Potato Leafhopper (PLH) is perhaps the most common insect pest in alfalfa. In fact, this yield-robbing pest is capable of inducing yield losses exceeding 50% during severe infestations. Potato leafhoppers cause damage by piercing the stem and underside of the leaf with its stylet and feeding on plant sap.

As it feeds, it injects phytotoxic saliva into the plant, which restricts the flow of carbohydrates within the phloem, and causes the wedge-shaped yellowing commonly called “hopper burn”.

Alfalfa is not the only crop to suffer the wrath of this tiny insect, but you probably figured that out by its name… the potato leafhopper. Besides potato and alfalfa, the PLH can also cause economic damage in soybean and clover.

Despite their small size, the adults are not difficult to find. They are small (1/8"), green, and hold their wings roof-like over their wedge-shaped body. The adults also have short, bristle-like antennae, and have a series of white spots on their back. Adults oviposit 1-6 eggs daily into plant stems, petioles and leaf veins and a total of 60-100 eggs are laid over the female's lifetime. When disturbed, leafhoppers run sideways and adults usually fly away. The major differences between adult and immature potato leafhoppers are the absence of wings and the smaller size of the leafhopper nymphs. Nymphs may also be lighter in color.

Adult leafhoppers migrate northward on prevailing winds from the Gulf of Mexico and typically arrive in Minnesota during May or June. Upon arriving, adults live for about a month and seek out host plants in which to oviposit and feed. Eggs hatch into nymphs after around 10 days and undergo 5 instars over a period of 1-2 weeks. There are likely to be overlapping generations in a field at any one time. Leafhoppers cannot over winter in Minnesota, and this pest is killed with the first frosts of winter (winter’s not all bad…). Each generation takes about a month (depending on temperatures), and there are several generations in Minnesota each season.

Leafhoppers do not typically reach economically significant populations until late June. Feeding damage can also present itself as stunting, cupping or curling of the leaves and slow plant development. Reductions in yield and protein content of alfalfa, as well as slow re-growth of infested stands, can result from high infestations of leafhoppers, suggesting root damage.

Researchers in Wisconsin and Purdue found that the residual effects of PLH damage could be measured for up to two years after a severe infestation. Whether we would experience a similar effect in our part of the state is unknown.

Once you see the hopper burn, most of the damage is done. The best strategy for preventing loss is sampling with a sweep net beginning in June, or directly after the first cutting, and continuing on a weekly basis throughout the summer. Take 10 sweeps from five sites throughout the field, but avoid the field margins. As you walk the field, determine the stem height since control measures depend on plant height.

Potato leafhopper populations are managed by several methods, including biological, cultural and chemical control options. Under the right weather conditions, the problem may take care of itself. Naturally occurring fungal pathogen can cause rapid population reductions during periods of cool, moist conditions.

If infestations exceed thresholds, and the crop is almost ready to be cut, perhaps the simplest strategy is cutting the alfalfa early. After cutting, adult leafhoppers fly out of the field, leaving the nymphs behind, which quickly die without food. With this control method, it’s important to cut the entire field in order to remove sites where leafhoppers can persist until the re-growth returns. After cutting, however, it’s important to monitor the re-growth to determine whether treatment is still necessary. The following factors may also influence treatment decisions: 

• Alfalfa growth conditions. Vigorously growing stands can withstand more damage.
• Value of the alfalfa stand. Young, heavy stands may be worth salvaging as opposed to older, thinning stands.
• Presence of other pests.

If several alternative pest insects are prevalent in the field, damage from potato leafhopper is likely to be compounded, and treatment may be necessary.
Current research is focused on the potential for resistant varieties. The glandular hairs on these varieties have been shown to reduce the number of nymphs. Glandular varieties typically out yield non-resistant varieties when leafhopper populations are high, but glandular varieties are not immune to damage, nor are they always superior in yield and quality.

To test this strategy in NW Minnesota, we began a cooperative project with Clearwater County farmer, Carl “Bunker” Cristofferson, to test a PLH resistant variety in an area with a history of PLH damage. In the study, we will measure PLH presence/population, crop damage, forage yield, forage quality, and stand persistence over the next two years.  

Should an insecticide treatment be warranted, remember that treatment thresholds for potato leafhoppers are dependent on alfalfa stem height:

less than 3" stem = 0.2 leafhoppers/sweep.
4 - 6" stems = 0.5 leafhoppers/sweep. 
7 - 12" stems = 1 leafhopper/sweep.
greater than 12" stems = 2 leafhoppers/sweep. 

Sources: MDA, UM, UWEX. 

Jim Stordahl, AFE—Technical Advisor