Life history and habits

Squash bugs overwinter as adults under plant debris near previously infested plants, with mild winter temperatures allowing a greater proportion of overwintering adults to survive. Overwintered adults resume activity during warm periods of spring, and cooler temperatures can delay adult emergence. As they leave overwintering sites, adults move to germinating squash to begin feeding and mating. Overwintered adults can harbor the bacterial plant pathogen Serratia marcescens that causes cucurbit yellow vine disease (CYVD). Like stink bugs (Hemiptera: Pentatomidae), squash bugs have glands that secrete an unpleasant odor when the insect is disturbed. Egg laying begins in June, roughly 7-10 days after the emergence of overwintered adults. Eggs are laid in clusters ranging from several to over 40 eggs, usually on the underside of leaves near the leaf veins. This species prefers to lay eggs on Cucurbita spp., primarily summer squash and pumpkin. In addition, survival tends to be higher on squash and pumpkin than other cucurbits such as cucumber (Cucumis sativus) and muskmelon (Cucumis melo). As eggs are laid, the adult female also deposits an adhesive secretion which adheres the group of eggs to the leaf surface. Nymphs develop through five instars. This species completes its life cycle in about 6-8 weeks, depending on temperature. Warm temperatures during the growing season allow most of the second-generation nymphs to successfully reach the adult stage. Two generations are produced annually in Colorado.

Injury

All hemipteran species have piercing-sucking mouthparts, and both nymphs and adults feed by sucking sap from plants. Most cucurbits are susceptible to attack by squash bugs. The manner of feeding of squash bugs, sometimes described as “lacerate and flush”, kills plant cells and produces many feeding punctures that disrupt the ability of plant tissues to transport water, leading to plant wilting. In Colorado, hard winter squashes are the most susceptible to the squash bug. Less commonly, summer squashes are injured. Related cucurbits, such as cucumber and melons rarely suffer significant injury.

Plants in the 2-4 leaf stage are highly susceptible to feeding when colonized by nymphs. Prolonged feeding can result in a condition known as “Anasa wilt”, followed by leaf necrosis, fruit rot, and death of the host plant. Feeding on the fruit itself can lead to sunken areas and blemishes. These injuries can result in secondary infections of microorganisms such as anthracnose, choanephora fruit rot, gray mold rot, and Rhizopus soft rot. Infections by these microorganisms can be especially problematic in stored fruits. Feeding injuries inflicted during flowering and fruit set reduce fruit yields by more than 50% in untreated fields of summer squash invaded at flowering and fruit set.

Infections of CYVD can cause significant economic losses in watermelon, pumpkin, cantaloupe, and squash. Currently, this disease is most prevalent in Texas and Oklahoma but is spreading to other midwestern and eastern states.

Monitoring

Before treating crops with insecticides, plants should be inspected to determine the density of squash bug adults and eggs. In summer squash, the recommended economic threshold is one adult squash bug per plant or one egg mass per plant. Egg surveys should be done at least once a week during June when egg laying is likely to begin. In watermelons, this threshold is increased to two adult squash bugs per plant. In large-scale growing operations, is recommended that 64 plants be sampled in summer squash and 54 plants be sampled in watermelon. It is worth noting that squash bug densities tend to be greater in summer squash than watermelon.

Recent research has shown that the squash bug is highly attracted to vittatalactone, which is a pheromone produced by another pest of cucurbits in North America, the striped cucumber beetle, Acalymma vittatum (Coleoptera: Chrysomelidae). Although this pheromone has not yet been incorporated into pest management programs for the squash bug, it may be a useful trapping component for multi-species control in cucurbit crops.

Cultural control

Early season management approaches tend to be more efficient. In small garden settings, hand picking squash bugs and destroying eggs can be very effective. Placing wood boards between rows of crops will provide attractive hiding places for squash bugs. The boards can then be stepped on to crush the insects. Removing debris and old squash plants will reduce the availability of overwintering sites for adults. Although crop rotation can help reduce the availability of host plants by planting the next year’s cucurbits as far away as possible from the previous year, squash bug adults are strong fliers, so crop rotation alone is unlikely to provide effective control. Using transplants rather than seeding directly can help compensate for some of the pest exposure since seedlings and small plants are more vulnerable to feeding injury.

