John Gavloski, Entomologist, Manitoba Agriculture and Resource Development – Spring 2021 Pulse Beat
Insect issues in pulse and soybean crops in 2020 were an interesting mix. Populations of insects like grasshoppers and cutworms that have been building in recent years continued to be of concern. Of the insects that move into Manitoba from the south, some, such as potato leafhopper, did arrive at levels that caused at least localized concerns. Others, such as soybean aphid, never did arrive at levels that were a threat to crops. Additionally, we continue to monitor levels of an invasive insect of peas and faba beans.
There has been an increase in grasshopper populations over the past few years and this trend of higher populations continued in 2020. Grasshopper populations have more successful development in dry years and generally increase over a series of dry years. If this trend of drier summers continues in 2021, pulse growers should keep an eye on grasshopper levels around and in their crops.
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. Rainy weather at critical points in the grasshopper’s lifecycle, such as when they are newly emerged or laying eggs, can help bring levels down. There are also fungal pathogens that can help control grasshoppers. One of these that can be noticeable is when grasshoppers are infected with Entomophaga grylli. This pathogen results in dead grasshoppers left clinging to the stems of plants. It is most effective under warm, humid conditions.
Although not quite as bad as in 2019, cutworms were still a concern in some pulse and soybean fields in 2020. In 2019 there were reports of reseeding because of cutworm feeding in soybeans, peas and faba beans, as well as extensive insecticide applications. There were no reports of reseeding of pulse or soybean crops because of cutworm feeding in 2020, but insecticides were used to control cutworms in some fields. Hopefully, the population cycles for our main species of cutworms are now declining after a few troublesome years. Cutworms remain an insect you want to make sure you are scouting for in 2021.
Seed treatments are one option for cutworm management in pulses and soybeans. Lumivia CPL is available as a seed treatment option in dry beans, chickpeas, faba beans, lentils and field peas. The active ingredient in this seed treatment is chlorantraniliprole, the same active ingredient as in the foliar insecticide Coragen. There are no seed treatments registered for cutworms in soybeans — just wireworms and seedcorn maggot.
Predators such as ground beetles and parasitoids, including bee flies, Tachinid flies and several species of parasitic wasps, can help manage cutworms. Wet soil conditions promote fungal diseases among cutworms and also force the larvae to feed at the soil surface, where they are subject to the attack of parasites and predators. High populations of these natural enemies can help regulate cutworm levels. To promote healthy levels of natural enemies, a recommended approach is to scout and get an idea of what levels of cutworms and cutworm feeding are like early in the season and control them if levels are high. When cutworm levels are naturally low, applying insecticides can be uneconomical, not only because of the insecticide costs but also by disrupting cycles of natural enemies.
Potato leafhoppers (Empoasca fabae) are an insect that does not overwinter in Manitoba. In some years, populations from the south arrive in Manitoba at levels that can be of concern in dry beans. In 2020, levels were near threshold in some dry bean fields in the central region of Manitoba, with at least a couple of fields receiving insecticide treatments for potato leafhopper.
The potato leafhopper does not overwinter in the north but migrates northward each season. The overwintering range is estimated to extend as far north as Missouri, Kansas, Kentucky, Virginia and Maryland. Each year in April and May, a northward movement starts. There is also a return migration to overwintering areas later in the season. A study published in 2015 looked at the average date of the first arrival of potato leafhopper in various states, including some northern states such as Minnesota and North Dakota. They found that over a 62-year period (ending in 2012), the average arrival date had become 10 days earlier over that period. Results suggest that continued warming could advance the time of potato leafhopper colonization and increase its impact on affected crops. The average date of first arrival in North Dakota was June 8th. This may be an insect to keep an eye on in dry beans.
Larvae of pea leaf weevil feed on the nodules on the roots 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, pheromone-baited pitfall traps for pea leaf weevil were set up in several locations in western Manitoba in the spring and early summer to determine the levels and range of pea leaf weevil in Manitoba. Pea leaf weevils were found in traps near Kenville and Minitonas in the northwest. In addition, a MPSG production specialist collected a sample of weevils from peas near Dauphin in September, which was verified as pea leaf weevils. All traps in the southwest were negative for pea leaf weevil. Pea leaf weevil appears to be established through a large part of the northwest region of Manitoba, but so far has not been detected outside this region.
Regular crop scouting is essential to ensure insects and other potential pests do not cause economic damage to your crop. In 2021, be vigilant for grasshoppers and cutworms and potential surprises from the south. If anyone suspects that they have found pea leaf weevil, please contact Manitoba Agriculture and Resource Development or a MPSG production specialist. We will continue to determine the range of locations and levels of pea leaf weevil in Manitoba.