times or stages of canola growth. For lygus bugs, that’s at the end of flowering, when there are plenty of canola pods with small, soft seeds inside.

“In general, that is the stage when lygus bugs are going to have the greatest risk and potentially affect the crop,” Carcamo said. “The key message is you have to get out there and sample at the correct stage.”

Gabert, a council agronomy specialist based at Innisfail, Alta., agreed.

“We can start finding lygus as soon as it warms up here on the Prairies. You’ll find the odd one creeping into your truck or in the ditch when you’re going to check the field at seeding time. But that’s not really an indication (you’re) going to have a problem in that field.”

Costamagna said the seedling stage is the most dangerous time for flea beetle damage.

“It’s a matter of timing. You’ll find (flea beetles) every year in every place, but they might not be (there) at the wrong time for your crop. They might completely miss the susceptible stage.”

In sufficient numbers, the insects can damage seedlings, but the plants quickly grow large enough to better withstand flea beetle pressure.

“(Canola) is an amazing plant in the sense that within a couple of weeks it grows to an enormous size,” he said. “There could be lots of (flea beetles then), and they would be inconsequential.”

According to Costamagna, insecticidal seed treatments limit seedling damage from flea beetles in the critical three or four weeks after emergence.

Scouting key

The three experts agreed that scouting is essential for flea beetle and lygus bug management, particularly at times when crops are most vulnerable.

Noting the canola council has resources for managing both pests on its website, Gabert said he recommends “aggressive” scouting for flea beetles. By the time producers notice significant crop damage, it might be too late.

“Typically, when a grower is really concerned about flea beetles, it’s because he’s come and looked at the field seven days too late. (They are) looking at a field that already has 50 per cent plus damage. It’s had that damage for quite some time, and the grower is reacting to some pretty ugly-looking crop,” said Gabert.

“(It) is really critical to get out and scout when the crop is struggling,” he added, because flea beetle pressure can accelerate quickly in warm, dry conditions.

The experts also emphasized the need to use recommended thresholds when making spraying decisions.

Costamagna said thresholds should be heeded to avoid excessive insecticide use. It’s a waste of money if there’s no economic return to applications and it also reduces the risk of insecticide resistance.

Carcamo said agro-ecosystems in Western Canada typically contain many natural enemies of insect pests, which help curb pest populations in crops.

“For example, for lygus bugs we have determined that we have fairly high levels of parasitism by a very tiny wasp called peristenus. It can inflict reasonably good mortality on lygus bugs in some areas, so conserving these beneficial insects is very important,” he said.

“That is why it’s crucial that we understand these economic thresholds.”

Canola producers can learn more about beneficial insects through the Field Heroes program, said Gabert. He commended that initiative for promoting protection of beneficial insects.

Carcamo said several cultural control tools within the framework of integrated pest management can help farmers ward off flea beetle and lygus bug problems in canola. One of them is early seeding. Including more crops in a rotation and extending crop rotations are other helpful measures.

“The more crops that you can integrate into your rotation, (the more) you are going to limit the amount of food available for an insect pest,” he said, adding a forage crop can also be useful in boosting beneficial insect populations.

“Crops like alfalfa, they can harbour a huge number of insects … which can be excellent sources for (insect) predators like ladybird beetles.”