Sampling with a Sweep Net

Sampling with a sweep net is commonly used to monitor 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 pest numbers; instead estimate plant damage visually). Sweep net sampling is also used for estimating numbers of lady beetles. A 15-inch diameter sweep net is the standard sampling tool used in alfalfa. The way in which this sweep net is used can greatly influence its effectiveness for collecting insects in alfalfa and, consequently, pesticide application decisions based on the number of insects caught. Therefore, standard methods have been developed for sampling so results are standardized.

To use a sweep net, swing it in a 180º 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 180º 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 five 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 to trap the insects in the bottom of the sweep net. Slowly invert the net bag, while releasing your grasp on the bag allowing the insects to escape and count thenumbers 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 five 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 starting from the field margin and working towards the interior 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 at least four areas 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.

Guidelines for Sweep Net Sampling

	Egyptian alfalfa weevil and alfalfa weevil	Alfalfa caterpillars and armyworms	Leafhoppers
When to start	In late 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 early summer (June) when plants reach adequate height.	In May to September 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. Sample field edges if an adjacent crop infested with leafhoppers is being harvested.
Divide field	4 sections; 5 sweeps per section (20 sweeps total)	4 sections; 5 sweeps per section (20 sweeps total)	4 sections; 5 sweeps per section
Special instructions	Continue to monitor weekly during spring or after a pesticide application: Central Valley through June Southern deserts until March Intermountain areas until mid-June. Keep records on monitoring form (PDF).	Identify type of caterpillar. Count armyworms 0.5 inch or longer. Record the number of healthy and parasitized (pull apart caterpillars and look for parasite larvae) Keep records on monitoring form (PDF).	Count number of adults and nymphs. Be sure to include field edges when sampling. Keep records on monitoring form (PDF).
Treatment thresholds	For sweep net sampling, spray when weevil larval count reaches an average of 20 larvae per sweep.	If cutting is not scheduled soon after monitoring, spray 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	If alfalfa is 2 or more weeks from harvest, apply pesticides if counts reach 5 leafhoppers per sweep (adults and nymphs). For fields scheduled to be harvested in 10 days to 2 weeks, spray if counts reach 10 leafhoppers per sweep.