These pests take money from alfalfa growers’ pockets
Alfalfa weevil and potato leafhopper are still primary alfalfa yield robbers around the country, say many state forage specialists and entomologists. Yet clover root curculio, threecornered alfalfa hopper and cutworms are worrying a number of the 17 experts answering the plea to tell what insects to watch for — and how growers can fight them.
Of those reporting, 14 cite alfalfa weevil or potato leafhopper — or both — as problem insects.
In general, experts warn growers to use potato leafhopper-resistant varieties and scout for the pest, cut early and apply insecticide if thresholds are reached. No alfalfa weevil-resistant varieties exist, so scout, cut early if possible and spray as thresholds are reached. For specifics, peruse these reports, organized by region and state.
Alfalfa weevil challenges California's Intermountain growers, causing about a 5% yield loss. Pea aphid and cowpea aphid, more isolated and sporadic, bring about a 1% yield loss, estimates Steve Orloff, University of California's Siskiyou County farm advisor.
Alfalfa weevil hits some fields every year and every few years or rarely in other fields. Typically only a first-cutting pest, on rare occasions it can also damage second cuttings. Cut early or spray with a pyrethroid insecticide. The university's threshold advice: treat when 20 larvae are in a sweep. Depending on the weather, it may be necessary to treat at lower populations. Cutting early can help, but under rare conditions larvae can become concentrated under windrows and damage the following crop's growth.
Monitor fields in spring; if populations reach damaging levels, decide whether to cut or treat depending on the alfalfa's growth stage and how long until it will be ready to cut.
Besides allowing diseases into roots, clover root curculio larvae make drought effects more problematic, says Glenn Shewmaker, University of Idaho extension forage specialist. He estimates 5-10% yield losses. Most fields more than two years old show some damage, and stand life may be shortened by one year. A 10% yield loss could be 0.6-0.8 ton/acre, equaling $120-160/acre for $200 dairy hay.
“It appears to me that the pest is fairly constant over years and will affect most fields,” Shewmaker says. Scout fields and dig up roots to identify damage. To control larvae, adults must be controlled in the previous year or years. He suggests growers rotate out of alfalfa sooner and use its nitrogen credits for non-leguminous crops.
Alfalfa weevil hits Montana's first-cut “money-making” alfalfa in two- or three-cut irrigated systems or dryland one-cut systems, says Dennis Cash, extension forage specialist, Montana State University. Severe infestations often are treated with insecticides early; growers at times ignore published thresholds. Many are anxious for a “silver bullet” to manage the pest. Monitor, use published thresholds and cut early for control, he says. Typical insecticides are Furadan, Mustang and Baythroid. Monitor when growing degree days reach 150. Sweep net larvae threshold is 20/sweep or two larvae/stem.
Besides sporadic outbreaks of cutworms, pea aphid and grasshopper, clover root curculio is causing problems. In a recent survey, sweep samples confirmed curculio adults early and late in the season, but only one generation per year. Incidence and severity of its root-feeding damage rose dramatically from spring of year two until fall of year four. New alfalfa stands treated with Furadan in spring and fall keep damage low, but when treatments end, the pest returns. Adults migrate season-long, so insecticides don't look promising.
Alfalfa weevil — every year in some fields and not in others — and pea and cowpea aphids — extreme outbreaks every 15-20 years — have been alfalfa's biggest pests, says Mylen Bohle, extension agronomist, Oregon State University. Clover root curculio, armyworm, cutworms and grasshopper appear occasionally, especially in eastern Oregon. Grasshoppers can cause 50-75% damage and invade a few years in a row.
Aphids, alfalfa weevil, beet armyworm and other caterpillars are the usual pests that Peter Ellsworth, IPM coordinator for the University of Arizona and Arizona Pest Management Center, sees in alfalfa. Pests that may take growers by surprise are cutworms, threecornered alfalfa hopper, blister beetle and whitefly.
Cutworms live in soil, are difficult to track and don't appear yearly. Know field histories and what neighbors may be contending with to help battle them.
With threecornered alfalfa hopper, Ellsworth says, “We occasionally get grower reports of very heavy densities, particularly in spots in the field. We don't know if they are significant enough to cause economic harm.”
Blister beetle doesn't cause direct economic stand, quantity or quality loss, but can carry toxins that injure or kill animals and hurt a grower's reputation. Watch for it on field edges, near deserts, in weedier stands and in fields left to full bloom.
Small, sucking whiteflies that transfer from other crops leave a sticky mess on hay and lower quality and yield.
Most large operations watch for alfalfa weevil and aphids and get help from beneficials, say Carol Sutherland and Jane Breen Pierce, New Mexico State University entomologists. Some producers make up to four insecticide applications per year, but most use less.
