Richard W. Straub
Department of Entomology
Cornell University, NYS Agricultural Experiment Station
Hudson Valley Research Lab
Highland, NY 12528
Charles J. Eckenrode
Department of Entomology
Cornell University - NYS Agricultural Experiment Station
Geneva, NY 14456
Many food historians place the earliest onion cultivation at the edges of the Mediterranean as long ago as 5,000 years. The National Onion Association says that onions were first grown in Iran and Pakistan. Others believe that it originated in central Asia. But with several hundred onion cousins growing wild across the temperate parts of the world, it is quite likely that they have been eaten for tens of thousands of years. In addition to its flavor and nutritional attributes, many healing powers have been attributed to this vegetable throughout the centuries - the plant or plant portions have been prescribed for a variety of ills including coughs, colds, stomach ailments, cancer and heart problems.
Onion, Allium cepa, is mentioned as being under cultivation in America as early as 1629, and is currently grown in all regions of the United States. Onion is grown either as green or bunching types, or as bulbs. Onions grown for bulbs are classified as either fresh or storage types. The fresh onions are spring and summer crops, high in water and sugar content and noted for their sweet, mild flavor. They do not store well and must be marketed relatively soon after harvest. Storage onions are available throughout the fall and winter. They are stronger in taste, usually smaller, have the traditional onion heat and store well. Both of these types are grown either on mineral soils or on high organic 'muck' soils, depending on the region.
Onion is subject to a number of disease pests (e.g., onion smut, downy mildew, pink root, neck rot, Botrytis leaf blight and many others). In comparison, there are fewer arthropod pests are fewer, but they can be serious detriments to onion production. Arthropod pests discussed here can be problematic in one or more areas of North America.
Root or Bulb Feeding Species
Onion maggot, Hylemya (=)Delia antiqua (Meigen) (Diptera: Anthomyiidae), is the most important and consistent insect pest of onion in Canada and in the northeastern and northcentral United states, particularly where onions are grown continuously on the same ground. This pest first reached North America from Europe in 1841, and since that time it has frequently devastated onion crops. Onion maggot attacks only member of the Alliaceae family, with onion being a preferred host, but it also infests garlic, leek, shallots and chives. Onion grown from sets and transplants, as well as from seed, are attacked.
Onion maggot overwinters as a puparium in the soil. Adults are somewhat smaller and more slender than house flies, but closely resemble them in general appearance. Three distinct broods occur annually in most temperate regions. Larval offspring from flies that emerge in the spring usually cause the most damage because the plants are most vulnerable to attack during the seeding stage. Flies live for 2-4 weeks and are capable of migrating about a mile in search of suitable hosts. Mating occurs a week to 10 days after emergence, and female flies oviposit on the soil near the plants or occasionally on the young leaves or plant necks. Each female is capable of laying several hundred eggs. Neonate larvae move down through the soil and enter at the base of the onion plant by feeding on the tissues in the vicinity of the basal plate using their hooked mouth parts. More than one larva may infest a single plant, and if plants are killed before larvae complete their development, maggots immigrate to nearby intact plants in search of additional food; more than 20 seedlings may be killed by a single maggot. There are three larval instars.
The first sign of onion maggot injury is the wilting of foliage, after which it subsequently becomes flaccid and collapses. This damage is particularly noticeable when the plants are in the seedling stage; the maggots frequently destroy groups of plants that result in a patchy crop. Larger onions may survive infestation, but distorted growth accompanied by the rotting of tissue renders them unmarketable. Considerable losses of bulbs at harvest have occurred because of maggot feeding during late-season when the bulbs are being field cured.
Effective control of onion maggot relies primarily on the proper application of in-furrow insecticides, although historically this pest develops resistance to insecticides rather quickly. Recently, a seed treatment with an insect growth regulator has shown promise. This treatment is effective at a relatively low amount active ingredient per acre. Efforts are currently being taken to attain registration for this new plant protectant. Because treatments at planting lack the persistence to control mid- and late-season maggot broods, adulticidal sprays of insecticides are often utilized. Because of the mobility of the flies however, the effectiveness of adulticides has been questioned.
