Alfalfa insects: What to look for and how to scout

Alfalfa insect populations and the pest species present will vary with weather and season.

For example, the migrant potato leafhopper is Minnesota alfalfa's most consistent insect pest, but rarely reaches damaging levels before the first cutting. The insects that most often cause first crop concerns in Minnesota include alfalfa weevil, pea aphid and plant bugs.

A 15-inch diameter, heavy-duty sweep net is a necessary tool for scouting insects in alfalfa.

Sweep nets quantify insect numbers when using economic thresholds for several key alfalfa insects and for initially detecting most alfalfa insects. The sweep net allows you to find insect pest populations early, before yield loss or obvious symptoms happen.

You can order sweep nets from several online suppliers. When purchasing a sweep net, consider the following:

1. Avoid the lightweight aerial nets designed to catch butterflies.

2. Specify heavy-duty.

3. A long-handled net is better. Marking the handle with inch increments will allow you to measure crop height with the same tool.

4. Aluminum handles are durable and light, but metal can rub off on your hands.

5. Purchase a net that you can get purchase replacement parts for. If your net doesn't wear out, you're not using it right.

Other important tools for alfalfa scouting are:

• A 10X to 15X hand lens

• Containers for saving specimens for later identification

• Identification guides

It pays to scout and base any insecticide applications on economic thresholds.

Although insurance insecticide applications might seem easier, they don’t always save crop yield but often waste crop input dollars.

Insurance insecticides might make your alfalfa insect problems worse because:

• Insecticides kill beneficial insects.
• Lost biological control could cause other insect problems to flare up.
• Overuse can lead to insecticide-resistant insect populations.

Always read and follow the insecticide label. Many insecticides are harmful to honey bees. Avoid applications to blooming alfalfa or when honey bees are active. If you must treat flowering alfalfa, notify beekeepers in the area.

Online suppliers

• BioQuip Products

• Ben Meadows

• Forestry Suppliers

• Great Lakes IPM, Inc.

• Gempler's

Correct identification and efficient sampling methods for beneficial and pest insects are two critical steps toward implementing an integrated pest management (IPM) program. IPM includes using all feasible control tactics (crop rotation, resistant hybrids and chemical control) to manage pests within a profitable, yet environmentally sound production system.

Accurate and timely evaluation of insect population (infestation) levels requires efficient sampling methods. Use estimates to compare with economic thresholds and when considering appropriate management tactics.

Insect mortality can result from:

• Starvation
• Desiccation
• Exposure (conditions that often occur when alfalfa is cut)
• Predators
• Parasites
• Disease
• Insecticide applications

Most insects are more vulnerable to these factors when they’re young.

The lowest labeled rates of insecticides can often provide effective control when treating young (immature) insects. This is why early detection is advantageous. Nevertheless, control action is still only appropriate when yield and quality savings justify control costs.

For high-yielding, high-quality alfalfa, check the crop weekly. In cooler weather, you can sample less frequently.

When the weather’s hot, increase sampling frequency. Insects develop, feed and reproduce faster under warmer conditions. You also need shorter sampling intervals when insect populations and/or damage approach economically damaging levels.

The scouting schedules on tables 3 through 6 summarize when to direct sampling toward particular insect pests.

Three sampling methods are recommended in Minnesota for scouting alfalfa insects:

1. Sweep net sampling
2. Stem sampling
3. Ground area sampling (e.g., square foot, yard or meter)

You need samples from several different locations in a field. This ensures sampled insect populations are representative of insects in the field as a whole. Five locations are usually advised, but more may be necessary in fields with a wide range of soil types, crop maturities, slopes, etc.

Choose sampling locations in proportion to the relative area in the field of each condition. Figure 1 illustrates one way to get a representative sample.

Sweep net sampling

This is the most convenient method of estimating many of the pest and beneficial insect populations in alfalfa. Although it provides only a relative estimate of insect density, it’s sufficient and cost-effective for most aboveground insect pests of Minnesota alfalfa.

Sweep net sampling isn’t recommended for determining absolute estimates of alfalfa weevil, cutworm, armyworm, grasshopper or aphid populations. However, sweep nets often are useful for detecting initial low-density populations of alfalfa weevil larvae and pea aphids.

What you need

You need a 15-inch-diameter, heavy-duty sweep net if the results are to be comparable with the provided thresholds. In Minnesota, threshold data is expressed in numbers per pendulum sweep.

How to sample

Swing the net from side to side on a more-or-less vertical axis sweeping down into the alfalfa, first in one direction and then the other (two sweeps). It’s important to have the net as low to the ground as practical (or the canopy’s top two-thirds in tall alfalfa) and to walk briskly when sampling.