More growers need to be aware of white fringed beetle. Known infestations of this hitchhiker are spotty in southern and eastern counties throughout the state's southern third. Some growers try to control the grub.
Watch for patches of weak plants with thin stems during the growing season, with symptoms similar to flooding, drought, winterkill or other root problems during summer. Plants die variably across fields. Dig up roots and look for surface scarring or tunneling through roots of larger plants. It's easy to diagnose if plump white legless grubs are present. The gray adult is slightly shorter than ½” with a pin-stripe pattern and a grayish-white rim on the forewings. It can be found under hay windrows in September or October.
No products are currently labeled for white fringed beetle in New Mexico alfalfa; those labeled for alfalfa weevil may the best option, with adults being likely targets. Crop rotation is fairly ineffective because of a broad host range, but it may allow for use of insecticides labeled for other crops.
Most growers know of potato leafhopper, but many don't scout and control it when threshold is exceeded. Second- and third-cut yields are most affected and variable from year to year. Scout weekly or more often if threshold is about to be reached. Harvest if at threshold and fields are past mid-bud stage, says Keith Johnson, forage specialist, Purdue University. Use resistant alfalfa if you don't intend to scout or if you want to reduce insecticide cost and use. If the crop isn't ready to be cut, apply insecticide. See recommendations on page 4 of extension.entm.purdue.edu/publications/E-220.pdf. For economic thresholds, see page 6 of extension.entm.purdue.edu/publications/E-220.pdf.
Aware of leafhopper, aphids and weevil, growers pay less attention to grasshopper and cutworms, says Michael Rethwisch, University of Nebraska extension entomologist.
Cutworms, difficult to sweep net, are in most fields; high infestations delay regrowth by two weeks. That, following a late-May/early June cutting, makes for a loss of 1.25+ tons/acre at an estimated $100/acre. Losses are geographically sporadic, and the entire field is not always affected. Cutworms are active every year but damage varies by year, field and cutting. Pyrethroid insecticides have been effective. Threshold is four worms per square foot for established alfalfa, two per square foot for seedling alfalfa.
Grasshopper causes 5-10% or more in yield loss when large areas are infested, but less than 1% most years. This past year, some summer fields lost ¼- to ½-ton/acre yield from the pest, equaling $20-40/acre with current prices. It is a widespread pest once every few years, but can affect more than one cutting in a year. Pyrethroids are effective; no economic thresholds have been set. Grasshopper can be controlled by treating field borders, but sometimes entire fields need treating.
Potato leafhopper probably causes more unseen damage than anything else. Most growers don't scout but should, say Mark Sulc and Mark Loux, Ohio State University extension forage specialist and extension weed specialist, respectively, and Ron Hammond, entomologist, Ohio Ag Research and Development Center.
Leafhopper reaches damaging levels each year in second- and possibly third-crop alfalfa. Uncontrolled, it reduces annual yield by up to 25%. Determine threshold with a 1:1 ratio of number of leafhopper adults and nymphs per 10-sweep sample to height of alfalfa. Threshold for 6” alfalfa is six hoppers (total of adults + nymphs) in 10 sweeps. For resistant varieties, raise threshold by three times; in resistant 6” alfalfa, it's 18 hoppers.
Alfalfa weevil injury is noticeable, but growers do little scouting and usually see feeding on tips after damage occurs. Infestations depend on weather and vary by year and location. They reach economically damaging thresholds, on average, one in five years, causing yield losses of 40-60% by essentially destroying first crop. Pyrethroids can give long residual control. See entomology.unl.edu/instabls/alfwlec.htm.
Potato leafhopper is economically more damaging, but clover root curculio probably infests the largest area and growers have the least knowledge of it, says Dan Undersander, University of Wisconsin extension forage specialist.
Leafhopper reduces yield by 30% in second and third cuttings each year in 50% of southern Wisconsin and 30% of central Wisconsin. Use resistant varieties and spray as needed. Threshold is one per inch of plant height per 10 sweeps for leafhopper-susceptible varieties and three times higher for resistant varieties.
Clover root curculio causes root damage that allows other diseases to invade. It occurs over about 15% of Wisconsin and, once present, is there for the life of the stand. No control is known other than rotation, he says.
Connecticut doesn't have many pure alfalfa stands and insects aren't usually a problem, says Richard Meinert, University of Connecticut extension educator. Occasionally, potato leafhopper damages second cuttings if winds come from the south rather than the west.
Potato leafhopper appears each year, says Les Vough, University of Maryland extension forage specialist emeritus. Yield losses can total 1 ton/acre, including carryover effects on subsequent cuttings. The pest primarily hits third and fourth cuttings in a five-cut system.
