The damage can be voracious and all at once, or steady, unseen over time, but forage-loving insects each year take a bite out of hay and grasslands.
There are various strategies, some of which involve turning nature around and decreasing the conditions for the insects, but sometimes there’s a need for an insecticide intervention.
At a forage workshop hosted by the Canadian Forage and Grassland Association at the University of Manitoba early this year, Manitoba Agriculture entomologist John Gavloski highlighted several key insect pests that can affect forage production and shared strategies for managing them.
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Grasshoppers
Grasshoppers are a common pest in Prairie fields. Canada is home to 129 species of grasshoppers, but very few of those cause crop damage. There are only four or five grasshopper species that are considered to be pests, and of those, three are responsible for most of the damage.
“The top three grasshoppers in our survey this year are two-striped, which has been our dominant species in recent years, followed by either clear-winged or migratory,” Gavloski says.
All three have a similar life cycle. They all overwinter as eggs, laid in clusters of 20 to 30.The individual eggs resemble grains of brown rice.
“If you can dig those up easily, then you know you’ve got a decent grasshopper egg population,” Gavloski warns.
The ideal time for control is when the grasshoppers’ wing buds become visible. At this stage, the hatch is likely complete.
Farmers can also lean on provincial grasshopper risk maps. Each of the Prairie provinces puts out a risk map in the fall based on samples taken, usually in August. The maps can help estimate the next season’s egg-laying potential.
While these maps can help, Gavloski warns they’re estimates and don’t guarantee outcomes for the following season.“They’re more vulnerable when they’re really tiny,” Gavloski says. “They get disease, they get bloated and they can drown.”
Economic thresholds: Estimating whether grasshopper numbers have reached the economic threshold can be challenging.
Gavloski estimates grasshopper populations by walking 50 metres along a field edge or ditch, selecting five random one-metre square areas and counting the grasshoppers that jump as he approaches. He then disturbs the plants to check for remaining insects.
These counts provide an estimate of the grasshopper population. The process doesn’t give very accurate results, but can probably let you know if you’re near the threshold.
“Usually, you can get a ballpark range,” Gavloski says. “If you think it was somewhere around 12 to 15, that is good. You don’t have to know that it was actually 13.”
For forage crops, the economic threshold for grasshopper control is generally considered to be eight to 12 grasshoppers per square metre.
Control methods: Gavloski recommends using what is called the Reduced Area and Agent Treatments (RAATS), which involve treating alternating swaths of land. This method works because grasshoppers move from untreated areas to treated areas. Research done in Wisconsin has shown RAATS can provide very good control while using half the insecticide.
“The results of the research showed they were getting 94 per cent control when they sprayed the whole rangeland, but they were able to get 81 per cent control by only doing half of the rangeland,” Gavloski says.
Insecticides such as Coragen (chlorantraniliprole) are commonly used to control grasshoppers. Coragen is effective, with relatively low toxicity to pollinators. Generic insecticide versions have made treatments more affordable.
Cutworms
Cutworms are a diverse group of pests in Canada. Gavloski spoke about three different categories of cutworms: climbing, surface-feeding and subterranean.
Climbing cutworms climb plants at night to feed, then retreat underground or under debris during the day. Unlike other cutworms, they don’t cut plants but leave feeding damage on leaves, which can be misleading when scouting. The dingy cutworm is the most common climber in Manitoba. It can be identified by small leaf-like markings on its body. To find them, dig around damaged plants.
Gavloski also pointed out that some infestations can appear in patches. The patchiness can depend on egg-laying patterns the previous season, but it can also depend on topography. In more rolling landscapes, the warmer, southern side of the slopes tends to attract the bugs.
Economic thresholds: Gavloski didn’t speak to economic thresholds in his presentation. Manitoba Agriculture’s website says economic thresholds haven’t been well researched for cutworms, but included some anecdotal thresholds for various crops.
For alfalfa (the only forage crop listed), the website suggests four to five larvae per square foot (30 cm) as a threshold. Otherwise, farmers can just make a judgement based on the level of plant injury. Visible, widespread damage that threatens plant stands would obviously signal the need for control.
Control methods: Management strategies depend on the species. Coragen Max is the only insecticide registered for cutworms in forages — and while Gavloski says more options are needed, the pesticide is effective.
