Distribution. This native insect pest occurs principally in the Rocky Mountain region of North America, from Arizona and New Mexico north to Alberta and Saskatchewan. However, it occasionally has been reported as far west as California and British Columbia and as far east as Minnesota and Quebec. Potato psyllid also occurs in Mexico. It overwinters only in the southernmost United States and in Mexico, so occurrence in northern regions is largely a function of wind-borne dispersal.
Host Plants. Potato psyllid is also sometimes known as "tomato psyllid," and these common names accurately indicate the most preferred hosts. However, this psyllid can develop on other solanaceous vegetables, particularly pepper and eggplant. Solanaceous weeds such as groundcherry, Physalis spp.; black nightshade, Solanum nigrum; buffalo bur, Solanum rostratum; and matrimony vine, Lycium halimifolium; also support reproduction. Matrimony vine is believed to be an important overwintering host in southern Arizona. Adults have been collected from numerous plants, including conifer trees, but this is not an indication of the true host range. With the possible exception of field bindweed, Convolvulus arvensis, non-solanaceous plants seem to be relatively unimportant, and even solanaceous weeds are not as important as crop plants. (See color figure 18.)
Natural Enemies. Several predators and parasites of potato psyllid are known, though there is little documentation on their effectiveness. Under rather artificial conditions the larvae and adults of lady beetles (Coleoptera: Coccinellidae), lacewings (Neuroptera: Chrysopidae), flower flies (Diptera: Syrphidae), big-eyed bugs (Hemiptera: Lygaeidae), minute pirate bugs (Hemiptera: Anthocoridae), and damsel bugs (Hemi-ptera: Nabidae) consume psyllid nymphs and adults (Pletsch, 1947). The wasps Metaphycus psyllidus Compere (Hymenoptera: Encyrtidae) and Tetrastichus trio-zae Burks (Hymenoptera: Eulophidae) parasitize potato psyllid. The latter species feeds externally on nymphs and attacks other psyllid species as well, and though sometimes abundant, is often absent from psyllid populations.
Weather. Weather is an important element of biology and damage potential. These psyllids seem to be adapted for warm, but not hot weather. List (1939a), for example, demonstrated how prolonged exposure to 32°C depressed egg production and egg hatching. Temperatures of about 21-27°C are considered optimal. Dispersal northward occurs when higher temperature is attained; temperatures of about 32°C and higher are deleterious to psyllids. Heavy precipitation may be detrimental, but this is less certain. Because psyllids disperse passively over long distances, wind patterns also are critical in determining their occurrence, and account for much of the variability in damage. Once invading psyllids have established, they multiply and cause greatest damage at moderate temperature, and populations fail to develop to damaging levels if the temperature is high.
Life Cycle and Description. A generation can be completed in 20-30 days, depending on temperature.
The number of generations varies considerably among regions. In Montana, only three generations can be completed before frost kills the host plants. In northern Utah, 3-4 generations are known, and in Colorado there are 4-7 generations. Once psyllids invade an area, however, prolonged oviposition by adults causes the generations to overlap, so it is difficult to distinguish generations.
Potato psyllid overwinters in southern Arizona, southern Texas, and northern Mexico, where populations build to high densities in early spring. By early summer the psyllids disperse from the spring breeding areas northward to Utah and Colorado and shortly thereafter to Nebraska and eastern Wyoming. In July, as the temperature becomes hot in these mid-summer breeding areas, psyllids disperse northward to the northern states and Canadian provinces. Psyllids reappear in the overwintering areas between October and November, presumably dispersing southward from northern locations.
A short fringe of wax filaments is present along the lateral margins of the body. Nymphs tend to remain essentially sedentary during their development, and prefer sheltered, shaded locations. Mean duration (range) of the instars was reported by Knowlton and Janes (1931) to be 2.8 (1-5), 2.4 (1-5), 2.5 (1-4), 2.7 (15), and 4.9 (3-9) days, respectively, for instars 1-5. Thus, nymphal development time required 12-21 days, though the average was about 15 days. (See color figure 183.)
Adult. The adults are quite small, and measure only about 2.5-2.75 mm long. In general body form they resemble miniature cicadas, largely because they hold their wings angled and roof-like over their body. They have two pairs of transparent wings. The front wings are considerably larger than the hind wings. The antennae are moderately long, about the length of the thorax. Body color ranges from pale green at emergence, to dark green or brown within 2-3 days, and gray or black thereafter. White or yellow lines are found on the head and thorax, and whitish bands on the first and terminal abdominal segments. Adults are active and easily disturbed, a distinct contrast to the nymphal stage. The pre-oviposition period of adults is normally about 10 days, with oviposition continuing for about 20 days. Total adult longevity is normally 25-35 days. Pletsch (1947) reported mean longevity of females to be 20, 23, 29, and 14 days when reared on tomato, potato, eggplant, and pepper, respectively. He also observed mean fecundity of 67, 258, 187, and 53 eggs on these same hosts. Knowlton and Janes (1931), working with potato foliage, reported mean fecundity in various trials of 300-400 eggs, with some individuals producing 1100-1350 eggs.
The biology of potato psyllid was described by many authors including Knowlton and Janes (1931),
List (1939a,b) and Wallis (1946, 1955). However, Pletsch (1947) provided the most complete summary and Cranshaw (1993) published an annotated bibliography. Keys to the North American Psyllidae were given by Tuthill (1943).
Feeding by nymphs of potato psyllid causes a disease known as "psyllid yellows" (Eyer, 1937; Arslan et al., 1985). Psyllids apparently produce a toxin or growth regulator that disrupts plant growth; a plant pathogen is not involved. Even a few psyllids can affect growth, but large populations are necessary for extensive disruption of plant growth. Symptoms of the disease include upward rolling of the young leaves, yellowing along the midrib and leaf margins, and sometimes induction of a purple color. Stem elongation is suppressed, plants are dwarfed, exceptionally small tubers and fruits are formed, and tubers sprout prematurely. In some areas, yield losses of 20-50% are not uncommon in the absence of treatment, and complete losses have been reported in potato. This insect is considered to be the principal pest of tomatoes in some Rocky Mountain areas. Greenhouse-grown tomatoes are also affected. Elimination of psyllids from infested plants results in partial recovery of plants. Other psyllid species are occasionally reported from potatoes, but only P. cockerelli occurs in large numbers and causes damage. Beet leafhopper, Circulifer tenellus (Baker), induces similar symptoms in tomato, but this disease (called "curly top") is caused by a plant virus.
Cultural Practices. Early-planted crops are more susceptible to injury than crops planted at mid-season or late, principally because only early crops are present when psyllids disperse into areas in the spring. However, part of the reason early season crops are more injured is that psyllids do not thrive under the hot weather conditions found later in the summer, and large plants from early-season plantings protect the psyllids from the sun and heat (Wallis, 1955). Sanitation also is important because if discarded potatoes are allowed to sprout they can serve as temporary hosts.
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