Melittia calabaza Duckworth and Eichlin Lepidoptera Sesiidae

Natural History

Distribution. Squash vine borer, M. cucurbitae, is found throughout the United States east of the Rocky

Mountains, and is also known from Central America. In Canada it occurs in southern Ontario. In the southwest, M. cucurbitae is replaced by M. calabaza (Eichlin and Duckworth, 1988). This latter species is poorly studied, but seems to have a biology similar to M. cucurbitae; the only known exception is host range, as noted below.

Host Plants. Squash vine borer feeds on wild and cultivated species of Cucurbita. Summer squash, Cucurbita pepo, and some winter squash, C. maxima, are most preferred for oviposition by adults, and most suitable for larval development. Some pumpkin, C. mixta, are intermediate in suitability. Winter squash derived from C. moschata does not support complete larval development. The weeds C. texana and C. andreana, are very suitable hosts, while C. okeechobeen-sis is intermediate and several cucurbit weed species are unsuitable (Howe and Rhodes, 1973). Other cucurbit crops such as cucumber and melon may be attacked, but this occurs mostly when the more favored hosts are not present (Friend, 1931).

Southwestern squash vine borer, M. calabaza, while generally having a host range similar to M. cucurbitae, was able to complete larval development in C. moschata and was not found in C. mixta (Eichlin and Duckworth, 1988).

Natural Enemies. The natural enemies of squash vine borer are not well-studied. General predators such as robber flies (Diptera: Asilidae) are reported to prey upon adults, and a wasp egg parasitoid, Tele-nomus sp. (Hymenoptera: Scelionidae) is known. Larvae seem to be relatively free of natural enemies (Friend, 1931).

Life Cycle and Description. The squash vine borer completes its life cycle in about 60 days. In the southern states it has two generations per year. In New York, New Jersey and other cold climates only a single generation occurs regularly. In Ohio, most borers diapause after the first generation, but some go on to complete a second generation (Smith, 1893). There has been considerable confusion concerning the number of generations in northern states, because in the field it is difficult to discern a single protracted generation from two overlapping generations. In North Carolina, the first generation is reported to occur from April to June and the second from July to September (Smith, 1910). In Connecticut, squash vine borer may not be observed until June or July and persist until September (Friend, 1931). Thus, each generation may remain active for two or three months.

  1. Oval eggs, reddish brown in color, are glued to the plant tissue and are slightly flattened at the point of attachment. They measure about 1.1 mm long and
  2. 85 mm wide. Each female is capable of producing 150-200 eggs. They are distributed singly on all parts of the plant except the upper surface of the leaves; most appear to be attached to the basal region of the vine, probably because this tissue is oldest and therefore exposed longer. The basal part of leaf petioles receives a moderate number of eggs. Some eggs are deposited in cracks in the soil near the base of the plant. Eggs hatch in 10-15 days and larvae soon bore into the plant. Those that hatch on the vine usually remain in this location to feed, whereas those hatching on leaf petioles eventually work their way to the base and enter the vine.
  3. Larvae are white with a dark-brown or black head. Initially, the larval body is markedly tapered posteriorly and equipped with numerous large hairs. As the larva matures it loses its tapered shape and hairy appearance and acquires a dark thoracic shield. There are four instars, with head capsule widths of about 0.4, 0.7,1.3, and 2.0 mm, respectively. The larva requires 4-6 weeks to complete its development and eventually attains a length of about 2.5 cm. The mature larva exits the plant tissue and burrows into the soil to a depth of 4-5 cm where pupation and formation of a black cocoon occurs. (See color figures 27 and 89.)
  4. The pupa is mahogany brown and the anterior end is equipped with a short but sharply pointed cocoon breaker, a structure used to cut a hole in the cocoon when the moth is ready to emerge. There is some disagreement whether squash vine borer always pupates soon after producing the cocoon. Friend (1931), working in Connecticut, reported that the fully grown larva overwintered in the cocoon, and pupated in the spring before producing the single northern generation. In contrast, R. I. Smith (1910) maintained that among two-generation populations in North Carolina the pupa overwintered. In any event, a distinctive, heavy-bodied moth eventually emerges from the cocoon.
  5. The wingspan of the moth is about 3 cm. The front wings are blackish and tinged with olive-green while the hind wings are colorless. The abdomen is usually dull orange and marked with black

Squash vine borer larva.