When possible, crop rotation of two or more years or the isolation of onion fields from one another can significantly reduce maggot populations for several seasons. Strict attention to good husbandry should include the destruction of volunteer and cull onions and the destruction of debris removed from onion storages, practices that eliminate possible sources of onion maggot infestation. Mechanical injury to the growing plant should be avoided because damaged foliage serves to attract ovipositing flies. The onion maggot has many potential natural enemies but none are known to be effective, largely because most commercial onion fields receive multiple foliar insecticide and fungicide treatments. Current onion cultivars differ little in their susceptibility to attack, although a few closely related crops such as leek are less damaged by this pest.
In Holland, the mass rearing and inundative release of sterilized flies has been implemented with some success. This strategy has also received a considerable amount of research attention in Ontario, Canada. It has been concluded that, while this technique is biologically feasible, under present conditions it is not economically competitive with soil insecticides.
Bulb mite, Rhizoglyphus robini Claparede (Astigmata: Acaridae), is a cosmopolitan species that has achieved pest status in countries as divergent as Australia, Israel, Japan and the United States. Its wide host range includes onion, garlic, lily, gladiolus, cereals and many other crops. It also feeds on organic matter in the soil. The first published record of R. robini as a pest of onion in the United States dates back to 1955, when it was briefly reported as a pest of commercial onion in New York. Since 1995, it has recurred sporadically in that state and has caused significant degrees of economic damage.
Most damage caused by bulb mite occurs at the roots and in the basal plate, but these mites have been found in pockets within the growing bulb tissue of otherwise healthy plants. The above-ground symptoms of damage are quite similar to those of onion maggot, and bulb mite damage is likewise most serious to seedling onions. Heavily damaged plants eventually lose their roots and topple. Damage is also manifest in infection from bacterial and fungal pathogens that may enter through mite-created wounds.
Although specific aspects of bulb mite biology and behavior in onion habitats are not yet clear, reports from other cropping systems are revealing. The bulb mite is extremely well adapted to its soil habitat: it prefers to feed on fresh tissue, but can survive on many organic materials including dead plants, dead insects and manure; it can survive up to five weeks submerged under water; it is capable of escaping adverse conditions, such as drought and extreme cold, by moving deeper into the soil. The second nymphal stage, or hypopus, may use its specialized sucker plate to attach itself to flying and crawling animals or to equipment, and thus be dispersed to new locations.
Populations of the bulb mite can increase rapidly. Under optimum conditions, females can live up to 40 days and produce 700 eggs; males can live for up to 73 days. It is estimated that under field conditions, a generation can be completed in approximately four weeks. No pesticides are registered for suppression of this pest on onion. Because it has such a wide host range, most crop rotations will not manage it. Although the mite probably exists at some level in most soil environments, it is believed that economic damage occurs when some disruption, such as reduction of natural enemies, or a sudden infusion of organic matter in the form of green mulch allows the resident population to increase to damaging levels. Research efforts are presently structured to monitor damaging levels of mites and to clarify the factors that cause these increases to occur.
Stem or Leaf Feeding Species
Onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), is a serious pest of onion in most production areas of the United States and Canada. Nearly all garden vegetable plants, some forage and grain crops, and many weeds serve as hosts.
Onion thrips, formerly thought to have rasping mouth parts, are now known to have asymmetrical appendages comprising a single mandibular stylet that is used to pierce plant tissue; maxillary stylets forming a feeding tube are used to remove plant liquids by cibarial pumping. Damage to plants results from the destruction of epidermal tissue and removal of cell sap from individual mesophyll cells. Feeding by nymphs is generally concentrated on young tissue between the leaf sheaths and the stem, but extensive feeding by adults on more mature tissue is not unusual. Affected leaves display whitish or chlorotic blotches, as if they have been sandblasted. If feeding is severe, particularly under drought conditions, the tips of leaves become brown. Prolonged feeding by infestations of thrips may eventually kill seedlings, reduce bulb size by decreased photosynthesis and increase the incidence of leaf and bulb rots.
