John Gavloski, Entomologist, Manitoba Agriculture – Spring 2022 Pulse Beat
ONE OF THE big factors affecting crops in 2021 was the weather. Generally, hot and dry conditions at times were stressful on crops. Weather conditions can also affect levels of some insects, as well as the ability of crops to compensate for feeding. A series of consecutive dry years provides favourable conditions for the potential pest species of grasshoppers to increase, which we have seen happening in Manitoba. Dry weather also provides favourable conditions for spider mite levels to increase. Grasshoppers were an issue in some pulse and soybean fields in 2021, and spider mites were a concern in some soybean fields. Feeding from lygus bugs was an issue in some pulse crops, and the known distribution of pea leaf weevil in Manitoba continues to spread both south and east. How big a concern this will likely be in Manitoba is hard to say.
On the positive side, soybean aphid levels were once again very low, and pea aphid was generally not at economic levels during the more vulnerable stages of peas. Cutworms were still a concern, although the level of damage to pulse and soybean crops in 2021 was not as great as the previous year. Hopefully, this trend continues.
GRASSHOPPERS – WILL THE INCREASE CONTINUE
There has been an increase in grasshopper populations over the past few years, and this trend of higher populations continued in 2021. If this trend of drier summers continues in 2022, pulse growers should once again keep an eye on grasshopper levels around and in their crops.
What can reverse the trend of increasing grasshopper levels? Rainy weather at critical points in the grasshoppers’ lifecycle, such as when they are newly emerged or laying eggs, can help bring levels down. There is also a fungal pathogen called Entomophaga grylli that results in dead grasshoppers left clinging to the stems of plants. It is most effective under warm, humid conditions.
Some predators of grasshoppers, such as certain species of blister beetles and bee flies, were abundant in some areas, which could help regulate levels somewhat. There are many species of both blister beetles and bee flies, and the larvae of some species within both these groups specialize in feeding on grasshopper eggs.
There were reports from agronomists of some fairly high levels of black or gray blister beetles in patches in some soybean fields.
These are both species that prey on grasshopper eggs as larvae. Adults feed on many plants and will sometimes be numerous in patches in crops. They generally don’t cause enough defoliation to be an economical concern, so control is generally not needed when you encounter these in pulse or soybean crops. On the plus side, in a year when grasshoppers are abundant, it is encouraging to know that their larvae feed on nothing but grasshopper eggs.
SPIDER MITES – A POTENTIAL CONCERN IN DRY YEARS
Spider mites first became noticeable in some soybean fields in late-July. Control was applied in some fields in early-August.
When scouting soybeans or dry beans, light-coloured stippling damage on the leaves, which is easier to spot than the mites themselves, may be the first indication of spider mite feeding. Confirm the presence of mites by tapping the leaves over something where they will stand out. A black piece of construction paper works well, as they can be easier to see against a dark background. The mites will look like specs of dust that move. Stippling can be common in the lower canopy and not be of economical concern. It is when stippling is becoming common in the middle of the canopy that it can be economical. If control is needed, stick to registered products. If you are applying insecticides for multiple pests, including spider mites, be aware that some insecticides (such as most pyrethroids) don’t work well on spider mites and may flare their populations. If rain is in the forecast, mite levels may naturally decline. Mites are particularly susceptible to fungal pathogens, which are favoured by moist conditions. This is one of the reasons dry weather encouraged mite outbreaks.
PEA LEAF WEEVIL – RANGE EXPANDING IN MANITOBA
Larvae of pea leaf weevil feed on the root nodules of pea and faba bean plants, and if there is excessive feeding, the reduction in nodules can lead to the plants not fixing enough nitrogen. Until recently, pea leaf weevil had been an insect that was moving eastward through the prairies but had not been found in Manitoba. Pea leaf weevil was found in Manitoba for the first time in 2019, after an agronomist in the northwest region sent in a sample for identification.
In 2020 and 2021, pheromone-baited pitfall traps for pea leaf weevil were set up in several locations in Manitoba in the spring and early-summer to determine the levels and range of pea leaf weevil in Manitoba. In addition, agronomists sent in samples of weevils from peas and faba beans for verification. Pea leaf weevils were only found in the northwest in 2020, but the known range expanded in 2021. A large part of the documented range expansion is because of some keen agronomists who have been collecting weevils from faba beans and peas in late-July and August.
In 2021, pea leaf weevils were found in some areas of the southwest and central regions. In early-August an agronomist sent in a sample of weevils collected near Sinclair, in southwest Manitoba, all of which were pea leaf weevil. Later in August, an agronomist with Manitoba Pulse & Soybean Growers (MPSG) collected pea leaf weevils from near Cypress River and Holland, expanding the known eastward distribution of pea leaf weevil into the central region of Manitoba. We still don’t know how significant of a problem pea leaf weevil will be in Manitoba though.
This coming year we will be doing a survey in late-May and early-June, when plants are in the second to sixth node growth stages, counting the number of crescent-shaped notches in the leaves, which the adult weevils make. This will help us determine the relative abundance of pea leaf weevil in various regions. The survey is easy to do and not too time-consuming, and agronomists and pulse growers are welcome to participate.
Regular crop scouting is essential to ensure insects and other potential pests do not do economic damage to your crop. For 2022, be vigilant for grasshoppers and start monitoring for them in late-May or early-June as the egg hatch is beginning. If it is another dry year, keep an eye on spider mite levels as well. If there are surprises that arrive from the south, such as soybean aphid, we will update you through the Manitoba Crop Pest Updates. If anyone is interested in participating in the pea leaf weevil survey or suspects that they have found pea leaf weevil in an area outside their known range, please contact myself, at Manitoba Agriculture or a production specialist at MPSG. ■