Squash vine borer larva.

flg&iSHr

Squash vine borer pupa and cocoon.

dots dorsally, though on some specimens the abdomen is entirely black. The hind legs are prominently tufted and are orange and black. Adults are capable of oviposition about three days after emergence. Moths are active during the daylight hours. (See color figure 254.)

Friend (1931) gave a detailed summary of life history. Good rearing techniques for this insect are unknown.

Damage

The damage caused by these insects is accurately described by the common name—squash vine borer. The larva spends almost its entire life feeding within the plant stem. Because larvae feed within plant tissues they are hidden from view and easily overlooked. However, upon close examination frass can be found accumulating beneath small entrance holes. The presence of one, or even several, larvae is not always deadly to the plant. However, up to 142 larvae have been removed from a single plant, and obviously such large numbers can disrupt the physiology of the plant beyond its ability to compensate successfully for the feeding injury. Often the first sign of infestation is wilting of the plant in the heat of the day while other plants remain turgid. If the vine is thin or heavily

Adult squash vine borer.

infested the portion beyond the feeding site of the larva(e) may be killed. Infestation of fruit can occur. Commercial cucurbit production rarely suffers significant damage by squash vine borer. These borers seem to plague small plots especially severely; almost all home gardeners have had some unpleasant experience with this pest.

Management

  1. Shallow pans painted yellow and filled with water can attract and capture squash vine borer moths. Although water pan traps are recommended by investigators in Ontario for population monitoring, it is not known whether such traps are attractive enough to be used as a method of population reduction (McLeod and Gualtieri, 1992). A sex pheromone produced by females has been identified (Klun et al., 1990). Male moths of M. cucurbitae may be attracted to traps baited with the insect's sex pheromone, but thus far pheromone technology has not been demonstrated to improve management practices. In fact, one study reported increased injury of squash plantings when pheromone was released; the biological basis for this is uncertain (Pearson, 1995). M. calabaza also is attracted to some components of the M. cucurbi-tae pheromone.
  2. Squash vine borer can be difficult to control with chemical insecticides, because it is protected from contact with most insecticides by its burrowing activity. Therefore, it is imperative that insecticide residues be in place during oviposition so that when the larva hatches from the egg it has an opportunity to make contact with a lethal dose.

Pollinators, particularly honeybees, are very important in cucurbit production, and insecticide application can interfere with pollination by killing honeybees. If insecticides are to be applied when blossoms are present, it is advisable to apply insecticides late in the day, when honeybee activity is minimal.

Cultural Practices. It has long been known that in the north, early planted crops experience minimal injury, so early planting is recommended. Also, the moths preferentially attack certain species such as summer squash; this can be used as a trap crop if the plants are then destroyed or sprayed, thereby protecting less preferred cucurbit species.

Cucurbit plants are very resilient, recovering well from injury. If accumulated frass can be located adjacent to vine tissue, indicating the presence of active feeding, larvae can be killed or removed with a knife without severely affecting growth of the remaining tissue. Vines also develop additional roots if covered with soil, which can compensate for damage by borers.

Covering the vines near the base of the plant discourages oviposition near critically important tissue. Dying vines should be removed and destroyed (Britton, 1919).

Biological Control. Using a medicine dropper or similar device, the bacterium Bacillus thuringiensis, and the entomopathogenic nematode Steinernema car-pocapsae, can be injected into squash vines to kill existing larvae or to prevent their establishment. The hollow, moist vines are especially conducive to the spread and survival of nematodes.

0 0

Responses

  • Nella
    How to remove squash vine borer adult plagues?
    8 years ago
  • kibra
    How cold can calabasa pumpkin plants survive?
    8 years ago

Post a comment