Eliminating weeds and organic mulches can reduce pest density. Minimizing the use of plastic mulches can help reduce squash bug infestations by eliminating refuge areas for this pest. However, plastic mulches may provide other benefits for the crop, such as increased weed control, cleaner fruits during harvest, and increased retention of moisture in soil. Therefore, decisions to eliminate plastic mulches should consider the needs of the crop system and is often impractical in cucurbits. Although eliminating row covers can help reduce the densities of adults and eggs in summer squash, the pest population can quickly rebound once the covers are removed. Since cucurbits require pollination for fruit production, this approach is not always recommended. It is worth noting that row covers are typically more effective when combined with mulches.

Since the squash bug prefers squash or pumpkin over other crops, these can be planted as trap crop for cantaloupe and cucumber. The trap crops can be treated with a systemic insecticide to improve the degree of control. Interplanting or companion planting may pose another effective strategy, in addition to the use of certain essential oils of clover, spearmint, lemongrass, rosemary, and geranium, which have shown repellent properties against other insects such as stink bugs. These may be developed into a push-pull strategy in the future.

In areas where this pest is prevalent, it may be worth opting for cucurbits that are less susceptible to infestations. The following varieties are listed in order of least to most susceptible to feeding injury and yield loss: Butternut, Royal Acorn, Sweet Cheese, Green Striped Cushaw, Pink Banana, and Black Zucchini. In addition, pumpkin, crookneck squash, and watermelon support squash bug survival more than cucumbers. The green striped cushaws and Waltham butternut are two winter squashes that are less attractive to this pest when compared with summer squash.

Biological control

It is recommended that resident populations of natural enemies be conserved by adopting practices such as avoiding overreliance on insecticides, providing floral resources for natural enemies such as parasitoid wasps, and planting insectary strips or hedgerows within or along annual crops. Many arthropods prey on the squash bug, including spiders, carabid beetles (Coleoptera: Carabidae), coccinellid beetles (Coleoptera: Coccinellidae), staphylinid beetles (Coleoptera: Staphylinidae), and bigeyed bugs (Hemiptera: Geocoridae). While these predators can help suppress populations of the squash bug, parasitoids are some of the most important natural enemies of this pest, most notably the tachinid fly Trichopoda pennipes (Diptera: Tachinidae), which has a distinct orange abdomen. This fly lays one or more eggs on the exoskeleton of late instar nymphs and adults of the squash bug. At times, parasitism rates up to 100% have been estimated in some areas. In addition to the fly T. pennipes, several species of parasitoid wasp in the families Encyrtidae and Scelionidae parasitize eggs of the squash bug, with estimates of over 60% parasitism rates in some areas. Collectively, the greatest rates of parasitism tend to occur later in the growing season. Therefore, it is likely that parasitic insects provide long-term suppression over multiple seasons, rather than quick knockdown of heavy squash bug infestations. Entomopathogenic nematodes have also shown promise as biological control agents of the squash bug. Laboratory assays have shown that the species Steinernema carpocapsae (Nematoda: Steinernematidae) can kill adults of the squash bug, and that the nymphs of infected females remain infective.

Chemical control

Infestations of the squash bug can be managed with insecticides applied while insects are in the nymphal stage. Foliar applications of pyrethroids or can effectively control nymphs. However, it should be noted that pyrethroids can have negative effects on resident populations of natural enemies and can lead to secondary outbreaks of other pests, most notably aphids. These insecticides are also toxic toward pollinators. To be effective, certain insecticides should be applied early in the season when eggs are first detected. A follow-up application should be made one or two weeks later.

To help address these concerns, other insecticides have been developed such as formulations of benzoylphenyl urea which is an insect growth regulator that appears to be effective against nymphs of the squash bug. Others, such as spinosyn, are not effective against this pest. In organic production, pyrethrins and azadirachtins can suppress densities of squash bug nymphs, but do not appear to consistently provide effective control under heavy pest pressure. Applications of pyrethrins and diatomaceous earth around the plant base can be an effective treatment in organic production systems.

It is important that insecticide labels be read and adhered to. In the case of organic production, the product should be approved by a USDA certifier. Doing so will avoid putting certifications at risk.

Additional reading