Ohio State University Fact Sheet

Entomology

1991 Kenny Road, Columbus, OH 43210-1000

Potato Leafhopper on Alfalfa

FC-ENT-0033-00

H. R. Willson
Associate Professor
Department of Entomology

J. B. Eisley
Research Associate
Department of Entomology

R.M. Sulc
Associate Professor
Department of Horticulture and Crop Science

Pest Biology and Host Injury

The potato leafhopper (PLH), Empoasca fabae L., is a very small, bright green leafhopper. It lives year round in the Gulf Coast region and is carried north each year via weather fronts. In Ohio, PLH begins to appear in alfalfa when 1st cutting of alfalfa is near harvest. The 1st cutting is not affected, but subsequent cuttings during the summer can be severely impacted by this pest. The life cycle of PLH includes the egg, five nymph stages, and the adult. About three weeks are required for PLH development from egg to adult.

PLH is a sucking insect. As leafhoppers insert their piercing-sucking mouth parts into plant tissue, the combination of a pulverizing feeding motion and injection of saliva blocks the normal flow of nutrients in the plant. Initial damage to alfalfa is exhibited in the form of wedge-shaped yellowing of leaf tips which is commonly called hopper burn and may be confused with boron deficiency symptoms. More important than the hopperburn symptom, is the stunting of plants, resulting in yield loss.

Figure 1. Hopper-burn symptom.		Figure 2. Adult potato leafhopper.

Heavy PLH infestations may completely stunt alfalfa stand development-especially early development of new stands and regrowth following harvest. Excessive stress on alfalfa due to PLH feeding activity may reduce yield of both the current crop and following crops. Yield reductions of 20 to 25% of the 2nd or 3rd cuttings due to PLH feeding are common in Ohio alfalfa production. In addition, stunting and slower regrowth of damaged alfalfa results in a less competitive crop, leading to weed invasion.

Harvest of an alfalfa crop will temporarily eliminate the presence of PLH nymphs and cause the dispersal of PLH adults from an alfalfa stand. About one to two weeks following harvest, regrowth will attract PLH adults, and nymphs appear soon thereafter. If an alfalfa stand is allowed to grow for more than three weeks, PLH nymphs mature to the adult stage and the numbers of PLH adults increase significantly. PLH adult numbers may increase at any time due to immigration from neighboring or distant sources such as late harvested hay fields. PLH activity on Ohio alfalfa tends to peak in July and decline in August. If weather conditions are favorable, PLH populations remain abundant until early September. If weather conditions are unfavorable for PLH development, PLH activity may decline-especially when damp weather favors development of a fungal pathogen that will decimate a PLH population.

Figure 3. Non-PLH resistant alfalfa adjacent to more dense growth of PLH resistant alfalfa.

Prevention of Yield Reduction Due to PLH Feeding

Losses in alfalfa yield due to PLH feeding activity can be reduced by (1) timely implementation of harvests, (2) growing of PLH resistant varieties, and (3) application of insecticide treatments when warranted.

Delayed harvesting of alfalfa stands encourages multiplication of PLH populations since PLH nymphs are able to reach the adult stage and reproduce. A timely harvest schedule will reduce PLH abundance. There is more opportunity for PLH to multiply to high levels under a lax 3-cut schedule than a 4-cut per season schedule.

Use of PLH resistant alfalfa varieties significantly reduces yield losses due to PLH feeding activity. The presence of glandular-hairs on PLH resistant alfalfa reduces PLH feeding and nymph development. The efficacy of PLH resistant varieties to tolerate PLH infestations has been improving since the introduction of PLH resistant varieties in 1996.

Under conditions of moderate to heavy PLH infestation, yields of PLH resistant alfalfa may be optimized by application of insecticides-especially during the season of stand establishment. However, yields of PLH resistant alfalfa will be higher than untreated non-PLH resistant alfalfa when significant infestations of PLH occur.

The timely application of foliar insecticide treatments to reduce PLH activity will optimize alfalfa yields of both PLH resistant and nonresistant alfalfa. Reduction of PLH activity by insecticide treatment is justified when assessment of PLH population abundance by sweep net sampling demonstrates the presence of economic levels of PLH activity.

Assessment of PLH population abundance requires the use of a standard sweep net, which has a 16-inch rim, a heavy cloth or synthetic net, and a 3-foot handle. When sampling alfalfa, 10 pendulum sweeps should be taken from each of three to five locations in a field depending on field size and topography. After taking each 10-sweep sample, contents of the catch should be inspected and the number of PLH adults and nymphs counted. Nymphs tend to collect near the rim of the net when conditions are damp. The presence of other alfalfa pests should be noted.

In the case of non-PLH-resistant alfalfa, corrective action is warranted when the number of PLH adults plus nymphs collected per 10 sweeps is greater than the stem height of alfalfa expressed in inches (e.g., a catch greater than 8 PLH per 10 sweeps of alfalfa having a stand height of 8 inches indicates a need for rescue treatment).

Treatment decisions may be adjusted based on (1) environmental conditions at the time of sampling (time of day, wind, etc.), (2) alfalfa growing conditions (vigor or stress), (3) relative abundance of other alfalfa pest populations, and (4) relative value of the crop. Adverse environmental conditions such as wet foliage or wind at the time of sampling may reduce the catch of PLH in a sweep net. In general, alfalfa should be sampled when a field is dry and wind conditions are calm.

A dominant factor governing the impact of PLH on alfalfa is the condition of the host crop. Vigorous growth may enable alfalfa to tolerate PLH feeding activity. In contrast, the presence of environmental stress due to heat or drought may make alfalfa more susceptible to PLH feeding. Thus, the economic threshold may be doubled or cut in half depending on the condition of the host crop.

Figure 4. Sweep net sampling alfalfa for PLH. Note pendulum action of sweeping.

The presence of other pests may be a factor in marginal situations. A problem may develop where a 2nd generation of alfalfa weevil, alfalfa plant bug, or aphids is present and the combination of two pest problems justifies treatment when treatment for only PLH may be in question.

The economic value of an alfalfa stand is also a key factor in a treatment decision. Treatment of a 1st or 2nd year stand of alfalfa may be justified, whereas treatment of an older alfalfa stand may not be warranted unless the PLH infestation is very heavy. In the case of a new alfalfa seeding, treatment may be warranted for marginal infestations of PLH to insure stand establishment.

Application of a foliar treatment to prevent yield losses in PLH resistant alfalfa may be warranted under conditions of abundant PLH activity. Research has demonstrated that severe PLH infestations of 5 to 10 PLH per sweep can significantly reduce yields of PLH resistant alfalfa-especially during the year of stand establishment.

The use of PLH resistant alfalfa is regarded as an alternative to the use of foliar treatments since PLH resistant alfalfa is less susceptible to PLH injury. However, consideration should be given to the need for insecticide treatment of PLH resistant alfalfa when PLH are abundant during the period of stand establishment. Where treatment of first year PLH resistant alfalfa is considered, the action thresholds used on nonresistant alfalfa should be increased about twofold.

A number of insecticide products are labeled for control of PLH on alfalfa. New products on the market often achieve a higher level of PLH control than older products. Although differences in PLH control among products have been demonstrated, yield loss reduction achieved by PLH control is usually obtained by timely treatment of a PLH infestation regardless of the product applied. Thus, the important factor in PLH loss prevention is timely treatment, which depends on a close monitoring of the abundance of PLH activity on an alfalfa crop.

Acknowledgments

R. B. Hammond, Associate Professor
Department of Entomology, Ohio Agricultural Research and Development Center, The Ohio State University
1680 Madison, Wooster, OH 44691