Classic weather pattern for potato leafhopper outbreak is here
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EmailSecond cutting alfalfa should be scouted early after regrowth appears.
The spring so far has been warmer and drier than normal. Michigan State University state climatologist Jeff Andresen said, “In 2021, regions in Michigan are experiencing the fourth driest late winter and spring for the last 128 years (since 1893).” Coupled with the recent rain, strong winds from the Gulf of Mexico during the last week in May, and a forecast for warmer and drier weather and you have the recipe for a potato leafhopper outbreak in Michigan. This is the ideal type of weather pattern that favors high potato leafhopper feeding and damage in both alfalfa and dry beans.
Alfalfa growers are encouraged to scout for potato leafhoppers once regrowth appears, since early feeding will often be more damaging. Hopperburn is the visible effect potato leafhoppers have on the plants (see photo). Unfortunately, once hopperburn is visible, the damage is done. It is already too late to prevent the effects of the damage. The picture above displays “severe” damage to alfalfa. With slight yellowing of the plants, yield reduction has already taken place.
Potato leafhoppers feed by sucking sap out of plants, injecting saliva as they feed. But unlike most other leafhoppers, potato leafhopper’s saliva results in abnormal cell growth and blockage of fluid transport in the leaf. Both nymphs (immatures) and adults cause this damage. Because of the damage, reduction of yield, forage quality, plant vigor and winter-hardiness will occur.
New seedings and field with regrowth less than three inches are particularly susceptible to damage from the pest. The recommendation from Michigan State University Extension is to utilize an integrated pest management approach to dealing with this insect.
MSU entomologist Chris DiFonzo says, “Potato leafhopper is the most important insect pest of alfalfa and dry beans in Michigan.” In alfalfa, sample using a sweep net and treat based on a combination of potato leafhopper number per sweep and average plant height. Regrowth (plants under 3 inches) is particularly vulnerable to potato leafhopper damage. As the crop grows, it can handle a greater number of leafhoppers.
The economic threshold values for alfalfa, based on the number of potato leafhoppers in 100 sweeps, are as follows:
- Under 3-inch alfalfa (regrowth): 0.2 adults per sweep = 20 per 100 sweeps
- 4-to-7-inch alfalfa: 0.5 adults per sweep = 50 per 100 sweeps
- 8-to-11-inch alfalfa: One adult and/or nymphs per sweep = 100 per 100 sweeps
- Greater than 12-inch alfalfa: Two adults and/or nymphs per sweep = 200 per 100 sweeps
For potato leafhopper-resistant varieties:
- New seedings, use regular threshold.
- Older stands, use three times the regular threshold.
How to scout alfalfa for potato leafhoppers
When using a sweep net, split the number of sweeps into smaller groups of ten until you reach 100. It is easier to count the elusive adults that fly quickly.
For control of potato leafhoppers, the preferred method is cutting. If a spray is used, it is better to spray smaller plant regrowth rather than taller growth. If sprays are used, only use approved insecticides labeled for potato leafhoppers and follow all label requirements. A major concern with spraying insecticides for control is that beneficial insects will also be killed along with potato leafhoppers.
Although no issues have been reported with potato leafhopper resistance to insecticides, the number of insects showing pesticide resistance is growing across the United States. Pesticide resistance is described as the decreased susceptibility of a pest population to a pesticide that was previously effective at controlling the pest. Research shows that resistance problems have increased because pesticides are applied more frequently and at higher dosage rates.
When alfalfa fields are at thresholds and a pesticide spray application is to be done, always follow label rates and application guidelines.
Going forward, continue to monitor alfalfa fields closely as conditions change through the summer. When wetter weather, or with days that have heavier morning dews occur, potato leafhoppers can diminish quickly due to the entomopathogenic fungi that can cause the population to collapse. When these conditions exist, and populations of potato leafhoppers drop, there is no need to spray.
For more information, contact Phil Kaatz at 810-667-0341 or email at kaatz@msu.edu.
