How to Manage Pests

UC Pest Management Guidelines

Sampling with a Sweep Net in Alfalfa


Sampling with a Sweep net

(Reviewed 11/06, updated 11/06)

In this Guideline:

Sampling with a sweep net is a common practice for many alfalfa pests when alfalfa plants are at least 6- to 10-inches tall. (For shorter regrowth, do not rely on sweep net sampling to determine population levels). Sweep net sampling is also used for estimating lady beetle populations. A 15-inch diameter sweep net is the standard sampling tool used in alfalfa. The manner in which this sweep net is used can greatly influence its effectiveness for collecting insects in alfalfa and, consequently, treatment decisions based on the number of insects caught. Therefore, standard methods have been developed for sampling so results from different individuals are comparable.

To use a sweep net, swing it in a 180° arc such that the net rim strikes the top 6 to 8 inches of alfalfa growth. Hold the net slightly less than vertical so the bottom edge strikes the alfalfa before the top edge. This will facilitate getting the insects into the net. Each 180° arc counts as one sweep. A common practice is to take a sweep from right to left, walk a step, and take another sweep, left to right.

After taking the desired 5 sweeps, quickly pull the net through the air to force all insects into the bottom of the net bag and grasp the net bag with a hand at about the mid-point. Slowly invert the net bag while releasing your grasp on the bag allowing the insects to escape and count the numbers of key species. Many slow-moving insects, such as weevil larvae, aphids, and caterpillars can be counted by turning the net onto a white pan or even the hood of a vehicle. Divide totals by 5 to get the average number of insects per sweep. To get a good representation of insect numbers in the alfalfa field, take sweep net samples in four different areas of the field.

If the numbers are so large that counting in the field is difficult, the bag contents can be placed into a plastic or paper bag and the counting done after cooling the sample to slow down the insect movement. Pest management decisions, however, are generally made before such high numbers occur. Collect samples from all portions of the field but avoid unusual parts of the field, such as field edges. The exception to this is when sampling leafhoppers, which tend to be concentrated initially on the field margins. The table below details specific sweeping guidelines for each pest.

  Alfalfa caterpillars and armyworms Egyptian alfalfa weevil and alfalfa weevil Leafhoppers

When to start In early summer (June) when plants reach adequate height. In early January or later, depending on location.

Sweep fields after weevil larvae appear (as evidenced by chewed leaves).

(If plants are too short to sweep, monitor terminals for damage.)

In July to August at the first sign of injury (wedge-shaped leaf burn at the tip of leaves).
How often Twice a week Twice a week Weekly until numbers approach the threshold.
Divide field 4 sections; 5 sweeps/section (20 sweeps total) 4 sections; 5 sweeps/section (20 sweeps total) 4 to 6 sections; 10 sweeps/section
Special instructions Identify type of caterpillar.
  • Count armyworms 0.5 inch or longer.
  • Record the number of healthy and parasitized (pull apart caterpillars and look for a parasite larva)
  • Keep records on monitoring form (100KB, PDF).
Continue to monitor weekly during spring or after a treatment:
  • Central Valley through June
  • Southern deserts until March
  • Intermountain areas until mid-June.
  • Keep records on monitoring form (100KB, PDF).
  • Count number of adults and nymphs.
    • Be sure to include field edges when sampling.
    • Keep records on monitoring form (100KB, PDF).
    Treatment thresholds If cutting is not scheduled soon after monitoring, treat when there is an average of:
    • 10 or more nonparasitized alfalfa caterpillars per sweep
    • 15 or more nonparasitized armyworms per sweep
    • or 10 or more per sweep of both species that are nonparasitized
    For sweep net sampling, treat when weevil larval count reaches an average of 20 larvae/sweep. If alfalfa is 2 or more weeks from harvest, apply treatments if counts reach 5 leafhoppers/sweep (adults and nymphs).

    For fields scheduled to be harvested in 10 days to 2 weeks, treat if counts reach 10 leafhoppers/sweep.


    [UC Peer Reviewed]

    UC IPM Pest Management Guidelines: Alfalfa
    UC ANR Publication 3430

    General Information

    C. G. Summers, Entomology, Kearney Agricultural Center, Parlier

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    Statewide IPM Program, Agriculture and Natural Resources, University of California
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