Leaf hoppers hit white beans & alfalfa crops

By John Madill, Kemptville College Agronomist

Potato leaf hoppers have become a problem on many crops this summer with alfalfa and white beans being hit hard. Threshold levels for spraying were reached in many areas early in July.

Dry edible beans

Potato leaf hoppers do not generally appear on dry beans until after late June when alfalfa has been cut. They may migrate from cut alfalfa fields or arrive in large numbers carried by favourable winds from the south. Females lay their eggs on the main veins and petioles or leaf stems. The life cycle may be as short as two weeks. Since populations can explode in a short time it is most important to monitor fields closely.

Since cultural methods are less effective, resorting to chemicals is the life line in edible beans. It is best to spray when the threshold is reached. Spray thresholds are dependant on the stage of plant maturity. For example, at the first flower stage two leaf hoppers per trifoliate leaf will trigger a spray recommendation. An alternative is to apply Thimet at the time of planting to achieve a systemic control. Leaf hoppers are less of a problem during cool wet weather when naturally occurring fungal pathogens help to control this insect pest. In hot dry weather the fungus is less active allowing the insect populations to explode when plants are growing less quickly.

Leaf hoppers are about an eighth of an inch long and they are pale green wedge shaped insects with wings. Hopper damage causes bean leaves to curl and pucker with the leaf edges eventually scorching. Hopper damage is not a good method of determining the need for control since much of the damage is done before symptoms become visible. Symptoms can also be confused with many other problems. Infestation and damage are intensified by hotter and drier than normal weather.

Alfalfa

Potato leaf hoppers are most severe on seedling alfalfa and on the young second cut of established stands. Most of the damage occurs from late June until mid August. Two varieties, one from each of Pioneer and Pickseed are on the recommended list with resistance using glandular hairs. These hairs exude chemicals that either poison or repel the insects. The hairs are not dense enough to discourage hoppers as in the case of soybeans. Consider using these varieties is in conditions where high insect numbers may be expected or in alfalfa underseeded where you are less likely to use chemical control.

Cultural methods can work well in alfalfa. Early cutting will expose insects to drying conditions. Early cutting will also remove damaged tissue and encourage the plant to grow new tissue to restart the plant. Cool wet weather will encourage a naturally occurring fungal pathogen that attacks the leaf hoppers themselves.

Insect sweeping is the best method of determining the number of hoppers present. As with white beans visual symptoms occur after much of the the damage has occurred. Sweep nets and other insect monitoring equipment is available at the Gempler’s Inc. web site

Be sure to check the chemical recommendations before spraying alfalfa or white beans since the recommended chemicals are not the same and the number of days to harvest differ.