Alfalfa (Lucerne) Pests and their Management

Alfalfa (lucerne), Medicago sativae, is one of the most important legumes used in agriculture. It is the principal roughage for ruminants, as well as being an important source of protein in animal diets. It is surpassed only by grass, corn, and soybean as an animal feed, and is especially important to the dairy industry. The USA is the world’s largest producer of alfalfa, but it also is an important crop in Australia, Europe, Argentina, China, South Africa, and the Middle East.

There are other uses for alfalfa, though they are minor. Alfalfa sprouts are a salad ingredient, alfalfa shoots are sometimes consumed as a leafy vegetable, and dehydrated alfalfa is sometimes formulated as a tablet to be consumed as a dietary supplement. Alfalfa is a cross-pollinated species. It relies on insects, often domesticated leafcutting bees, honey bees, alkali bees, and various wild bees, for pollination. Wind pollination does not occur because the blossom is structured in a way that physical “tripping” to expose the stigma to the anthers is required. Bees manipulate the blossom when foraging for nectar and pollen and thereby “trip” the blossom, an action that results in the bee being struck in the head. An interesting aspect of pollination is that some bees learn to avoid the tripping process to avoid being struck, thereby robbing the flower without pollination occurring. Older honey bees are good at avoiding tripping, but naïve young honey bees trip the blossom and provide pollination.

Alfalfa is normally harvested before, or at, the initiation of flowering, which maximizes protein content of the harvested hay. Because pollinators are often present in alfalfa fields during the bloom period, care must be taken when using insecticides for pest suppression to avoid products that are highly toxic to pollinators, at least if seed production is a concern. However, most alfalfa is grown only for forage, and without regard for seed production. Thus, insecticide use may include the bloom period, though if pollinator populations are reduced, other crops that require pollination may be inadvertently affected.

Alfalfa is unusual as a field crop in that it is a short-lived perennial, living 3–12 years. It may be harvested from once to 12 times per year, depending on climate and growing conditions. It has deep roots, and is resistant to drought, though in arid climates it is irrigated. It is tolerant of cold, growing well in cool and cold climates. It does not tolerate hot, humid climates, however.

Alfalfa often is cut and dried before it is baled and stored. To speed up the process of drying, alfalfa is commonly flailed or passed through a set of rollers to break or crush the stems, facilitating the drying process. The crushing process is called crimping and sometimes can cause problems for horses because blister beetles (Coleoptera: Meloidae) are incorporated into the hay (see below, blister beetles). Dried alfalfa is tied into bales of various sizes, including large cylindrical bales, and stored under shelter, or packaged in plastic, to avoid moisture. If the alfalfa is to be fed to cattle, however, it is not dried, and instead it is finely chopped and stored in trenches, silos, or bags where it can ferment and maintain high nutrient levels. Cattle are not very susceptible to poisoning by blister beetles.

Alfalfa has undergone considerable breeding to produce strains that have not only suitable agronomic conditions, but also are disease and pest resistant. Nevertheless, insects can damage alfalfa nearly everywhere it is grown. Some of the important pests are listed in the table, and the most important are discussed below.

Alfalfa Weevil, Hypera postica (Gyllenhal) (Coleoptera: Curculionidae)

In many regions, this is the most important pest of alfalfa. It is found in Europe, the Middle East, Central Asia, and North America. Alfalfa weevils overwinter as adults in the soil of weedy, brushy areas near alfalfa fields. They disperse to alfalfa in the spring and oviposit within the stems. The eggs are oval and yellow. Early instars developing from these eggs are slate colored, but develop a bright green color and a white stripe down the middle of the back as they mature. Larvae have a black head capsule. They display four instars and will grow to about 8–10 mm in length. Aſter feeding for 3–4 weeks, larvae spin loosely constructed cocoons on plants or in litter on the soil, pupate, and emerge as adults in 1–2 weeks. Adults are 5–6 mm long, have a long snout, and have a dark stripe down the back. They are light brown at emergence and darken in several days. The number of generations varies according to climate, but eventually they leave fields for grassy, brushy, weedy areas where they become inactive until the onset of winter.