Since cutworms are most active at night, insecticides should be applied late in the afternoon or evening.
Unfortunately, the effectiveness of insecticides to control subterranean species is “hit-and-miss,” Gavloski says. “Because they’re not coming above the ground to feed, they can be a real nuisance to manage with insecticides.
“If the problem is small — say, 10 or 20 acres have a lot and the rest of the fields aren’t bad — you can patch-spray for cutworms.”
Cereal armyworm
The pest Gavloski refers to here shouldn’t be confused with the better-known bertha armyworm, which is mostly a concern for canola growers. As the name suggests, cereal armyworms are more likely to be found in forage grasses than alfalfa stands. It is also known as true armyworm or simply as “armyworm.”
Adults don’t overwinter in Canada but migrate from southern areas, often carried by wind currents in spring and summer, and while they have a particular affinity for timothy, most forage grasses are excellent hosts for armyworms.
“When the adults arrive, they’re looking for a lush, dense grassy stand to lay their eggs into,” Gavloski says. “So a perennial grass is just ideal … that’s where the eggs often end up.”
Armyworm larvae have distinct stripes, with a V-shaped marking on their heads. The colours can vary, but those striped patterns are usually visible. The adult moths are light brown with white dots on their wings. Armyworm larvae feed primarily on grasses and are often found in forage stands, especially in early spring.
“During the day, they’re trying to hide, so they’re underneath the debris and may go into the cracks in the soil,” Gavloski says. “So you have to look in the cracks, peel away the debris and do your counts.”
In cereal and grass crops, begin scouting in late June and examine at least five areas, focusing on patches of lodged plants and grassy weed infestations. Fields or sections with significant bird activity should also be monitored, as birds often feed on armyworms. Additionally, check the backs of larvae for parasite eggs, which can indicate natural control.
Economic thresholds: The threshold for control varies by crop. For forage grasses, it’s generally five larvae per square foot (929 cm2). However, as mentioned, they love timothy. Instead of feeding on the leaves before moving on to the heads like they do with other grasses, with timothy, they attack the heads first. As a result, they may require intervention even if the threshold isn’t met.
“Sometimes, it will pay to control them at levels below a threshold, depending on where they’re feeding and how much damage they’re doing,” Gavloski says.
Control methods: Insecticide applications should be timed for the evening or late afternoon, as armyworms primarily feed at night.
As with cutworms, chlorantraniliprole is the only active ingredient registered for armyworms. The products Coragen Max, Coragen (the pre-Max version) and a new generic version called Shenzi are available for farmers to use.
Alfalfa weevil
The alfalfa weevil is a major economic pest for alfalfa crops, but will also feed on other legumes such as clover, making it a major concern for many farmers growing forages.
Adults overwinter under plant debris and soil in and around alfalfa fields and emerge in spring, feeding on alfalfa leaves. Round, elongated holes in the leaves are an indication of adult alfalfa weevils feeding.
When females are ready to lay eggs, they chew a hole in the stem and deposit from one to 40 eggs. When they hatch, the larvae will feed upon the stem before moving to fresh buds and leaves. After the larval stage, they make silky cocoons that, if the infestation is widespread, can be visible as a white haze over the field.
“People often first notice this driving by their field and see the crop is looking kind of frosted,” Gavloski says. “If you see this, you’ve probably got lots of feeding, and you probably should have been in there earlier.”
As a general guideline, he recommends scouting for the bugs in early June and continuing weekly checks.
Economic thresholds: Alfalfa weevils cause significant damage, particularly to young alfalfa crops. The general action threshold for larvae varies based on crop height. For hay crops under 30 cm, the threshold is one larva per stem. For crops between 30 and 40 cm, it’s two larvae per stem. If three larvae per stem are found, control measures are generally needed.
Control methods: For hay crops, cutting the plants early is the main strategy used to control the insects because the larvae will starve or desiccate after cutting. Several insecticides are available for use, but many only provide partial suppression.
“Our insecticides don’t always work well for alfalfa weevil,” Gavloski says.
Natural predators, such as parasitoid wasps (bathyplectes and oomyzus), can help control alfalfa weevil populations. These predators can sometimes reduce the need for chemical control. n
