Based on what went on in Manitoba fields this past season, producers may want to be on the lookout for several insects in 2020 that could potentially make a reappearance.
At the top of the list are flea beetles, cutworms and grasshoppers, according to Manitoba’s provincial entomologist, John Gavloski.
Speaking at the recent Manitoba Agronomists Conference in Winnipeg, Gavloski said these three pests were of major concern in Manitoba in 2019, and army worms and thistle caterpillar also had a number of significant localized populations that prompted producers to spray in some areas.
Flea beetles on canola
Most canola seed is treated with some form of neonicotinoid, which gives approximately three weeks of protection, but a number of producers last year, who were concerned about flea beetles based on previous years’ experience, seeded their canola later, into mid-May, to try and reduce the risk for flea beetles.
“It’s not a bad strategy. In some years, that can work quite successfully, but it didn’t work that well for many producers this last season because after seeding, the seed wasn’t germinating because of the dry conditions, or the seed was germinating and just sitting in that cotyledon stage for weeks,” says Gavloski. “Some people who, in addition to the neonicotinoids, had enhanced seed treatments like a Lumiderm on the seed, still ended up doing foliar spray.”
Some producers also had to reseed because of flea beetle injury. “I had people call me saying that they had reseeded and ended up spraying the reseeded canola for flea beetles, something that doesn’t usually happen,” said Gavloski.
Some producers with extreme flea beetle issues sprayed up to five times, but many producers had to spray more than once, so flea beetles were definitely a major problem this past year, largely because weather conditions were conducive.
“Anything that keeps the seedlings in that seedling stage for a prolonged period of time is going to increase the risk and that’s what happened this past year,” said Gavloski.
Although delayed seeding last year wasn’t effective to reduce flea beetle risk in many cases, numerous studies in Canada and the United States have shown it can be an effective tool to avoid flea beetle damage.
“The big factor is, anything that gets you from the time you seed to the three- to four-leaf stage quickly is going to help reduce the risk,” said Gavloski. “Some years, seeding later can help, but other years it’s just not going to do it.”
A number of cultural controls can be effective in reducing the risk for flea beetles, including seeding as shallow as available moisture allows, direct seeding and increasing seeding rate.
“Anything that you can do again to get quick germination, so you want to seed into moisture and if you can seed shallower, you might get quicker germination and quicker early growth that certainly can help,” said Gavloski. “A study done in Alberta looked at flea beetle damage in flax direct seeded into stubble versus plots that were worked up. They got less flea beetle feeding when there was some stubble, potentially because flea beetles like an open, hot, dry environment to feed in, so they’re more attracted to the open areas where there was no stubble than to plots where there was stubble.”
Increasing seeding rate provides more cotyledons for the same number of flea beetles to feed on, so it spreads the damage out more, added Gavloski.
Cutworms were widespread across Manitoba in 2019 and again, producers did a lot of spraying and reseeding in some cases. Cutworms are not restricted to any particular commodity, and affected canola, sunflowers, corn and soybeans among other crops.
Different species of cutworm meant that producers were dealing with this pest on an ongoing basis from early spring to early summer.
“We had two main species; the dingy cutworm, which overwinters as a partially grown larvae, which is there feeding on seedlings as they’re coming up in the spring, and redback cutworm, which overwinters as an egg, so you usually don’t start noticing them until late May, early June,” said Gavloski.
In a few cases there was also a third species – pale western cutworm – which does most of its feeding below ground and rarely comes above ground.
“The dingy cutworm and redback cutworm you can kill with an insecticide quite easily; the pale western cutworm is hit and miss,” said Gavloski. “Some people were telling me they were spraying and not really getting good results. The pale western might be one reason for that, but there could be other reasons too depending on when they were checking for them and when they were sprayed.”
Some commodities can compensate for cutworm feeding better than others.
“One that can compensate somewhat is canola,” said Gavloski, citing a study at the University of Manitoba that found there’s not a direct correlation between plants lost due to cutworms and yield loss. “The remaining plants are able to fill in, produce more pods, more seeds per pod and heavier seeds, so don’t make the assumption that if you lose 10 per cent of your plants, that’s 10 per cent yield loss.”
Gavloski recommends that producers use acceptable plant stand counts to help with cutworm decision-making in a crop like canola where there are some compensatory capabilities.
Forecasting the likelihood of cutworm problems in 2021 is tricky because there are multiple species and they are not easy to monitor. There are pheromones available for traps but no one knows how to properly interpret the data from them.
“We really don’t know what 300 cutworms in a trap in the fall means,” said Gavloski. “People have tested it, tried to figure out what it means, but the data was not good, there was too much variability. So, we don’t use pheromone traps to try and do any kind of predictions.”
The best advice Gavloski can offer to producers is to watch their fields in the spring. Based on what happened last year, the way cutworm cycles usually work is they build up, are bad for three to four years and then the curve goes the other way.
“That’s if you’ve got natural enemies that are regulating populations,” said Gavloski. “We had a really bad year last year. I’m hoping that we’re peaking and are going to go down the other side of the curve, but I would say be vigilant for cutworms for at least another year or so.”
Natural enemies that can help control cutworm population include parasitoids such as parasitic wasps, bee flies and tachinid flies, as well as predators like ground beetles and some fungal pathogens.
Again, with grasshoppers, there are multiple species and two of the most dominant in 2019 were the two-striped and migratory grasshoppers. The population of grasshoppers has been building consecutively for three years, which has coincided with three drier-than-normal years, conditions that favour grasshopper populations.