Resistant varieties and insecticides offer control. For recommendations, see extension.umd.edu/publica tions/EB237online/InsectManagement/InsectsAlfalfa/PotatoLeafhopperRecInst.cfm.
Potato leafhopper causes losses of at least $15 million/year, usually in second and third cuttings, says John Tooker, entomologist with Penn State University. Plant resistant varieties, scout in early to mid-June, and harvest early if some plants are stunted and yellow, if greater than 60% of plants are in the bud stage, or it's been more than 28 days since the last cutting. For IPM information, see www.ento.psu.edu/extension/factsheets/potatoLeafhopperAlfalfa.htm.
Alfalfa weevil can be less severe, usually in first cuttings. One larva in thirty 12”-tall plants can cut yield by 3 lbs/acre. For alfalfa selling for $160/ton (based on late-November 2008 local auctions), the economic threshold is around 50 larvae per 30 plant stems. Weevil damage decreases with plant height; therefore, one larva in thirty 16”-tall plants translates to a loss of 0.75 lb/acre. To determine when to scout, see cas.psu.edu/spotlight/pa-pipe.html. Once plants reach 16”, harvest rather than chemically treat.
Should either pest exceed economic threshold, consider an insecticide. For options and economic thresholds, see agguide.agronomy.psu.edu/.
Potato leafhopper is Vermont's primary alfalfa pest. In areas hardest hit, leafhopper-resistant varieties are planted and few growers apply insecticides, says Sid Bosworth, extension agronomist, University of Vermont.
Look out for the alfalfa snout beetle, a root-feeding insect that wiped out alfalfa in northern New York 20 years ago. Recently reported in a county just across the northwestern border of Vermont, the beetle takes out stands quickly. Symptoms may look like winterkill.
Alfalfa weevil can be found each year in every alfalfa field. But watch for a “sneaky” pest that can cause losses once very few years — the threecornered alfalfa hopper, says Kathy Flanders, extension entomologist, Auburn University.
Scout from June to September and apply insecticide when the hopper is girdling 10% of lateral stems in fields — about three hoppers per plant. For insecticide recommendations, see www.aces.edu/pubs/docs/A/ANR-0500-A. The hopper overwinters in pine forests, reinfests alfalfa each spring and damages later cuttings. See insects.tamu.edu/fieldguide/aimg84.html and wiki.bugwood.org/Threecornered_alfalfa_hopper for more information.
Alfalfa weevil is in most fields most years; it caused losses of $347,000 in 2006, says David Buntin, grain and forage crop entomologist at the University of Georgia. More sporatic pests are the potato leafhopper, aphids and threecornered alfalfa hopper. Losses from these pests vary but they usually cause less than $40,000 in damage state-wide in any year.
Early cutting to control alfalfa weevil isn't an option; at least one insecticide application is needed in most established fields each year.
Few varieties adapted to the Deep South have potato leafhopper resistance, and pyrethroid insecticides and dimethoate are recommended. Some varieties have pea aphid and spotted alfalfa aphid resistance. Early cutting can control these pests if alfalfa is within a week of cutting. If spotted alfalfa aphids appear in seedling stands, treat. Control with dimethoate and cyhalothrin products. Threecornered alfalfa hopper usually isn't treated in Georgia.
For Buntin's detailed report, click here.
Alfalfa weevil and potato leafhopper are Kentucky's key pests, says Lee Townsend, extension entomologist, University of Kentucky.
“Growers are less aware of the potato leafhopper because of its small size and subtle damage symptoms as well as the need of a sweep net to sample for them accurately,” he says.
Greatest losses occur in spring-seeded stands, with stunting and reduced vigor in most fields each year. Fall-seeded fields can be damaged as well as established fields on second and/or third cutting(s) if cutting interval extends beyond 35 days. Harvest at 30-35 days, if possible. Sweep net before that, especially with new seedings. Use pyrethroid insecticides if leafhoppers exceed thresholds. See www.ca.uky.edu/entomology/entfacts/ef115.asp for treatment guidelines.
Growers here battle alfalfa weevil in early spring and occasionally see outbreaks of potato leafhopper in mid-summer, says John Andrae, forage specialist at Clemson University. For management information, see entweb.clemson.edu/cuentres/cesheets/forage/ce163.htm.
Potato leafhopper is a problem every few years, but not consistently enough to spray, says Gary Bates, forage specialist, University of Tennessee. Alfalfa weevil can cause 20-30% yield loss or more if untreated and primarily hits first cuttings. Various insecticides work; spray when about 30-50% of plants show some weevil damage.