Don’t let leafhoppers ‘burn’ hay

Potato leafhopper is a tiny insect that can feed on leaves of alfalfa plants, lowering both yield and quality of forage. Leafhopper populations don’t typically build up to damaging levels during the first crop in Iowa. But after you harvest that first cutting, keep an eye out for signs of this pest.

Infestations vary year to year, depending a lot on the weather. “Fields should be monitored weekly after you harvest the first cutting of alfalfa,” advises Steve Barnhart, Iowa State University Extension forage agronomist. “Watch the regrowth. If conditions favor development of this pest, it can also show up after the second cutting and subsequent cuttings.”

Each year, these small, green insects reach Iowa by flying in from overwintering populations in the southern United States. Once established in alfalfa fields, populations can increase rapidly with feeding damage caused by both adults and nymphs. Scouting alfalfa for leafhoppers on a weekly basis can help prevent significant yield and quality losses.

Don’t wait until you begin to see “hopper burn” symptoms before you start using a sweep net to estimate insect numbers, says Barnhart. By then, losses have already begun. Hopper burn is the yellowing of leaves, especially on new growth.

Key Points

Potato leafhopper is a small, bright-green insect that can reduce alfalfa yield.

Feeding on alfalfa leaves can also reduce the quality of forage for harvest.

Alfalfa leaf yellowing or “hopper burn” is the telltale symptom of this pest.

Severe damage causes stunted plants. Stunting and leaf loss result in poor hay quality, reduced tonnage and occasionally the loss of an entire cutting. Long-term losses include reduction in plant vigor, reducing winterhardiness. Stunted alfalfa allows light penetration into the canopy, encouraging weed growth.

Female adults deposit two to three eggs per day in plant stems, explains Erin Hodgson, ISU Extension entomologist. Pale, green nymphs emerge in seven to 10 days depending on the temperature; the fastest development occurs at 86 degrees F. Nymphs look similar to adults except they are smaller and are wingless. Adults are lime green, an eighth inch long, and have a broad, wedge-like head. Adults live for four to seven weeks.

As a result of the extended egg-laying period, at least two overlapping generations occur in Iowa every year. Leafhoppers are active and easily disturbed; adults jump or fly away while nymphs quickly move sideways and backward.

Hodgson recommends the following:

Scouting. Fields should be monitored weekly after the first cutting until the end of the season. A sweep net is the most effective way to sample for nymphs and adults. Fields should be sampled when dry and in calm conditions.

Sweep vigorously through foliage, using a 180-degree motion for one sweep. For each field, stop at four to five locations and take 25 sweeps per location. Count the number of nymphs and adults at each location and estimate the number of potato leafhoppers per sweep for each field. Nymphs will be near the sweep net ring and adults will be at the bottom of the net.

Management. Healthy and vigorous stands are able to better tolerate leafhopper (and other insects) feeding. Heat or drought-stress can make alfalfa more susceptible to insect feeding. Protecting alfalfa from leafhoppers usually involves a three-pronged approach.

The use of glandular-haired, leafhopper-resistant alfalfa varieties can significantly reduce yield losses. Using resistant varieties doesn’t mean fields will be hopper-free, but these plants should be able to tolerate moderate populations compared to conventional varieties. Newly planted resistant fields may not show tolerance immediately, but should express tolerance after becoming established. Consider using leafhopper-resistant varieties if the local area is consistently infested.

The cultural control tactic of cutting management can disrupt leafhopper populations as they develop in alfalfa. Delaying harvest will allow nymphs enough time to become adults and start reproducing. Young nymphs will be destroyed or starve before regrowth occurs. Timely cutting will force adults to move to nearby crops, but they often move back into a field after alfalfa regrowth. It is important to start scouting seven to 10 days after each cutting to monitor for re-infestations.

Insecticide applications can protect yield and are economically justified with regular scouting and the use of action thresholds. The fluctuating values of hay and control costs are important considerations for making a treatment decision.

Leafhopper-resistant alfalfa

Seed companies sell both leafhopper resistant alfalfa varieties and susceptible varieties. Without insecticide treatment, the resistant varieties outperform susceptible alfalfa when leafhopper populations are high. Resistant alfalfa, however, is not immune to damage by leafhoppers. In fact, the presence of leafhoppers in resistant alfalfa is common and does not mean that resistance has failed.

Resistant alfalfa may still show hopper burn and suffer yield loss when leafhopper populations are high. The advantage is resistant alfalfa can tolerate larger leafhopper populations than susceptible varieties before control becomes necessary. Thus, scouting resistant alfalfa is still recommended and requires a slightly different set of decision guidelines for deciding whether or not it will pay to apply an insecticide to control a leafhopper infestation.

Source: ISU Extension

This article published in the May, 2012 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.