Manitoba farmers have, in recent years, found themselves hosts to three new uninvited guests.
That was the message from John Gavloski, Manitoba Agriculture and Resource Development entomologist, to the Ag in Motion Discovery Plus virtual farm show this summer.
He said all three of these new pests are types of beetle, and all three first arrived in western Manitoba, and are working their way eastward.
The first to arrive in the province was the cereal leaf beetle. It was first found in Manitoba in 2009 and of the three, it has had the most time to propagate. The cabbage seedpod weevil was first found in Manitoba in 2017. The most recent arrival, the pea leaf weevil, was only discovered last year.
Cereal leaf beetle
Cereal leaf beetles are normally six to eight mm long with blackish-blue wings, a black head and a reddish-orange thorax and legs. As the name suggests, they feed on cereals – oats, barley, wheat, rye, forage grasses and even some grassy weeds, like wild oats. While the adult beetles do feed on the leaves, the larvae are of more significant concern.
“The best time for scouting is just prior to, or as you go into the boot stage, when the head is just starting to emerge,” says Gavloski noting this is usually in June or early July. He explains that, prior to the boot stage, the threshold is three larvae per plant, but that once the flag leaves emerge, the threshold is one larva per leaf on average.
Cabbage seedpod weevil
The adult cabbage seedpod weevil will feed on canola buds and flowers but the adult feeding habits aren’t what cause the economic damage. However, the adults do lay eggs in the seed pods and the larva will feed right inside the pod. “Each larva will feed on several seeds within that pod, so you have direct seed loss happening,” Gavloski explains.
Unfortunately, once the larvae are in the seed pods, little can be done to halt the infestation. The farmer or agronomist needs to be focusing on looking for the adults as they move into the canola prior to laying their eggs. “To do that you use a sweep net,” says Gavloski. He recommends doing a minimum of four sets of 10 sweeps. The threshold for an infestation that will have a noticeable economic effect is anything more than 25-40 weevils per 10 sweeps on average.
Pea leaf weevil
The only visible evidence above the ground to indicate pea leaf weevil, is the characteristic semi-circular notches on the plant’s young leaves after the adult weevil has fed. But once again, it’s not the adult’s feeding habits that cause the economic damage.
“The larvae are in the soil and they’re feeding on nodules growing on the roots of peas and fava beans (their main two host plants),” Gavloski said.
Because the nodules are involved in nitrogen fixation in the plant, a significant infestation will cause nitrogen deficiency in the crop.
Unfortunately, the pea leaf weevil is tricky to pin down.
“The adults are very good at hiding when you look for them,” Gavloski said. “They drop to the soil and they play dead; they’re tough to count.”
As a result, agronomists have to count the notches on those early leaves in the pea seedlings. The threshold for an adverse economic outcome is when more than about 30 per cent of the seedlings have those notches in the clam leaves.
Although spraying pesticides or using pesticide-treated seeds have been effective in containing infestations, Manitoba Agriculture is working on some more innovative containment strategies.
To control cereal leaf beetles’ populations, they have introduced a parasite called a tetrastichus julis. This tiny wasp that only attacks cereal leaf beetles, bores into the larvae and lays its eggs then they feed on the larva once they hatch.
In an ironic twist, the cereal leaf beetle larvae creates a fecal shield to protect themselves from predators.
“Basically they coat themselves in their own poop,” says Gavloski. But he says it has the opposite effect on these special wasps. “They are actually attracted to the smell given off by the fecal shield – it helps the wasps find the larvae.
“We’ve done releases in the northwest and central regions,” says Gavloski. “Things have been quite successful. Quite often when we look at the larvae to see what the percentage of parasitism is, we’re well over 50 per cent.”
However, what is of particular interest is that in some areas where they haven’t released the wasps, they are seeing significant numbers on the larvae.
“Somehow the wasp has moved with the populations,” said Gavloski.