Damage is caused by the larval stage which feeds on leaves; damage ranges from pinholes to skeletonization of leaves. Adults generally cause minor damage. Peak damage is usually just prior to the first cutting or aſter the first cutting, as both larvae and adults feed on new growth; this can seriously affect regrowth of the stand. Also, cool, cloudy weather exacerbates damage done by the alfalfa weevil. Cool and cloudy weather conditions slow the regrowth rate of alfalfa, and also increase the daily feeding period of the weevil because both larvae and adults tend to hide under crop residue during bright sunlight and will not actively feed during such periods.

Weevil larvae can be found early in the Spring. It is important to scout for live larvae and injured terminals on the first crop, but also subsequent crops. Sweep net sampling can be used to detect weevil presence. Several species of wasps can be effective in maintaining weevil populations below economic threshold levels. Among the effective parasitoids are Bathyplectes curculionis (Thomson),

B. anurus (Thomson) and B. stenostigma (Thomson) (Hymenoptera: Ichneumonidae); Microctonus aethiopoides Loan and M. colesi Drea (Hymenoptera: Braconidae) Oomyzus incertus (Ratzenberg) (Hymenoptera: Eulophidae); Dibrachoides dynastes (Forester) and Peridesmia discus (Walker) (Hymenoptera: Pteromalidae); and Anaphes luna (Girault) (Hymenoptera: Mymaridae). A fungal pathogen, Zoophthora phytonomi Arthur (Phycomycetes: Entomophthoraceae), attacks weevil larvae and can control populations in several days, though it is most effective under moist conditions. These biological control agents are extremely effective control measures in all but major outbreak periods. However, when fields show damage on 35–40% of plant tips more than 7–10 days prior to harvest, chemical suppression is often initiated.

Early harvest (first crop) is very effective in killing larvae, and is preferred to chemical control if the planned harvest is less than 7–10 days away. If harvesting is used to control alfalfa weevil, the stubble and debris should be examined closely for adults and larvae, and stems should be examined for feeding signs. It may be necessary to spray stubble, though in many areas producers can avoid insecticide use consistently through timing of harvest.

Root Weevils, Sitona spp., Atrichonotus taeniatulus (Berg), Others (Coleoptera: Curculionidae)

Root weevils such as clover root curculio, Sitona hispidula (Fabricius), in North America; and sitona weevil, Sitona discoideus Gyllenhal, small lucerne weevil, Atrichonotus taeniatulus (Berg), in Australia, and whitefringed beetle, Naupactus leucoloma Boheman in Australia and South America, can be significant pests of alfalfa. Although the adults commonly feed on the foliage, the principal damage is due to larval feeding on the roots of the alfalfa plant.

Eggs are laid in fall or spring, on the soil surface or lower parts of plants. Eggs hatch in the winter or spring. White, legless larvae move into the soil and feed on roots until they pupate. Pupae are found just below the soil surface. Adults emerge in the summer months and live up to a year. The adults are brown or black, blunt-snouted weevils up to about 10 mm long. There is one generation per year. Adults migrate by crawling, and thus infest new areas rather slowly.

The adults feed on alfalfa leaf margins, leaving crescent-shaped notches, and chew on stems and leaf buds of seedlings, but this tends to cause minor loss. Most damage is caused by the larvae. First larval instars feed on root nodules and lateral roots; later instars feed on the taproot. Feeding on the taproot can girdle the plant, resulting in plant death. Such damage also weakens the overall vigor of a stand, perhaps contributing to winter-kill and increased susceptibility to disease.

It is difficult to control larvae because they are in the soil and largely protected from insecticide. Suppression aimed at adults usually requires multiple applications. It is inadvisable to plant alfalfa into a field which has previous ly been infested, to plant into fields previously supporting legume crops, or to seed alfalfa next to established stands.

Blister Beetles (Epicauta spp.) (Coleoptera: Meloidae)

There are several species of North American blister beetles that can be of concern in alfalfa. They are a problem not because of their food habits (they tend to feed mostly on blossoms) but because they contain the toxin cantharidin within their bodies. When alfalfa is harvested, if the hay is crimped it may contain crushed blister beetles that may prove toxic to horses that ingest the hay. The most abundant blister beetle in alfalfa fields generally is the black blister beetle, E. pensylvanica (De Geer). However, the species that is most toxic is E. vittata (Fabricius).

