Insects in Pulse and Soybean Crops in 2022
John Gavloski, Entomologist, Manitoba Agriculture – Pulse Beat 96, Fall/Winter 2022
WEATHER WAS ONCE again a big factor in how insects interacted with pulse and soybean crops. Seeding was quite late in many areas this year, mainly because of excessive rainfall and overland spring flooding in some areas. This resulted in some crops remaining in susceptible stages to some insect pests later in the season than normal. Few insect issues were reported for dry beans this year, and pea aphids were the main insect concern in peas. Soybean aphids and grasshoppers were the main insect concerns in soybeans. The range and levels of pea leaf weevils continues to be tracked.
Pea aphids and soybean aphids are very different in how they colonize peas and soybeans respectively,as are the levels of each that are considered economical.
A PLETHORA OF PEA APHIDS
Levels of pea aphids were high in many fields of peas, with insecticide applications occurring in many fields. Aphid feeding on peas in the flowering and early pod stage can cause lower yields due to less seed formation and smaller seed size. Protein content and other quality issues do not appear to be affected.
Sampling and thresholds: Sampling to determine aphid levels should be done when about 50 to 75% of the pea plants are in flower. The economic threshold is two to three aphids per 20 cm plant tip, or nine to 12 aphids per sweep (90 to 120 aphids if doing a 10-sweep sample). If the economic threshold is exceeded, a single application of insecticide when about 50% of plants have produced some young pods will protect the crop against yield loss and cost-effective. Most of the damage that aphids do to peas is to the pods before they start to fill. If most of the pods have already started to fill, spraying would be too late to have an economic impact.
SUPERFLUOUS SOYBEAN APHIDS
The first report of any soybean aphids was on July 5. Soybean aphids reached economic levels and control was needed in some fields in August. There were reports of insecticides being applied for soybean aphids in the Eastern, Interlake and Central regions. Until this year, soybean aphids had not been at economic levels in Manitoba since 2017.
Thresholds to use for soybean aphids: The action threshold for soybean aphids (where insecticide application is recommended to prevent economic
loss) is:
- 250 aphids per plant on average,
- and the population is increasing,
- and the plants are in the R1 (beginning bloom) to R5 (beginning seed) growth stages.
The reason that “and the population is increasing” is part of the threshold is because the economic injury level, where control costs will equal yield loss, is actually about 670 aphids per plant. The action threshold, where control is suggested, has been set much lower than 670 to allow time for an insecticide to be applied before increasing populations could potentially reached 670 per plant. The population doubling time for soybean aphids is on average about seven days.
Should the action threshold for soybean aphids be adjusted for higher crop values?
There was some discussion this year on whether the economic threshold for soybean aphids should be adjusted because of the high price of soybeans, but this was discouraged.
When the value of the soybean crop has increased or is high, it is not advised to use an action threshold below 250 per plant and the population increasing. There is already a large gap between the economic injury level and the action threshold suggested. Setting an action threshold at lower aphid densities increases the risk to producers by treating an aphid population that is growing too slowly to exceed the economic injury level in seven days, eliminates generalist predators, and exposes a large portion of the soybean aphid population to selection by insecticides, which could lead to the development of insecticide resistance. Continue to use the action threshold presented above.
Natural enemies such as lady beetles, hover fly larvae and aphid mummies, which are parasitized aphids, were noticed in some pea and soybean fields. Where these are abundant, consider them in your decisions since they can regulate aphid populations to below economic levels.
GRASSHOPPERS GALORE, BUT SOME GOOD NEWS
Grasshopper levels were once again high in many areas. There were reports of insecticide applications to control grasshoppers in soybeans from the Southwest, Central and Interlake regions. Some of the applications were to borders, and in other instances whole fields were treated.
Late in the season, a high level of infection by a pathogenic fungus, called Entomophaga grylli, was noticed in grasshopper in some areas. This fungal pathogen results in dead grasshoppers left clinging to the stems of plants. It is most effective under warm, humid conditions.
A very high level of infection by E. grylli was noticed in a canola field in the Central region near MacGregor. Many dead grasshoppers were easy to spot clinging to tops of the plants as you looked over portions of the field, and the dead grasshoppers were so thick that in some areas they were found clinging to other dead grasshoppers. Hopefully this pathogen results in some lower levels next year in areas where it was abundant.
Some predators of grasshoppers, such as certain species of blister beetles and bee flies, were also 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 of both these groups specialize in feeding on grasshopper eggs.
PEA LEAF WEEVILS
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 failing to fix enough nitrogen. 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 and samples collected by agronomists were used to determine range and relative levels. Pea leaf weevils were only found in the Northwest in 2020, but the known range expanded considerably in 2021. In 2021, pea leaf weevils were found in some areas of the Southwest and Central regions, as far east as fields near Cyress River and Holland.
In 2022, a survey was done when peas were in the 2nd to 6th node growth stages. The survey counted the number of crescent-shaped notches in the leaves made by the adult weevils. This helped determine the relative abundance of pea leaf weevil in various regions. The map bottom-left shows the results.
Highest levels of pea leaf weevil are currently in the Northwest region, although how economical these populations are is uncertain.
Regular crop scouting is essential to ensuring insects and other potential pests do not do economic damage to your crop. Stay vigilant for grasshoppers and pea aphids, and if insects arrive from the south, such as soybean aphid, we will update you through the Manitoba Crop Pest Updates.
Anyone interested in participating in the pea leaf weevil leaf notching survey, or who suspects that they have found pea leaf weevil in an area outside their known range, please contact John Gavloski, at Manitoba Agriculture, or Laura Schmidt at MPSG.
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