Adults overwinter in legume and cereal grain fields from which they immigrate to onion and other preferred crops such as cabbage, usually during early- to mid-summer. In some instances, perhaps because the overwintering host crop has been harvested, mass immigrations into onion fields have occurred over very short time periods. Eggs are laid in onion leaves and hatch in four to 10 days, depending on temperatures. Males are wingless and scarce, and females regularly reproduce without mating - allowing for the rapid increase of populations when conditions are favorable. Depending upon the time of immigration into onion fields and weather conditions, there are from three to five overlapping generations per season.
Effective management of onion thrips relies primarily on foliar sprays of insecticides based on some treatment threshold, usually from two to four nymphs per leaf. Because nymphs feed between the leaf sheaths and the stem, they are difficult to contact with insecticide sprays - consequently high pressure, high gallonage sprays are recommended. Commercial onion cultivars differ little in their susceptibility to attack, although research reports suggest that onion types with glossy, spreading foliage are less suitable for thrips development that are the more common upright, waxy foliage types with tight leaf interfaces. Because of their mobility, crop rotation or isolation from immigration sources have little effect on onion thrips infestations.
The black cutworm, Agrotis ipsilon (Hufnagel) (Lepidoptera: Noctuidae), is a cosmopolitan insect known to attack at least 49 species of plants. Infestations of onion and other vegetable crops often occur exceptionally early in the spring. Evidence suggests that the parents of these early season populations are either very early immigrants from southern regions, or have passed the winter in the vicinity as pupae. Infestations of onion are sporadic from year to year, and no reliable methods have been found for predicting the occurrence of this pest.
Black cutworm moths are attracted to fields containing weeds, especially winter or spring annual and perennial weeds such as chickweed, purslane, shepherd's purse, pepper grass and yellow rocket. Therefore, weedy fields tilled shortly before seeding are probable sites for infestation. Moths are to a lesser degree attracted to cereals, such as oat or wheat, that are often used as winter cover crops in many onion growing areas.
Eggs of black cutworm are deposited singly or in small clumps on the leaves or stems of attractive hosts. Neonate larvae settle just below the soil surface to feed. As larvae consume the below ground portion, they continue to feed and pull the above ground portion into the feeding cell - thus plants appear to be sinking into the soil. A single larva may partially consume a number of onion plants during the seedling to five-leaf stages. Infestations of onion are usually limited to the earliest spring plantings.
Management of black cutworm often relies on high volume insecticide sprays directed at the bases of plants. This method is of questionable value because larvae are difficult to contact with the toxicant and they are usually of advanced maturity before detection and treatment. A likely cultural management option would include fall plowing to reduce winter or spring annual and perennial weeds that serve as ovipositionally attractive plants the following spring.
Links to Onion Websites
- National Onion Association
- University of California, UC IPM Online, Statewide Integrated Pest Management Program, How to manage pests - onion and garlic
- Texas A&M University - Onions in Texas
- APS, Compendium of Onion and Garlic Diseases
- New Mexico State University - Onion Breeding Program
- Oregon State University: PNW Weed Managment Handbook - Onions
- Purdue University, Allium cepa L.
- Eckenrode, C. J. and J. P. Nyrop. 1995. Onion maggot management in New York, Michigan, and Wisconsin. New York's Food and Life Sci. Bull. (Plant Sciences). No. 144. 2 pp.
- Ellis, P.R. and C.J. Eckenrode. 1979. The onion maggot and its control in New York. New York's Food and Life Sci. Bull. (Plant Sciences) #79. 4pp.
- Griffith, Fred and Linda. 1994. Onions, onions, onions. Chapters Pub. Ltd., Shelburne, VT, 05482. 384 pp.
- Hoffmann, M.P, C.H. Petzolt and A.C. Frodsham. 1996. Integrated pest management for onions. New York State IPM Prog. Pub. #119. 78pp.
- Hodson, W.E.H. 1928. The bionomics of the bulb mite, Rhizoglyphus echinopus Rumoze and Robin. Bull. Entomol. Rsh. 19:187-200.
- Lewis, T. 1973. Thrips: their biology, ecology, and economic importance. Academic Press. London.
- Sherrod, D.W. J.T. Shaw and W.H. Luckmann. 1979. Concepts on black cutworm field biology in Illinois. Environ. Entomol. 8:191-5.