William E. Snyder, Department of Entomology, Washington State University - Pullman
This article examines the biology and management of squash bugs in organic farming systems.
The squash bug (Anasa tristis; Fig. 1) is a major pest of cucurbit crops in North America (Worthley, 1923). The large, distinctive adults (Fig. 1A) overwinter in residue from the previous year’s cucurbit crops, or in other debris in or near the field. For this reason, crop rotation can contribute to controlling the squash bug. Movement of adult squash bugs begins and the spring and continues throughout the summer (Palumbo et al., 1991). The adults feed on all aboveground plant parts using piercing-sucking mouthparts, sucking sap and disrupting the flow of nutrients and water through the plants tissues. The adults lay clumps of distinctive bronze-colored eggs on the plant foliage (Fig. 1A) which hatch into whitish colored nymphs (Fig. 1B) that feed like the adults (Fig. 1C). Adults lay eggs continuously through the growing season so all stages can occur on plants at the same time. Squash bugs only complete one generation each summer farther north in the U.S., but may sometimes complete two generations farther south (Fargo et al., 1988; Decker and Yeargan, 2008).
Figure 1. Squash bug (A) hatching egg mass, (B) older nymph and (C) adult. Photo credit: Bill Snyder, Washington State University.
Squash bugs usually damage cucurbits through their direct feeding on the plant. Eventually, feeding damage by heavy squash-bug infestations can cause vines to wilt, turn black, and die. These symptoms superficially resemble bacterial-wilt disease that is transmitted by cucumber beetles but are caused by insect feeding rather than a pathogen. However, in recent years some parts of the U.S. have reported cucurbit damage due to cucurbit yellow vine disease, which is caused by a bacterium (Serratia marcescens) that is transmitted by the squash bug (Bruton et al., 2003; Pair et al., 2004). Plants infected with the yellow vine pathogen exhibit stunted growth, wilting, and yellowing foliage.
Figure 2. Predators attacking squash bugs include (A) ground beetles and (B) damsel bugs. Photo credits: (A) Bill Snyder, Washington State University; (B) Ken Yeargan, University of Kentucky.
Adult squash bugs are relatively large and release an odiferous chemical when disturbed. For these reasons there are few known predators of adult squash bugs. The hard eggs of the squash bug resemble seeds and are heavily preyed upon by ground beetles (Fig. 2A) that otherwise feed mostly on weed seeds (Snyder and Wise, 2001). In test plots in Kentucky, predation by ground beetles reduced squash bugs on yellow crookneck squash and increased squash yields. So, these predators have the potential to exert considerable control. The nymphs are also fed upon by predatory damsel (Fig. 2B) and big-eyed bugs (Snyder and Wise, 2001; Rondon et al., 2003; Decker and Yeargan, 2008). Predation of squash bugs can be increased by employing farmscaping strategies that conserve predators.
A tachinid fly parasitoid, Trichopoda pennipes, attacks nymphs and adults of the squash bug (Worthley, 1923). The female fly lays an egg on the underside of large squash bug nymphs or adults; the larva feeds and develops inside the bug and eventually kills it. Parasitism rates by the tachinid fly can approach 100% in particular fields in particular years (Pickett et al., 1996). Unfortunately, these high rates of parasitism are usually seen only later in the season, after squash bugs have done most of their damage (Decker and Yeargan, 2008). Worse still, parasitized squash bugs continue to feed normally until their death. Several parasitoid wasps (in the families Encyrtidae and Scelionidae) attack squash bug eggs (Olson et al., 1996). However like the parasitoid fly, high egg parasitism rates generally are only seen very late in the season (Decker and Yeargan, 2008). So, both adult and egg parasitoids may be most important in lessening the year-to-year buildup of squash bugs on a farm, rather than achieving pest control within any single year.
There is little evidence that organic-approved insecticides are very effective against squash bugs (see below), so cultural controls may be the best option for many organic farmers. Cultural controls include crop rotation, good field sanitation, careful variety selection, the use of transplants rather than direct seeding, row covers, mulching for predator conservation, and perimeter trap cropping.
Squash bugs often overwinter near the previous year's; cucurbit crop, so one way to reduce pest problems the next year is to plant cucurbits as far away from last year's crop as possible. However, the bugs are strong fliers, and crop rotation alone is unlikely to completely control squash bugs.
Removing crop residue at the end of each growing season can deny squash bugs overwintering sites. This can be accomplished by tilling debris into the soil, or by gathering and hot-composting the residue.
Squash bugs are able to survive much better on some cucurbit varieties (e.g., pumpkin, crookneck squash, and watermelon) than others (e.g., cucumber) (Bonjour and Fargo, 1989; Bonjour et al. ,1990, 1993), so, where markets will allow, avoiding the most susceptible species/varieties can help reduce squash bug problems. Bauernfeind and Nechols (2005) reported differences in relative resistance of cucurbit varieties to squash bug feeding, with varieties ranking, from least to most damage and yield loss, as follows: Butternut, Royal Acorn, Sweet Cheese, Green Striped Cushaw, Pink Banana, and Black Zucchini.
Seedlings and small plants are most susceptible to squash bug feeding damage. Using transplants avoids exposure to squash bug feeding during the most susceptible plant stages. This also reduces the total time that cucurbit plants are in the field each season, providing less time for squash bug densities to build.
Floating row covers provide the most reliable defense against squash bugs, when left in place until flowering begins. Rrow covers must eventually be removed to allow bees and other pollinators to visit the flowers. Disadvantages of row covers include their high cost and the fact that they block access to the crop for weeding. Mulches may be used with floating row covers to reduce weed problems.
Straw mulch can help control squash bugs by providing cover and alternative prey for ground beetles, which are key pests of squash bug eggs (Snyder and Wise, 1999, 2001). However, mulches also provide cover for squash bugs themselves (Cranshaw et al., 2001), so the indirect benefits of mulching due to greater biological control may not always offset direct benefits to the squash bugs themselves. However, straw mulches serve other purposes in the production system as well, such as weed control, water conservation, and soil protection.
Generally speaking, vigorous older plants appear able to tolerate squash bug damage at fairly high pest densities, without a reduction in yield (e.g., Edelson and Roberts, 2005).
Ringing the main crop with a highly-attractive trap crop appears to be an effective way to control another devastating cucurbit pest: cucumber beetles. One study (Pair, 1997) indicates that this approach may also be effective for squash bug control.
Field trials have reported only spotty success using organic-approved insecticides to control squash bugs, and chemical options may be relatively limited. Organic-approved neem and pyrethrum formulations may provide some control.
NOTE: Many of the controls described on these links are NOT APPROVED FOR USE IN CERTIFIED ORGANIC FARMING, although the details of local squash bug biology are relevant to organic farming systems.
IMPORTANT: Before using any pest control product in your organic farming system:
- read the label to be sure that the product is labeled for the crop and pest you intend to control, and make sure it is legal to use in the state, county, or other location where it will be applied,
- read and understand the safety precautions and application restrictions, and
- make sure that the brand name product is listed in your Organic System Plan and approved by your USDA-approved certifier. If you are trying to deal with an unanticipated pest problem, get approval from your certifier before using a product that is not listed in your plan—doing otherwise may put your certification at risk.
Note that OMRI and WSDA lists are good places to identify potentially useful products, but all products that you use must be approved by your certifier. For more information on how to determine whether a pest control product can be used on your farm, see the article, Can I Use This Input On My Organic Farm?
This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.