Epilachna borealis Fabricius Coleoptera Coccinellidae

Natural History

Distribution. Squash beetle is reported to occur throughout much of the eastern United States from Massachusetts to Kansas in the north and from Florida to central Texas in the south. However, its impact as a pest is generally limited to the Atlantic Coast, from Connecticut to Georgia. It appears to be a native insect. A related and similar-appearing species, E. tredecimno-tata (Latreille), feeds on cucurbits throughout Mexico and Central America, and has been found in western Texas, and southern New Mexico and Arizona.

Host Plants. Squash beetle larvae feed only on cucurbits, and successful larval development has been reported on various squashes, cucumber, watermelon, cantaloupe, and gourds. In addition, wild cucurbits can serve as hosts. Underhill (1923) indicated that prickly cucumber, Echinocystis lobata; and one-seeded bur cucumber, Sicyos angulata, were readily attacked in Virginia. Adults are less restrictive in their diet; in addition to cucurbits, they have been observed to feed on the blossoms and pods of lima beans and cowpeas, on lima bean foliage, and on fresh corn silks.

Natural Enemies. Because this insect is a relatively minor pest, it has not been thoroughly studied, and therefore its natural enemies are not well-known. However, general predators such as stink bugs (Hemi-ptera: Pentatomidae) and assassin bugs (Hemiptera: Reduviidae) are known to attack larvae. Underhill (1923) reported some evidence of attack by tachinids (Diptera: Tachinidae), and he further noted that 2533% of eggs were sometimes consumed by predatory ladybeetles (Coleoptera: Coccinelliae) and lacewings (Neuroptera: Chrysopidae). Smith (1893) noted that squash beetle larvae frequently attack eggs, so cannibalism may be an important mortality factor.

Life Cycle and Description. Squash beetle can complete its life cycle in 25-48 days, but the average is 32 days. Two generations are produced per year in Virginia, but only one in Connecticut (Britton, 1919).

  1. The yellow eggs are elongate, measuring about 1.8 mm long and 0.7 mm wide. They are laid on end in clusters averaging 45 eggs per mass (range 12-60 eggs). Females deposit eggs at 4-5-day intervals, and generally produce about 300-400 eggs during their life span. Eggs hatch 7-8 days after oviposition.
  2. The larvae are yellow and have six rows of large spines running the length of the body; the spines

Squash beetle larva.

are often darker or tipped with black. Underhill (1923) indicated that the branching characteristics of the lateral row of spines could be used to distinguish the four instars. The number of branches on each spine was given as 0, 4-7, 10-12, and 16-18 for instars 1-4, respectively. The duration of the larval stage is 16-18 days, with average instar durations of 3.4, 3.2,4.0, and 6.0 days for instars 1-4, respectively.

  1. Pupation occurs on the underside of cucurbit leaves, or on adjacent weeds, often in a shaded location. The pupa is about 9 mm long, yellow or orange in color, with branched brown spines attached to the old larval covering and clustered near the tip of the abdomen. Duration of the pupal stage is about seven days.
  2. The adult is 8-10 mm long, with the females averaging larger than males. Beetles are yellow to dull red, with 12 large black spots on the elytra and four small black spots on the thorax. The spots on the elytra are arranged in three transverse rows. Adults begin production of eggs about eight days after emergence. Beetles normally abandon cucurbit fields in August or September, seeking shelter under plant debris or structures in preparation for overwintering. Cracks in the rough bark of trees are especially pre-
Abdomen Epilachna
Squash beetle pupa.
Coleoptera Coccinellidae
Adult squash beetle.

ferred as a resting site for overwintering though they may also seek shelter under leaves and pine straw. Beetles seem not to penetrate more that 12-15 m into forested areas, preferring trees near the edge. In Virginia, they move back into the cucurbit fields in May or June. After overwintering, they may live up to several weeks without food while awaiting the availability of cucurbits. The beetles usually begin egg deposition within two weeks of feeding, so eggs from the overwintering beetles are abundant in June. The adults from this first generation begin to lay eggs in July, producing the second generation in August. By late summer, the overwintering beetles have died, but both first and second generation adults are available to overwinter.

Underhill (1923) gave a complete description of squash beetle biology, and the report of Brannon (1937) is informative.

Damage

The larvae and adults feed on the underside of cucurbit foliage, though adults may also feed on the upper surface. Larvae make a circular cut in the leaf before consuming the lower layer of epidermis within the circle. The circular cut or trenching behavior is believed to inhibit production of sticky phloem sap by the plant, which inhibits feeding (McCloud et al., 1995). The plant tissue is not removed in a conventional manner, but crushed between the jaws, and the remnants left in ridges. Adults sometimes feed on the rind of the fruit. Squash beetle feeding behavior is similar to Mexican bean beetle, but these two species are unusual among the family Coccinellidae, which is normally regarded as a very beneficial group of insect predators.

Management

Insecticides. Squash beetle is relatively easy to kill with foliar insecticides, though usually this species does not warrant control.

Cultural Practices. Squash beetles are not strong fliers, so relocation of cucurbit fields away from overwintering sites is recommended. Prompt destruction of cucurbit vines in the late summer may kill many of the late-developing larvae and pupae, reducing the number of beetles that will overwinter successfully. Row covers should effectively disrupt their life cycle.

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