Most blister beetles are recognized by the shape of their body. They are narrow, cylindrical, and soft. The region between the head and wings is distinctly narrower than the wings, and is usually narrower than the head. Most species have one generation per year, although some have two. Blister beetles overwinter as larvae. The adults begin to emerge in the Spring and adults deposit their eggs where grasshopper egg pods may occur, as larvae feed on the grasshopper eggs.

If grasshoppers are not abundant, then blister beetles are unlikely to be abundant. When both are numerous, it is advisable to harvest alfalfa early, before bloom, as this is the only time that beetles are attracted to the crop. There is some yield loss associated with this approach, of course, and an alternative it to treat the crop with insecticides. If insecticide is used, alfalfa should be harvested as soon as possible aſter the pre-harvest interval expires, to get hay out of the field before it is re-infested. A principal problem with blister beetle management is that the beetles tend to aggregate. Thus, there may be relatively few beetles in field, but a large number in one location, and these may be crushed together and concentrated into one or a few bales of hay. Thus, they are hard to detect by standard sampling methods. When alfalfa hay is purchased for horses, it is advisable to acquire early-crop hay, or hay from areas free of high grasshopper populations. Alternatively, inspection of the hay as it is fed to horses can reveal the presence or absence of beetles.

Potato Leafhopper, Empoasca fabae (Harris) (Hemiptera: Cicadellidae)

Potato leafhopper is indigenous to eastern North America. Adults are about 3.5 mm long, wedge-shaped, winged, and green. Nymphs are similar in appearance, but are smaller, yellowishgreen to fluorescent green, and wingless. Each Spring, potato leafhoppers migrate north from southern states where they overwinter. Timing of the first and subsequent arrivals in the north is heavily dependent on weather patterns. Adults lay eggs in stems and leaf veins; eggs hatch in 6–9 days in mid-summer. Each generation takes approximately 30–35 days to mature, resulting in several generations.

Adults and nymphs both feed on alfalfa with piercing-sucking mouthparts, sucking plant sap and injecting a toxin into the plant. Damage is called “hopperburn,” and is a yellow wedge-shape area beginning at the tips of leaves. The leaves may eventually turn entirely yellow or reddish. Plants may become stunted. Leaf hoppers cause yield loss, reduced nutritional quality of alfalfa, and reduced plant vigor that results in increased winter-kill and slower regrowth of the crop the next spring.

In some regions of the USA, the potato leaf hopper is the worst insect pest of alfalfa, and can cause losses of 80% or more if not controlled. Leaf hoppers are not generally a problem in the first crop in an established stand, but as the population increases, all subsequent crops will need to be monitored for infestation. The characteristic hopperburn will not appear until some yield and quality loss has occurred, so it is important to scout for leaf hoppers weekly on the second and subsequent crops. Scouting may be concluded 7–10 days prior to harvest.

Potato leaf hopper economic thresholds are based on plant height. Scouting is accomplished by sweep net sampling. As an example, following are treatment thresholds recommended for Minnesota, USA.

Although the potato leaf hopper has natural enemies, they often get leſt behind when the adults disperse. Thus, a combination of crop monitoring and insecticide suppression is often the principal management strategy. Chemical control of potato leafhopper is effective, but should not be used if harvest is within seven days of harvest. Cutting will kill a large percentage of nymphs, and will force adults out of the field. Cutting is the control of choice if thresholds are reached within seven days of harvest. Additionally, early harvest may be an alternative to insecticides when thresholds are reached late in the year.

Aphids (Hemiptera: Aphididae)

Several aphids are pests of alfalfa, including pea aphid, Acyrthosiphon pisum (Harris); blue alfalfa aphid or bluegreen aphid, Acyrthosiphon kondoi Shinji, cowpea aphid, Aphis craccivora Koch; green peach aphid, Myzus persicae Sulzer; and spotted alfalfa aphid, Therioaphis maculata Buckton.

All these aphids are small, measuring 3 mm or less. Their color varies, depending on species. They may or may not be winged. In most climates, in early Spring nymphs hatch from eggs that were laid in the fall; these aphids are all female. Females can reproduce without mating when conditions are favorable, and they do so in Spring and Summer. In the Summer, the entire life cycle takes only a few days. Males appear in late Summer, and mate with females to produce eggs capable of overwintering.

Aphids use piercing-sucking mouthparts to remove plant sap, and prefer to feed on young growth. Aphid feeding can result in stunted or wilted plants. The plants may also turn yellow.

Aphids commonly attain high densities in alfalfa, but in most years natural enemies keep aphid populations at levels that are not economically important. Many natural enemies of pea aphids exist, including green lacewing larvae (Neuroptera: Chrysopidae), damsel bugs (Hemiptera: Nabidae), and parasitic wasps (Hymenoptera, various families), lady beetles (Coleoptera: Coccinellidae), and disease (fungi).

Plant Bugs (Hemiptera: Miridae)

Several species of plant bugs affect alfalfa, but the most common are tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), and alfalfa plant bug, Adelphocoris lineolatus (Goeze). Adult tarnished plant bugs are brown, winged, and 4–6 mm long; nymphs are green, wingless, and the third and subsequent instars have black spots. Adult alfalfa plant bugs are light green, winged, and 7.5–10 mm long; nymphs are green, wingless, and have red eyes. Tarnished plant bugs overwinter as adults; alfalfa plant bugs overwinter as eggs in plant tissue. During the growing season, the entire life cycle takes 20–50 days, depending on temperature. There are two to five generations per year.

Plant bugs suck sap from plants and inject toxic saliva into the plant. They cause leaves to crinkle, plants to be stunted, and flower buds to abort. They are abundant in all but the earliest portions of the season. Although traditionally considered mostly a seed pest, plant bugs also contribute to forage yield reductions. If bugs are abundant more than seven days prior harvest, chemical control may be warranted.

Grasshoppers (Melanoplus spp. and Phaulacridium spp.) (Orthoptera: Acrididae)

Everywhere alfalfa is grown, grasshoppers and locusts will feed on the crop. However, they are only casually associated with alfalfa, attacking the crop only when abundant. None feed preferentially on alfalfa. In North America, the principal pests are Melanoplus spp., and in Australia Phaulacridium spp. is the major grasshopper pest. These economically important grasshoppers overwinter as eggs. Populations disperse into cultivated fields or pastures as their populations build through the season.

Most egg laying occurs in late summer and fall in production areas; most species prefer uncultivated, grassy or weedy areas, and lay eggs 1–3 cm below the soil surface.

Grasshoppers are generally considered a minor pest except during periods of great abundance, and then they can do great damage. An exception is Australia, where wingless grasshopper has become an increasingly severe pest of alfalfa. Damage has increased in Australia due to widespread cultivation of alfalfa, which is more suitable than grasses for nymphal growth and survival. Grasshopper nymphs and adults damage alfalfa by chewing on leaves from the margin inward in an irregular pattern. Attacks are often on new growth, but will occur on any stage. The margins of fields are most likely to be damaged.

In North America, grasshopper infestations are more severe in warm and dry years. Warm, dry weather immediately following egg hatch favors survival of nymphs, because nymphal growth rates and survival are lower in cool, wet weather. Long, warm autumns prolong the egglaying season, and result in larger populations in the next growing season. It can take 3–5 years for populations to build to economically important levels. In Australia, drought also is implicated, but mermithid nematodes are a critical element in grasshopper biology. Absence of rainfall, and clearing of drier, higher elevation pasture impedes the ability of the nematodes to parasitize the grasshoppers.

Grasshoppers are naturally suppressed by numerous natural enemies, but when weather conditions favor the grasshoppers their populations increase quickly. The natural enemy population increases as their food supply becomes more available, but the lag in natural enemy abundance can result in crop damage by the grasshoppers. Weedy fence rows, irrigation ditches, and fallow fields are important sources of grasshoppers. Weed populations should be managed, which may require tillage or burning to make these habitats less productive for grasshoppers.

Cutworms, Armyworms and Budworms (Lepidoptera: Noctuidae)

The caterpillars of several moths can become abundant enough to cause significant loss to alfalfa. Among these are the armyworms Mythimna spp., Persectania spp., and Pseudaletia unipuncta Haworth; variegated cutworm, Peridroma saucia (Hübner); army cutworm, Euxoa auxiliaris (Grote); granulate cutworm, Agrotis subterranea (Fabricius); beet armyworm, Spodoptera exigua (Hübner); budworm, Helicoverpa punctigera (Wallengren), and many others. The important species vary among regions, though they are similar ecologically.

The larvae of cutworms, armyworms, and budworms range in color from greenish-yellow to brownish-black. Larvae are 2–5 cm long at maturity. The wings of the adults vary from tan to dark brown with mottling or stripes. Pupae are 1–3 cm long and are reddish-brown to black in color. There are one to six generations per year. Larvae overwinter in the larval or pupal stage, depending on species.

Larvae feed on stems and leaves of plants, and can limit regrowth aſter harvest. Larvae will also cut the stems of seedlings. Their occurrence as economic pests is sporadic. Although these insects have many natural enemies, when they are abundant insecticides are the preferred approach to population reduction.

Lucerne Flea, Sminthurus viridis (Collembola)

Sminthurus viridis, the lucerne flea or clover springtail, is an insect relative (hexapod) belonging to the order Collembola (the springtails). It is bright green with a roughly spherical body and may swarms in large numbers on living plants, including alfalfa or lucerne, thus the first part of the common name. The second part of the common name was given for its jumping ability and its minute size, not because it is a flea or related to fleas. This species has a patchy distribution in Europe and North Africa, and has been accidentally introduced to Australia, where it is most injurious. It also affects lupine flowers, lentils, beans, and field peas. Immature lucerne fleas consume small patches of foliage, whereas adults consume the entire leaf except for the veins. Early season spraying of insecticide is the most common recommendation to curb their damage.

Mites (Acari)

Mites generally are not major pests of alfalfa, but under arid conditions or along the margins of fields they can be quite damaging. The most important are clover mites, Bryobia spp. (Acari: Tetranychidae), and redlegged earth mite, Halotydeus destructor Tucker (Acari: Penthaleidae), in Australia, and twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae) in North America. They rupture the cells of leaf tissue, imparting a silver or yellow appearance, and reducing yield.

Pest Management in Alfalfa

Alfalfa is an excellent crop for the practice of modern pest management tactics because (i) it is quite tolerant of damage; cosmetic injury is not important; (ii) it is a perennial crop, providing harborage throughout the year for an immense assemblage of insects, including predators and parasitoids;

(iii) it is an important crop, so extensive research on the pests have been conducted; (iv) it is amenable to various cultural manipulations, and produces multiple crops over a large portion of the year; and (v) it is a favorite crop for rotations, so there is ample opportunity to integrate its culture with the culture of other crops.

The principal tactics used for alfalfa production include scouting and use of an economic threshold for decision making, natural and classical biological control, cultural control, and chemical control. The economic threshold varies among insect species, geographic locations, crop management practices and economic conditions, but most locations have established such benchmarks for initiating chemical control. A large number of insecticides are registered for this crop, so growers have ample opportunity to select products according to their need and budget. A modest level of host plant resistance apparently exists in alfalfa, and although resistance is effective mostly against aphids, there is also some success with alfalfa weevil and leaf hoppers.

A large number of beneficial arthropods have been moved around the world in an effort to attain biological suppression of invading alfalfa-feeding insects. In some cases this has met with success. For example, alfalfa blotch leafminer, Agromyza frontella (Diptera: Agromyzidae), was considered a serious pest when it first invaded the eastern USA, but following release of wasp parasitoids it fell to minor pest status. Similarly, the status of spotted alfalfa aphid, pea aphid and blue alfalfa aphid was affected by importation of beneficial insects. A native entomopathogenic fungus, Zoophthora phytonomi, has adapted to the invasive alfalfa weevil and sometimes provides good suppression. Pea aphid is affected by the fungus Erynia neoaphidis under favorable weather conditions. Generalist predators such as lacewings, lady bird beetles, nabids, soft-winged flower beetles, big-eyed bugs, and minute pirate bugs are often active in alfalfa, and provide good suppression of aphids, thrips, and also consume eggs and young larvae of caterpillars.

