Introduction

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Field Scouting and Insect Management Charts
Field Scouting in Alfalfa
Sap Or Fluid Feeders
• Lygus bugs/Alfalfa plant bug
º Typical Damage: Field blooms poorly or not at all. Flower buds blasted, whitish, and dry; flowers dropping off
before fully open. Collapsed seed.
º When and How to Monitor: Look for plant bugs when monitoring alfalfa in June through mid-August. Make five
180º sweeps with a 15-inch (40 cm) insect net through alfalfa canopy at each sampling site. Record total number of
plant and lygus bugs (both nymphs and adults) captured. Calculate average number per sweep.
º Economic Threshold: Hay: Control not recommended. Seed alfalfa at bud and early bloom: 8 lygus bugs/sweep;
4 alfalfa plant bugs/sweep; or 5 bugs if the plant bug population is a combination of lygus bugs and alfalfa plant
bug. If insecticides are used, attempt to spray before the onset of bloom. Protecting insect pollinators in seed
production fields is very important.
• Potato Leafhopper
º Leafhoppers are most severe in new seedings and in regrowth under hot dry weather.
º When and How to Monitor: Take 20 180º sweeps from 5 areas of the field. Avoid field edges. Determine the average
number of potato leafhoppers per sweep.
º Economic Threshold: For 9 cm stem height = 0.2 adult leafhoppers per sweep; 15 cm stem height = 0.5 adults per
sweep; 25 cm stem height = 1 adult or nymph per sweep; 36 cm stem height = 2 adults or nymphs per sweep.
• Pea Aphid
º Typical Damage: Suck juices from plants; stunt growth; cause premature drying.
º When and How to Monitor: Look for when monitoring in July through August. Take 5 sweeps at each location.
Monitor fields closely during periods of slow plant growth.
º Economic Threshold: 100-200 aphids/180° sweep when crop is moisture stressed, or until mid-August.
Defoliators
• Alfalfa Weevil
º Typical Damage: Feed on developing buds and leaves. Stunt growth.
º When and How to Monitor: Start scouting fields in mid-May. Look for shot holes initially, then clipping along the
edges of leaves and pinhole damage. For determining if levels are at threshold in hay crops, collect 30 stems in
an M-shaped pattern, place them inside a white pail and beat them against the side to knock off larvae. Do not
include younger first and second instar larvae (3 mm or less) in the counts. Determine the average height of the
crop as well.
º Economic Threshold:
º Alfalfa Hay: One of the best control strategies is to cut fields for hay early. If early cutting of the hay crop is not
possible, treatment thresholds are based on the following measurements of plant height and levels of larvae: