Adelphocoris lineolatus Goeze Hemiptera Miridae

Natural History

Distribution. Alfalfa plant bug is native to Europe, and was first observed in North America in 1917 at Cape Breton Island, Nova Scotia, but it failed to spread. It then was accidentally imported to the United States with alfalfa, and introduced near Ames, Iowa in 1929. Thereafter, alfalfa plant bug spread across the United States and Canada, and was observed Minnesota in 1934; Illinois, Missouri and South Dakota in 1935; Nebraska and Wisconsin in 1936; Kansas and Manitoba in 1939; and Saskatchewan in 1947. It now is found in the northern United States and southern Canada from the Rocky Mountains to eastern Canada and the northeastern states. The southern limits to its range currently are Kansas in the midwest, and Virginia in the eastern states.

Host Plants. As suggested by its common name, this insect feeds principally on such forage legumes as alfalfa, birdsfoot trefoil, red clover, sainfoin, and sweet clover. However, it is an occasional pest of vegetables, including asparagus and potato. Often the occurrence of alfalfa plant bug in vegetable crops can be traced to nearby forage legumes or weeds which were harvested or otherwise became unattractive to the bugs. Hughes (1943) reported that alfalfa plant bugs normally deposited eggs only in alfalfa and sweet clover. However, in the absence of such plants, successful oviposition occurred in red clover; timothy; soybean; bindweed, Convolvulus sp.; and redroot pigweed, Amaranthus retroflexus.

Natural Enemies. A common parasitoid of mir-ids, Peristenus pallipes (Curtis) (Hymenoptera: Braconi-dae), is recorded from alfalfa plant bug. Day (1987) reported an undetermined species of mermithid nematode. Other parasitoids are known in Europe, and a program to import beneficial insects has been implemented. One imported species, Peristenus digo-neutis Loan (Hymenoptera: Braconidae), has been succcessfully established in New Jersey (Day 1996). A key for identification of parasitoids affecting Adel-phocoris was provided by Loan and Shaw (1987).

Life Cycle and Description. Two generations occur annually in Minnesota and, apparently, over most of this insect's range in North America. Total development time from the egg to the adult stage is estimated to require 33.5 days. Another two weeks are usually required before eggs are produced, so total generation time is about six weeks. The eggs are the overwintering stage. Hatching first occurs in May, and by mid-June first generation adults are present. Nymphs of the second generation can be observed in July, and adults in August. By mid-August overwintering eggs are produced. The two generations may overlap considerably, and in cool summers the development of the second generation is delayed so that second generation adults are not produced until September. All stages except the eggs are killed with the onset of cold weather.

In contrast, there is principally one generation per year in northern Saskatchewan. Most of the eggs laid by the spring generation enter diapause; only a few, probably less than 5%, develop into nymphs. A few second generation adults are produced, but there is not adequate time before the onset of cold weather for egg production to occur (Craig, 1963).

  1. The eggs typically are deposited in alfalfa stems. During the first generation, they are usually deposited 25 cm or so above the soil surface, but the overwintering eggs are deposited in the less succulent alfalfa stems close to the soil surface. The eggs of alfalfa plant bug are about 1.38 mm long and 0.33 mm wide. They are slightly curved in shape, with one side concave and the opposite side convex, and are yellow in color. They are laid closely together in small groups, and about 10-30 eggs are deposited daily. The tip of the eggs protrudes slightly from the oviposition site. The incubation period of the eggs is 11-17 days, averaging 15 days.
  2. The nymphs are light green, and all except the first instar bear black spots on the femora. There are five instars. Wing pads are not evident during the first two instars, but in the third instar they become visible. The wing pads increase in length at each of the next molts, attaining the second abdominal segment in instar four, and the fourth abdominal segment in instar five. Mean development time (range) was reported as 4.9 (4-7), 2.9 (1-8), 2.5 (2-5), 3.2 (2-4), 4.9 (4-6) days for instars 1-5, respectively. These values can be prolonged significantly, perhaps doubled, during periods of inclement weather. Total nymphal development time is usually 18-20 days, but sometimes prolonged to 30 days. (see color figure 141).
Miridae Europe
Adult alfalfa plant bug.

Adult. The adults are yellow-green, and marked with brown. This color pattern serves to distinguish them from the other common Adelphocoris species: rapid plant bug, A. rapidus (Say); and superb plant bug, A. superbus (Uhler). These latter species are principally dark-brown. The general body form (viewed from above) of alfalfa plant bug and other Adelphocoris spp. is fairly elongate, with the overall length nearly three times the maximum width. The Eygus spp., the other common plant bugs, tend to be much broader in general body form, only about twice as long as wide. Alfalfa plant bug is also quite large in size, usually measuring 7.4-10.0 mm long, with a mean length of about 8.0 mm. The antennae are yellowish-brown basally and reddish-brown distally. The prono-tum bears two small dark spots. The edges of the scutellum are brown. The legs are yellowish-brown and the femora marked with black spots.

The biology of alfalfa plant bug was given by Hughes (1943) and Craig (1963).

Damage

Alfalfa plant bug removes the sap from plants, causing blossom and seed abortion, local lesions, tip die-back, and plant growth distortions. On alfalfa, the symptoms are similar to those induced by tarnished plant bug, Eygus lineolaris (Palisot de Beauvois), but not as severe (Grafius and Morrow, 1982), and also resemble the yellowing induced by potato leafhopper, Empoasca fabae (Harris) (Radcliffe and Barnes, 1970). It is principally a seed feeder, however, and is most damaging to alfalfa grown for seed.

Management

  1. Dispersion patterns and sampling protocols for alfalfa plant bug were studied in birdsfoot trefoil by Wipfli et al. (1992). The population of both nymphs and adults tends to be aggregated. The number of sweep net samples necessary to accurately determine population density varies from about 5 sets of 10 sweeps, to 20 sets of 10 sweeps, depending on bug density. Males of alfalfa plant bug have been shown to be attracted to traps baited with virgin females of tarnished plant bug, suggesting that a sex pheromone is involved in mate location (Slaymaker and Tugwell, 1984).
  2. Foliar applications of chemical insecticides are used to suppress alfalfa plant bug. Conventional and ultra low-volume applications are effective, but populations can recover quickly. Recovery is due both to reinvasion of treated fields, and hatching of eggs developing in plant stems, a relatively protected location (Pruess, 1974). Thus, careful timing of application in advance of egg deposition, or use of residual insecticides, may be desirable.

Cultural Practices. Harvesting of alfalfa hay can induce relocation of plant bugs, especially if the population consists mostly of adults. Harvesting before development of adults greatly diminishes survival of plant bugs (Harper et al., 1990). Vegetable crops located adjacent to alfalfa are susceptible to invasion by dispersing bugs, but the immigration rate is diminished by such barriers as roads and irrigation ditches (Schraber et al, 1990).

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