Potato leafhoppers are doing more harm than alfalfa growers may realize
Hot, dry weather has led to high numbers of leafhoppers in 2020.
Tropical Storm Cristobal roared into Michigan from the U.S. Gulf states in June 2020 and many remember the massive flooding and water damage near Midland, Michigan. However, the storm brought more than just huge amounts of rainfall. The storms also brought potato leafhoppers along with the rain. The period since the storm passed through has had hot, dry weather. This is the ideal type of weather pattern that favors high potato leafhoppers feeding and damage in both alfalfa and dry beans.
There are numerous reports of alfalfa fields with significant levels of hopperburn. Unfortunately, once hopperburn is visible, the damage is done. It is already too late to prevent 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. Unlike most other leafhoppers, potato leafhopper’s saliva is toxic and results in abnormal cell growth and blockage of fluid transport in the leaf. The visual symptom in many plants is a characteristic yellowing called hopperburn (see photo). 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 fields with regrowth less than 3 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 Christina 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 threshold values are:
- Under 3-inch alfalfa (regrowth): 0.2 adults per sweep = 20 per 100 sweeps
- 3-to-8-inch alfalfa: 0.5 adults per sweep = 50 per 100 sweeps
- 8-to-12-inch alfalfa: one adult and/or nymphs per sweep = 100 per 100 sweeps
- 12-to-14-inch alfalfa: two adults and/or nymphs per sweep = 200 per 100 sweeps
When using a sweep net, split the number of sweeps into smaller groups of 10 until you reach 100. It is easier to count the elusive adults that fly quickly.
The preferred method for controlling potato leafhoppers is cutting. If a spray is used, it is better to spray smaller plant regrowth rather than taller growth. 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 the 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.
Continue to monitor your alfalfa fields closely as conditions change going into August. With wetter weather, or with heavier morning dews, 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 firstname.lastname@example.org.