Economic thresholds of insects in pulses
by Megan Madden
While a pulse grower's goal is to use preventative measures to keep insect infestations below significant levels as part of a sustainable integrated pest management plan, that is not always how the season works. Crop rotation can be beneficial in not providing the same host in back-to-back years but since fields are not islands, they can be affected by other surrounding crops. Often, immediate, in-crop control is required when these insects will have an impact on your crop to the point of affecting your bottom line. How do you know whether or not to treat for a specific pest?
Your first reaction might be to spray as soon as you see insects in your crop. However, control methods can be expensive, and time and labour intensive, and also affect beneficial insects if the pesticide is unwarranted. Economic thresholds can fluctuate depending on many factors including the pest, the crop type, markets, pesticide cost, and even growth stage. A healthy, fast-growing crop may be able to withstand more pests without sacrificing yield loss.
Cutworms, pea leaf weevil, wireworms, aphids, and grasshoppers (in dry years) are what pulse growers need to be most concerned about according to Scott Hartley, Provincial Specialist, Insect and Vertebrate Pest Management, with the Saskatchewan Ministry of Agriculture.
“Cutworms were a significant, widespread problem in 2016 after a decline following some peak years (circa 2012/13). Cutworms can affect all pulse crops and high populations can greatly reduce stands over many acres,” says Hartley.
When scouting for cutworms, first look on the tops of hills and south facing slopes, as the warmer drier soils make the cutworms easier to spot and they may not be noticed in low-lying areas until the insects become larger and more numerous. The standard economic threshold for cutworms in pulses is 30 per cent stand reduction, or two to three larvae per square metre in the top seven centimetres (cm) (three inches) of soil.
Pea crops have only a few insect pests that affect plants to the point of financial detriment, but the few that can affect pea plants need to be monitored to avoid yield loss. Both peas and faba beans are hosts to the pea leaf weevil, which can cause significant losses in an abundant year. “Generally it is not the leaf feeding by the adults in the spring that cause significant damage,” says Hartley. “However on some years, if plants are small and pea leaf weevil populations are high, feeding pressure can warrant control. It is the larval feeding on the nitrogen fixing nodules on the roots that results in most yield loss.”
The standard economic threshold of pea weevil is one in three plants showing feeding damage on the clam leaf, or 30 per cent of the plants showing feeding damage. Scouting for pea leaf weevil can be a challenge, as the adult is elusive. They will sometimes play dead, dropping off the plant and hiding in the soil when they sense movement, making this soil coloured insect difficult to see. You should begin scouting immediately after the crop has emerged, and look at the newest leaf for active feeding. Choose 10 plants in an area and look for feeding damage, repeating this at multiple locations in the field. If more than three out of 10 plants are affected at multiple locations in the field you may want to consider an insecticide, depending on node staging. By the time the pea plants reach the sixth node they will outgrow any pea leaf weevil damage. A foliar application is too late at that time as most of the eggs will have been laid and are unaffected by insecticides.
Wireworms can affect all pulse crops, though the species may differ. Converse to cutworms, wireworm scouting should take place lower as they will be less abundant on hilltops, preferring the more moist soils found in low-lying areas. While wireworms do not have an established threshold, a common guideline is 32 per square metre or greater, and to use a seed treatment the following year.
Hartley comments that aphids have been most problematic in pea crops in Saskatchewan, but in the past three years he has also seen significant infestations of aphids in lentils. “Aphids can also be problematic in beans and faba beans but have not been of note yet here in Saskatchewan,” he adds.
Early seeding can help to avoid infestations because the crop can mature before they become an economic issue, as plants mature they are less attractive to aphids. Threshold levels are accepted as two to three aphids on the top 20 cm of the plant tip but it should be noted that this threshold was established with older varieties of peas. Newer varieties are thought to be able to withstand considerably higher levels.
“Grasshoppers are mostly a problem in lentils due to feeding on developing pods and therefore have a direct negative effect on yield. The economic threshold is only two per square metre,” says Hartley. “In high grasshopper years there were reports of grasshoppers nipping off chickpea pods but I cannot quantify the damage. Grasshoppers will eat anything green so could still be a problem in other pulse crops to some degree. Although they will eat peas, it is not a preferred crop and it will lower their biotic (egg-laying) potential.”
Hartley reassures those growers who had harvest ‘16 overlapping with seeding ‘17. “Over-wintered crop may provide an easily accessible food source and may also provide better over-wintering habitat for some insects but generally it is spring conditions that will have a bigger influence on insect