Distribution. Pea moth is native to Europe, and was first observed in North America in 1893 at Toronto, Canada. Probably, the insect had been in residence for a number of years before attracting attention. It quickly spread across the northern United States, causing great damage in the Great Lakes region in the early 1920s, and reached Washington in 1926. It inflicted considerable damage in both Washington and British Columbia in 1933. Now found throughout the northern United States and southern Canada, it is rare in the Prairie Provinces, and infrequently is a serious pest anywhere in North America. It remains a serious problem in Europe, however, and is known from Japan.
Host Plants. This insect feeds only on plants in the family Leguminosae. The only crop injured is pea, but several other legumes are suitable hosts, including Canada pea, Vicia cracca; common vetch, Vicia angusti-folia; hairy vetch, Vicia villosa; lupines, Lupinus spp.; Scotch broom, Cytisus scoparius; spring vetch, Vicia sativa; sweet pea, Lathyrus odoratus; yellow vetchling, Lathyrus pratensis; and possibly other members of these plant genera. Hanson and Webster (1936) reported that the most suitable host was pea, followed by sweet pea, and then the vetches. Larvae feeding on lupines and Scotch broom experience high mortality.
Natural Enemies. Several natural enemies have been imported from Europe and introduced into Canada and the United States. Although some species failed to establish, the parasitoids Ascogaster quadri-dentata Wesmael, Phanerotoma fasciata Provancher (Hymenoptera: Braconidae), and Glypta haesitator Gravenhorst (Hymenoptera: Ichneumonidae) became established. Ascogaster quadridentata proved to be very important, causing over 70% parasitism in British Columbia. Although the parasitoids undoubtedly are important in checking the abundance of pea moth, changes in cropping practices also are important. Specifically, the dramatic decline in the production of dried peas is of major significance, because the major host plant is now much less abundant. Also, harvesting of peas while they are green results in the destruction of larvae before most are able to mature. Clausen (1978) summarized the parasitoid introduction program. Pea moth seems to be free of important diseases. However, it has been shown to be susceptible to infection by a granulosis virus isolated from a related insect, codling moth, Cydia pomonella (Linnaeus) (Payne, 1981).
Life Cycle and Description. Generally there is a single generation per year, though in Washington a few insects apparently go on to form a second generation. In England, a small proportion of the insects have been observed to have a two-year life cycle, larvae spending two winters and a summer in the soil; this likely occurs in North America as well. Oviposition occurs in June and July, followed by development of larvae until September. Larvae overwinter, with pupation occurring in the spring.
Pea moth larva.
Pea moth larva.
nearly uniform brownish gray, but is bordered with a light-colored fringe with a thin, dark inner line. The moths tend to be active only in the afternoon, and a threshold of about 18°C must be exceeded before flight occur. Flight also is reported to be favored by high humidity. Female moths emerge with nearly mature ovaries, mate upon emergence, and commence egg laying almost immediately. Females disperse freely in search of suitable oviposition sites, but males are less dispersive. Males are attracted to a female-produced sex pheromone that has been identified, and synthesized, for use in population monitoring.
Wright and Geering (1948) provided detailed biology of pea moth, though the observations were made in England. The work of Hanson and Webster (1936) in Washington, though less complete, likely reflected American and Canadian conditions. Lewis and Sturgeon (1978) gave important infomation on egg ecology.
The larvae damage the pea crop by tunneling into the pod and feeding on one or more seeds in each pod. Often the larva does not consume the entire seed before attacking another. Silken webbing and fecal material also are found in the pea pod. In addition to yield loss caused by destruction of pea seed, and quality loss caused by contamination of undamaged seed with insect-damaged seed, additional loss may occur when the larvae contaminate peas harvested for processing (freezing or canning). The latter problem is minimal with this insect because it is not hidden within the pea seed.
Damage potential varies with intended use of the peas. Pea moth damage is of little consequence for peas grown as livestock food because the quality standards are low. For processing peas, the damage potential is significant, but commercial processors have established procedures to eliminate damaged peas and insects. Pea grown for seed is another instance where damage potential is great, because even a small level of damage, 1.0-1.5%, is sufficient to inflict loss to farmers due to downgrading of the seed quality.
Sampling. In England, a great effort has gone into refinement of a pheromone-based monitoring system to farmers of pea moth problems and to time insecticide applications carefully. At least two pheromone traps are placed in each field, and when 10 or more moths are collected in either trap for two consecutive nights the farmer can be assured that moths are present in sufficient number to warrant control measures (Gratwick, 1992). After consulting weather data and estimating the time of egg hatch, farmers can apply insecticide to kill first instar larvae at hatch. Usually a delay of 10-15 days is recommended between initiation of sustained moth catches and application of insecticide to allow for oviposition and egg hatch. This system has found good acceptance in England (Wall et al., 1987; Wall, 1988).
Release of large quantities of the sex pheromone can also be used to disrupt chemical communication in pea moth and disrupt mating (Bengtsson et al., 1994). This potential crop protection system is not yet operational.
Insecticides. Insecticides are applied to foliage to kill first instar larvae before they enter the pea pod, so timing is critical. This is accomplished most easily with determinant pea varieties, because the synchrony of fruiting allows growers to protect pods with fewer applications of insecticides. With indeterminant varieties, or when planting of determinant varieties is staggered, more applications are required to protect the crop from damage. If crops remain in the field in June, they may require two insecticide treatments at about 10 day intervals to protect them adequately, though early crops may escape injury. Application of granular formulations of systemic insecticides is reported to be less satisfactory than foliar application (Thompson and Sanderson, 1977).
Cultural Practices. Several cultural practices help reduce damage from pea moth (Wright and Geering, 1948). Tillage can destroy the larvae overwintering in the soil. Disking the soil twice is commonly recommended, especially if the field, or a nearby field, is to be planted to peas during the following growing season. Destruction of wild vetches is desirable to decrease the potential of pea moth to develop on these alternate hosts. Thick vegetation, either in the form of leafy pea varieties or weedy vegetation, exacerbates pea moth problems, probably by providing shelter for the moths. Thus, weeds should be destroyed and pea varieties with minimal foliage should be favored.
Peas that mature early may also escape attack, so early planting or selection of early maturing varieties is desirable. Crops that are harvested before mid-June may escape attack. It is also beneficial to either pick all peas or to quickly destroy remaining pods after harvest, as this can minimize successful development of pea moth in the field. Peas that are grown for livestock food seldom receive insecticide applications, so they may serve as a source of infestation. Cultivation of fresh market and processing peas at a considerable distance away from livestock peas is suggested.
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