Some workers use 180-degree sweeps, which are more horizontal and wider, but most thresholds used in the Midwest are based on pendulum sweeps.

After 10 or 20 pendulum sweeps, quickly shake the net contents to the bottom of the bag. Firmly grasp the net bag above the sample to prevent escape. Then, empty the sample into a plastic bag for later identification and counting, or count them in the field.

You can also count insects in the net. To do this, allow insects to escape very slowly, as you gradually open the base of the net. It’s useful to carry small vials to collect insects requiring further identification.

Stem sampling

Use stem sampling to monitor tip injury in alfalfa, and get more accurate estimates of alfalfa weevil and pea aphid abundance than sweep samples.

The only tool you need for this method is a carton or bucket to place stems in, or a tray to shake the aphids or larvae from the stems onto.

Ground area sampling

Estimate grasshoppers as the number per square yard, and cutworms, armyworms and post-harvest alfalfa weevils as the number per square foot.

A 1-square-foot sampling frame is useful for square-foot sampling of insects and measuring alfalfa stand density. It should be made of heavy wire, plastic or wood, and painted a bright color to reduce the likelihood of losing it.

Weather affects sampling efficiency

Don’t take sweep samples when the alfalfa is wet, or under extremely windy conditions; the results will be meaningless.

Sampling in still air is preferable to sampling in a light breeze but often isn’t possible. Wind reduces the efficiency of sweep-sampling adult potato leafhoppers (PLH). Sampling in winds of more than 10 miles per hour isn’t advised.

Insects are more active and, in turn, more visible and catchable in warm weather. Cool conditions reduce the efficiency of sweep sampling adult PLH. Other insects are similarly affected; consider this when interpreting the number of insects caught.

Spring growth

• Estimate the percentage of terminals injured by alfalfa weevil and check for live larvae.
• Do a sweep net sample for pea aphid. If abundant, do a stem sample.
• See if unusual numbers of spittlebug nymphs, plant bugs, grasshoppers, cutworms or other occasional pests are present.

Second and third growth

• Check stubble to see if alfalfa weevil larvae are feeding on and retarding growth.
• Do a sweep net sample for potato leafhopper.
• Do a stem sample for pea aphid two weeks before harvest if large numbers are detected in sweep net samples.
• See if there are unusual numbers of cutworms, plant bugs, grasshoppers or other occasional pests present.

Fourth growth (if applicable)

• Do a sweep net sample for potato leafhopper.
• Do a stem sample for pea aphid if large numbers are detected in sweep net samples and alfalfa is drought-stressed.
• Observe if alfalfa weevil adults or larvae are present, or unusual numbers of cutworms, plant bugs, grasshoppers or other pests.

Newly seeded alfalfa

• Intensify all surveys because seedling stands are more susceptible to injury than established stands.
• Minnesota research indicates that in some years, you may need up to two insecticide treatments to prevent yield and quality losses to PLH on spring-seeded alfalfa during the establishment year.
• Good PLH management during the first year may also be important for the stand’s long-term health and productivity.

When

Potato leafhopper (PLH) migrates into the state each spring (May 1 to June 15). Consequently, only monitor the second and third cuttings or recently established stands.

You can conclude PLH monitoring seven to 10 days before harvest, as the PLH causes little injury to more mature growth. Closely check the first growth of spring-planted seedling alfalfa for PLH, as it’s much more sensitive to PLH injury.

How

Research from Cornell University indicates that, on average, three to five sets of 10 sweeps each will provide accurate estimates of PLH density. In very early regrowth, when it’s difficult to standardize sweeping efficiency, take at least four 10-sweep samples per field.

Tables 3 through 6 show the sequential sampling plan for PLH. As indicated, you must take a minimum of three 10-sweep samples (Figure 1). The plan is very useful when PLH populations are low or high, with an average time savings of 50 percent compared to a traditional 100-sweep sample (ten 10-sweep samples).

Like our traditional thresholds for PLH (Table 7), the sequential plan accounts for differences in plant height.

When using the plan, if the total (cumulative) number of PLHs you catch falls in the “continue sampling” column, move to at least one more site to take an additional sample before making a treatment decision. Whenever the total number of PLHs falls in the “don’t treat” categories, you can stop sampling and advise the producer accordingly.

Weather

Cool conditions reduce the efficiency of sampling adult PLH. At 60 degrees Fahrenheit, the catch is 56 percent of that at 77 degrees Fahrenheit. Table 8 shows the wind’s effect on the efficiency of sweep-sampling adult PLH.

Also, 180-degree sweeps are much more severely affected by wind. Use pendulum sweeps, and don’t sweep-sample when winds are more than 10 miles per hour.

