The squash bug: A major insect pest of pumpkins and squash
September 9, 2009 - Author: Dan Pavuk, Michigan State University Extension
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
The squash bug, Anasa tristis, is a major, widespread insect pest of pumpkin and squash throughout most of the United States, southern Canada and Central America. This insect is also very difficult to control in pumpkin and squash plantings. The key to effective control of the squash bug is early detection and control of the immature, or nymphal, stages of this pest.
Description and life history of the squash bug
Adult squash bugs measure approximately 0.5 to 0.75 of an inch in length, and are dark to grayish brown in color (Figure 1). The back of the adult bug is flattened, and the wings do not cover the orange and brown stripes of the insect’s abdomen. After mating, the female bugs lay eggs in clusters of seven to 20 in rows on the undersides of leaves in the angle formed by two leaf veins (Figure 2). Egg laying usually starts in mid-June in the Midwest and three to four weeks earlier in the southeastern United States. The eggs are about 1/16 inch in length and orange to yellow, but they turn to a metallic bronze color in a few days. Squash bug eggs will hatch in seven to 10 days under typical summer conditions. Newly hatch nymphs are wingless and pale-green to white with reddish-brown heads and legs. As the nymphs grow, they become gray and their wing pads become more and more developed (Figure 3). There are usually five nymphal stages, ranging in size from the smallest (0.1 inch) to the largest (0.5 inch), and the nymphs develop into adults in four to six weeks. All stages of the squash bug may exist on a single plant because the females lay eggs over a long period of time.
Figure 1. Adult squash bug.
Figure 2. Squash bug eggs on squash leaf.
Figure 3. Squash bug nymphs and adults on pumpkin leaf.
Female squash bugs that mature in late July in the Midwest do not mate or lay eggs. As the fall season approaches, adult males and females seek shelter in plant debris located in the field, or move into nearby woods or edges of fields to overwinter. Any nymphs that are present in the late fall die; only the unmated adult individuals overwinter. These overwintering adults begin infesting pumpkin and squash plantings the following spring. The adult stage of the squash bug is extremely mobile and can move quite easily across a planting or even to other fields. The adults spend most of their lives in the plant canopy around the stems or on the undersides of leaves. Nymphs are also secretive, remaining on the undersides of leaves and around the stems of the plants, so it is often difficult to detect a squash bug infestation.
How squash bugs damage plants
Squash bugs are sap suckers; they damage plants by removing sap which causes leaves to wilt and collapse. This insect has a preference for squash and pumpkins rather than other cucurbits. Places on the leaves where the bugs feed develop small, yellow specks that eventually turn brown. If the feeding is intense enough on a leaf, the entire leaf will turn brown and die. If the bugs feed on the vines of the plants, the vines will wilt from the point of feeding to the end of the vine. Leaves on the damaged vine turn brown, then black, and dry up. If large numbers of overwintering adults enter a newly transplanted or emerged field, their feeding can cause severe damage and stand loss. Squash bugs may also feed on squash and pumpkin fruits, and if populations of the bugs are large enough, the fruit may collapse or be unmarketable.
Cucurbit yellow vine decline and squash bugs
The squash bug has recently been identified as the vector of the bacterium, Serratia marcescens that causes cucurbit yellow vine decline in the United States. The bacterium is inoculated into a cucurbit plant by the piercing-sucking mouthparts of the squash bug and enters the phloem of the plant. Disease symptoms usually take four weeks or more to be manifested in infected plants. Because the bug prefers to feed on squash and pumpkin, the disease will ordinarily appear in these plants before it does in watermelon and cantaloupe. Symptoms of yellow vine decline include a general yellowing of the entire vine within a two to three day period. There is no root rot or crown lesion associated with this disease as is found in other types of vine declines. Taking a cross-section of the crown at ground level will show a yellowish-brown discoloration of the phloem tissue. Infected plants usually collapse completely approximately 10 to 14 days before the fruit matures. Plants cannot be rescued once they are infected, so it is important to control squash bugs if they are present in the field.
Squash bug management
Squash bugs are generally difficult to control, especially on larger plants, because the bugs feed on the undersides of the leaves and when the plants are large, the bugs have a very effective shelter from insecticides. Additionally, larger nymphs and adults are less likely to be effectively controlled than are small nymphs. Cucurbit plantings should be monitored throughout the growing season for the presence of squash bugs. The two most critical periods when plants should be protected from these bugs is at the seedling stage and at early flowering. Although large, damaging populations of squash bugs early in the growing season is unusual, occasionally the overwintering adults will become numerous on seedlings and will need to be controlled. One or two applications of a pyrethroid insecticide at this time will provide excellent control of adult squash bugs. The second critical control period during early flowering is the time when squash bugs will typically become a major problem. If the bugs are allowed to increase during early flowering, they will become very numerous, damage plants, and reduce yield. Foliar insecticides become necessary for control of squash bug nymphs if the average number of egg masses per plant, either before or at flowering, is greater than 1 to 1.5 per plant.