Cultural manipulations are the most important tactics for management of alfalfa pests. In particular, early harvesting can provide acceptable or even nearly complete control of alfalfa weevil, alfalfa blotch leafminer, several caterpillars, aphids, and leaf hoppers because when the crop is cut the insects are exposed to lethal levels of heat and dryness, or the environment becomes so unsuitable that the insects move elsewhere. Crop rotation is most important for root feeding pests, many of which take several years to develop damaging populations. Strip cropping is commonly recommended because the uncut areas retain populations of natural enemies, allowing the beneficial insect to move into newly harvested alfalfa as it regrows and becomes infested with pests. Farmers rarely embrace this approach, however, opting for operational efficiency over economic pest control.

Some pests of alfalfa (lucerne), and locations where they are considered to be damaging

Feeding behavior	Primary taxon	Common name	Scientific name	Location
Above-ground, chewing	Coleoptera	Sitona weevil	Sitona discoides	Australia
		Small lucerne weevil	Atrichonotus taeniatulus	Australia
		Vegetable weevil	Listroderes obliquus	Australia
		Alfalfa weevil	Hypera postica	Europe, Asia, N. America
		Clover leaf weevil	Hypera punctata	Europe, Asia, N. America
		Clover head weevil	Hypera meles	Europe, N. America
		Blister beetles	Epicauta spp.	N. America
		Flea beetles	Epitrix, Systena, Disonycha spp.	N. America
	Orthoptera	Grasshoppers	Melanoplus spp.	N. America
		Wingless grasshopper	Phaulacridium spp.	Australia
	Lepidoptera	Armyworm	Mythimna spp.	Australia
		Armyworm	Persectania spp.	Australia
		Armyworm	Pseudaletia unipuncta	N. America
		Variegated cutworm	Peridroma saucia	Europe, Asia, Africa, N. America
		Army cutworm	Euxoa auxiliaris	N. America
		Granulate cutworm	Agrotis subterranean	N. America, S. America
		Black cutworm	Agrotis ipsilon	N. America, Europe, Africa
		Beet armyworm	Spodoptera exigua	Asia, N. America
		Fall armyworm	Spodoptera frugiperda	N. America, S. America
		Budworm	Helicoverpa punctigera	Australia
		Corn earworm	Helicoverpa zea	N. America, S. America
		Alfalfa looper	Autographa californica	N. America
		Lucerne leafroller	Merophyas divulsana	Australia
		Alfalfa caterpillar	Colias eurytheme	N. America
		Webworms	Loxostege spp.	N. America, Europe
	Collembola	Lucerne flea	Sminthurus viridis	Australia, Europe, Africa
	Diptera	Alfalfa blotch leafminer	Agromyza frontella	Europe, N. America
Above-ground, sucking	Acari	Redlegged earth mite	Halotydeus destructor	Australia
		Clover mite	Bryobia spp.	Australia
		Twospotted spider mite	Tetranychus urticae	No. America
	Collembola	Lucerne flea	Sminthurus viridis	Europe, N. Africa, Australia
	Hemiptera	Pea aphid	Acythosiphum pisum	Europe, Asia, Australia, N. & S. America
		Blue alfalfa aphid	Acythosiphum kondoi	Mediterranean, Australia, N. & S. America
		Spotted alfalfa aphid	Therioaphis maculata	Mediterranean, Australia, N. America, Asia
		Potato leafhopper	Empoasca fabae	N. America
		Lucerne leafhopper	Austroasca alfalfae	Australia
		3-cornered alfalfa hopper	Spissistilus festinus	N. America
		Meadow spittlebug	Philaneus spumarius	N. America
		Tarnished plant bugs	Lygus spp.	Europe, N. America
		Alfalfa plant bug	Adelphocoris spp.	Europe, N. America
	Thysanoptera	Flower thrips	Frankliniella spp.	Europe, Asia, N. America
Below-ground	Coleoptera	Clover root curculio	Sitona hispidula	Europe, N.
		Alfalfa snout beetle	Otiorhynchus ligustici	N. America
		African black beetle	Heteronychus arator	Africa, Australia
		Whitefringed beetle	Naupactus leucoloma	Australia, S. America
		Small lucerne weevil	Atrichonotus taeniatulus	Australia