“When we get these periods of extended hot, dry summers, consecutively, it tends to build the grasshopper population up,” said Gavloski. “Going into next year, a lot will probably depend on the weather. If we get another hot, dry summer, anticipate some additional grasshopper issues.”
Many producers are asking, with the late rains and early snowfall across most of Manitoba this past fall, has that helped to kill off grasshopper populations, but Gavloski says probably not.
“Grasshoppers all overwinter as eggs and eggs are very resilient,” he said. “Grasshopper eggs can handle flooding well, so with the heavy rain in October and the early snow there might have been fewer eggs laid because if you don’t have nice sunny days, the grasshoppers are less active, but the eggs that are in the ground are likely going to make it through.”
It needs to get to around -15 C where the grasshopper eggs are in the soil, an inch or two down, Gavloski said, and if that doesn’t happen, there won’t be much winterkill. “Grasshopper eggs are often in ditches, and areas where snow accumulates, so don’t count on winterkill for grasshoppers,” he said.
Similarly, heavy rains in April and early May won’t likely provide much control either, although if heavy rains came in June it could decimate a grasshopper population, so it all depends on when the rains come next year.
Gavloski recommends producers consider management techniques that minimize harm to grasshopper predators and parasites, such as bee fly larvae, blister beetles, ground beetles and field crickets.
Manitoba Agriculture and Resource Development’s website says,’Preserving natural enemies by only using insecticides based on threshold guidelines is an important component of managing grasshoppers. When only low levels of grasshoppers are present, broad-spectrum insecticide applications can do more harm than good by reducing the number of natural enemies, which leaves grasshopper populations free to increase without natural population checks.’
Other cultural controls include seeding early and choosing vigorous crops that develop rapidly before the grasshopper eggs hatch, which will tolerate feeding better than later-seeded crops. Crops that are less preferred such as peas and oats can be used as guard stripe around more preferred broadleaf crops like canola, sunflowers and soybeans.
When insecticides are necessary, spraying the crop margin or border area surrounding the crop may be adequate, and insecticides with long residual activity is most effective, particularly if not all the eggs have finished hatching.
Insecticide applications are most effective when applied after the eggs hatch and while the nymphs are in the third to fourth nymph stage and still concentrated in their breeding areas. The lowest dosage given on the insecticide label should be used when the grasshoppers are small and the vegetative cover is low.
If a crop or vegetation outside the field contains flowering plants, it’s best to apply insecticides that are not harmful to pollinators either prior, after flowering, or as late in the day as possible, and only if needed. Most insecticides registered for grasshoppers in Canada are broad spectrum and harmful to pollinators, the exceptions being chlorantraniliprole (Coragen) and the bran baits Eco Bran and Nolo Bait.
Grasshopper bran baits are a good alternative to foliar sprays where practical. They are low cost and do not harm pollinators or most beneficial insects. The bait can be spread by application equipment designed to spread granular herbicides or seeds.
The thistle caterpillar is a butterfly that migrates from the southern U.S. and Mexico. Gavloski usually refers to thistle caterpillar as a one in 10-year problem, but it seems to be bucking the trend all of a sudden.
“About every 10 years on average, we have an issue with thistle caterpillar,” he said. “It’s been an exception recently; we’ve had two bad years in the last five years so that seems to be out of the norm. Last year, was probably the worst that we’ve seen in quite some time.”
Thistle caterpillar is mainly a problem in soybean and sunflowers, as well as weeds and thistles. Soybeans can compensate for defoliation, so in most years, Gavloski advises holding off spraying to determine how bad it gets.
“Usually the soybeans do compensate quite well from thistle caterpillar defoliation,” he said. “The problem this year in some of the areas, drought also factored in, so we had thistle caterpillars on small plants, there was a lot of defoliation and the plants just weren’t able to grow and compensate the way a growing soybean plant would be able to. So, the thistle caterpillar combined with the dry conditions created some economic issues this year.”
Natural predators for thistle caterpillar include tachinid flies which look like large houseflies with hairy, bristly tips to their abdomen.
“When you go out into any kind of crop in June and July and your sweep net is full of these flies, that’s good news, you want them around,” said Gavloski.
Will thistle caterpillars be an issue in 2020?
That’s hard to predict, said Gavloski, because they purposefully migrate and it depends on what the populations are like where they overwinter, mainly in California, Arizona and Mexico.
“We usually don’t get two bad, back-to-back years, they are usually a one in 10-year problem, so it all depends on if they migrate in or not,” said Gavloski.
There are no insecticides registered in Canada for controlling thistle caterpillars.
Army worms, sometimes called cereal army worm or true army worm, don’t overwinter well in Manitoba, but the adult moths blow in from the southern U.S., so it’s hit or miss whether or not Manitoba gets a significant population or not.
Last year they blew in mainly in the eastern part of Manitoba, into the southern Interlake and down towards Carman and Morden, so there were economical populations, but it was regional.
Control of army worms starts with biological controls.
“We had lots of army worms in late June or July and by mid- to late July, I was getting a lot of phone calls and emails with pictures of eggs all over wheat heads and oats,” said Gavloski. “They weren’t eggs, they were pupal clusters of a wasp called Cotesia that are natural parasitoids. Each of these wasps will lay anywhere from 20 to 60 eggs in an individual caterpillar.”
Tachinid flies are another parasitoid of army worms, and producers when they scout should look out for their eggs on army worms. “If you start seeing caterpillars with white little eggs behind the head, good chance those are tachinid eggs, that’s a good thing too,” said Gavloski.
Other predators are ground beetles and rove beetles, and for cultural controls, destroying grassy weeds, one to two weeks before seeding, will minimize the risk of attracting egg-laying moths and subsequent infestations.