When sampling in a light wind, a small, hand-held anemometer will enable you to adjust catch figures upward according to wind speed.

What to count

Sweep sampling doesn’t sample potato leafhopper nymphs as effectively as adults, but there’s currently no more convenient method for commercial use.

Open the bag carefully, when examining the sweep net contents, as many adult leafhoppers will readily escape. Total the numbers of both adults and nymphs to determine the average number of leafhoppers per sweep. Place more emphasis on samples taken during the 3- to 7- and 8- to 12-inch plant growth stages.

When

Check for weevils before harvesting the first crop and post-harvest, on the stubble after the first cutting. The earliest larvae tend to be found on south-facing slopes, as early as May 1 in years with an early spring.

Generally, larvae are rare or absent by the time of the second cut. Some larvae may hatch from fall-laid eggs, but these rarely reach economic levels.

How

Different parts of the United States use a variety of methods to determine whether economically damaging levels of alfalfa weevil are present.

Pre-harvest

The Illinois method, which measures and integrates crop growth stage, intensity of weevil infestation and progress of the infestation is the most accurate but hasn’t yet been calibrated for Minnesota conditions. Meanwhile, monitoring tip injury is often recommended as a pre-harvest method.

This method is relatively simple and appears to be adequate for management purposes. It provides a good rule-of-thumb estimate for the alfalfa weevil’s pre-harvest damage potential.

1. Collect 50 to 100 alfalfa stems (10 to 20 randomly selected stems from five locations).

2. Examine whether they show obvious feeding damage – pinhole or more severe feeding – in the rapidly growing tip leaves and leaf buds.

3. Divide the number of stems with recent tip injury by the total stems collected to convert it to a percent.

4. Compare it with the below threshold.

Alfalfa weevil thresholds

• Before first cutting: 35 percent (weak stand) plants with feeding damage. 40 percent (vigorous stand) plants with feeding damage or two live larvae per stem.

• After first cutting: 8 or more larvae per square foot, (6 per square foot on sandy soil), or larvae are suppressing regrowth.

Don’t confuse the percentage of tips showing feeding damage with the percentage of stems defoliated.

Post-harvest

For post-harvest, monitor regrowth and potential stubble infestation. After the hay has been picked up, search the stubble and early regrowth in 20 samples (four randomly chosen, 1-square-foot samples from each of five locations).

Because harvesting often concentrates weevil larvae in the windrow areas of the field, it may be useful to sample first (or only) from windrows. When regrowth after harvest is sufficiently long to make square-foot counts too difficult, revert to monitoring tip injury.

When

The immature nymphs and subsequent adults arise from overwintered eggs (northern states). Early spring populations are often heavily parasitized by braconid wasps.

Generally, it’s not necessary to monitor or control pea aphids earlier than two to three weeks before alfalfa harvest. Aphids usually are maintained below economically damaging levels by natural enemies.

Locally higher densities are generally reduced by predators, parasites and disease. However, under dry, warm conditions, pea aphid infestations may rapidly increase. Control may be necessary if aphids are abundant on seedling alfalfa or early regrowth.

Alfalfa can tolerate fairly high pea aphid populations. You don’t need to take stem counts until about a quarter cup of aphids are found per 20-sweep sample.

How

The simplest and most accurate method of taking actual counts of pea aphids is to estimate numbers per stem. This is similar to stem sampling for monitoring tip injury due to alfalfa weevil.

Shake six to 10 stems (randomly selected from each of five field locations) in a carton or over a tray, and calculate the average number of aphids per stem. Very carefully cut and handle the stems because pea aphids readily fall from the plant when disturbed.

Three species attack Minnesota alfalfa: tarnished plant bug, alfalfa plant bug and rapid plant bug. The tarnished and alfalfa plant bugs are the more consistent pests.

Both the immature nymphs and adults (wings) suck plant juices from the stems, leaves and flower buds. Subsequent damage may be characterized by crumpled leaves (feeding notches from either side of the leaves), stunted plants or aborted flower buds.

When

Plant bugs overwinter as adults. Significant nymphal populations usually aren’t present until late May to early June, as the spring crop approaches maturity. Populations are usually highest on the second and third regrowth cycles.

How

Although plant bugs are often considered significant pests in seed production fields, research from the University of Wisconsin suggests forage yields may also be reduced via the bugs’ direct feeding on vegetative growth.

Action thresholds resulting from this research range from three to five bugs per pendulum sweep (nymphs and adults of both species combined). If the alfalfa plant bug is the dominant species, lower the threshold to two to three per sweep.

Check for plant bugs when sampling potato leafhopper. For plant bugs, four to five sets of 10 sweeps each should provide a representative sample.