Scouting For Potato Leafhopper in Alfalfa

Potato Leafhopper (PLH) is one of the most damaging alfalfa insect pests in Ontario. While PLH is seldom a problem on the first-cutting of established alfalfa, the insect can start damaging new seedings by mid-June. The damage done by PLH is not as dramatic as that of an alfalfa weevil outbreak, but is more chronic and widespread. The injury often goes undetected and affected plants do not recover until the stems have been harvested. New seedings are particularly susceptible to PLH damage.

Leafhoppers usually arrive in Ontario in late May. The adult is one-eighth of an inch (3 mm) long, lime green and wedge-shaped. The juvenile leafhoppers or nymphs are about 0.8 mm (1/32nd of an inch) long when they hatch. They resemble adults, except they are wingless and are often found on the underside of the leaves. Both the adults and nymphs feed on alfalfa. They insert their mouthparts into a leaf midrib and suck sap juices from the plant. PLH inject saliva containing a toxin into the plants as they feed. This distorts and blocks the tiny tubes that distribute nutrients within the plant. The result is the characteristic "hopperburn", which starts as a wedge-shaped yellowish pattern on the leaf tips. The alfalfa will be stunted and may also turn reddish.

Economic losses occur before plant symptoms develop, so it is important to identify the presence of large leafhopper populations before the damage occurs. If yellowing has already occurred, it is too late to prevent damage this cutting. Scouting with a sweep net will help you determine whether early harvest or spraying is needed. In new spring seedings, start scouting fields when the plants are 10 to 15 cm (4-6 inches) high. Scout at intervals of 5 to 7 days. In established stands begin scouting after first cut when plant regrowth is 5 to 10 cm (2 to 4 inches) high.

To determine the number of leafhoppers, including adults and nymphs, take 10 sweeps and divide by 10. Do this in 5 representative areas of the field and note the height of the alfalfa. Recommended action thresholds are as follows:

Recommended Action Thresholds

Refer to OMAFRA Publication 812, "Field Crop Protection Guide" and product labels for insecticide information. Keep in mind that there is a "days to harvest after application" to consider, ranging from 2 to 21 days.

Related Links

• OMAFRA Publication 812, "Field Crop Protection Guide

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