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Potato Leafhoppers & Spider Mites in Beans (7/19/12)

Populations of potato leafhoppers and spider mites are increasing in dry edible beans and soybeans in eastern North Dakota.

Fields need to be scouted by sampling at least 5 sites while walking a W-pattern and inspecting about 10 plants per site. Avoid sampling the edges of field.

Potato leafhopperPOTATO LEAFHOPPERS: Potato leafhopper was one of the many migratory insect pests that invaded North Dakota this year. Adults are quite mobile and move from field to field, and may migrate from freshly cut alfalfa fields. Other crops attacked include potato, dry bean and soybean. The small (⅛ inch), pale green, wedge-shaped adults move rapidly by jumping. Nymphs are paler green, lack wings and exhibit a characteristic sideways walk when disturbed. Nymphs can be found on the undersides of leaves.

Damage by leafhoppers is referred to as hopper-burn. Foliage becomes dwarfed, crinkled and curled. Small triangular brown areas appear at the tips of leaves, gradually spreading around the entire leaf margin. Nymphs are generally more damaging than adults, since they feed for several weeks on the leaves where they hatched.

Sweep nets are useful for confirming potato leafhopper presence in a field. Fields need to be scouted to determine whether economic populations are present. Examine the underside of leaves for adult and nymph leafhoppers. Follow these economic thresholds to help make insecticide spray decisions for potato leafhoppers:

Dry bean = 1 leafhopper (either adult or nymph) per trifoliate leaf
Alfalfa
= 1-2 leafhoppers per sweep when alfalfa is 8-14 inches high
Soybean
= 5 leafhoppers per plant in vegetative stage and 9 leafhoppers per plant in early bloom
Potato
= 10-20 adults per 20 sweeps, or 1 nymph per 10 leaves

Two-spotted spider mitesSPIDER MITES: It is not surprising to see spider mites showing up with the hot dry weather. Mites are small and magnification is required to see them. A quick sampling procedure to determine whether mites are present is to hold a piece of white paper below leaves, then beat them to dislodge the mites. The mites appear as tiny dust specks; however, they will move after being knocked off the leaf. Another method is to pull plants and examine the undersides of the leaves for mites and webbing.  Begin from the bottom of plants and move upwards into the canopy. Feeding damage by mites first appears as small yellow spots (stippling). As feeding activity increases, leaves become yellow, bronzed or brown, and eventually shed from the plant. Be sure to scout during full pod (R4) through beginning seed (R5) stages since these crop stages are the most important contributors to soybean yield. Mite infestations typically are first noted near field edges.

Spider Mite Threshold: There is no specific threshold that has been developed for two-spotted spider mite in dry beans or soybeans. Treatment is advised when heavy stippling on lower leaves with some stippling progressing into middle canopy. Mites may be present in middle canopy with scattered colonies in upper canopy. Leaf yellowing is common on lower leaves.

Pest Management: If spider mites are a problem along with leafhoppers, the only pyrethroid that will work is bifenthrin (Tundra, Sniper, Brigade, Fanfare, Bifenture, etc.) in dry beans and soybeans. While other pyrethroids, such as lambda-cyhalothrin (Warrior, Silencer, etc.) will control leafhoppers, they will cause spider mites to flare up and then you may well have to spray again with bifenthrin or an organophosphate (OP) insecticide.

Two active ingredients of OP insecticides for control of leafhoppers and spider mites are chlorpyrifos and dimethoate. However, chlorpyrifos (Lorsban and generics) is NOT registered for foliar application in dry bean. It is only registered as a preplant broadcast or at-plant T-band for control of seed corn maggot in dry beans. However, chlorpyrifos is labeled for both leafhoppers and spider mites in soybeans. Dimethoate will control both leafhoppers and spider mites in dry beans and soybeans, but has a shorter residual than bifenthrin. We think it’s realistic to expect about a 7 to 10 day residual from bifenthrin (if it is hot, residual may be decreased), a 4 to 7 day residual from chlorpyrifos, and a 3 to 5 day residual from dimethoate. It is extremely important to scout and monitor for recurring leafhopper and especially spider mite populations after spraying. Check your fields five days after treatment and again at regular intervals to make sure your control is holding. If newly hatched spider mites are observed after 5 days, a second treatment may be necessary with a different insecticide mode of action. For example, if you use bifenthrin (pyrethroid) for the first application, use a non-pyrethroid product, such as dimethoate or chlorpyrifos (OP), for the 2nd application.

DRY BEANS AND SOYBEANS:

Bifenthrin 2EC product rates:

Leafhoppers = 1.6 to 6.4 fl oz per acre, 14 day PHI
Spider mites = 5.12 to 6.4 fl oz per acre, 14 day PHI
*If you use bifenthrin, use the high rate for spider mite control.

Dimethoate 4E products

Leafhoppers and Spider mites = 0.5 to 1 pint per acre, no PHI (do not feed to livestock)

SOYBEANS ONLY

Bifenthrin 2EC products
Chlorpyrifos products

Leafhoppers = 1 to 2 pts per acre, 28 day PHI
Spider mites = 0.5 to 1 pt per acre, 28 day PHI

Janet J. Knodel, Extension Entomologist

janet.knodel@ndsu.edu

Patrick Beauzay, Research Specialist

patrick.beauzay@ndsu.edu

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