The squash bug, Anasa tristis, is a perennial challenge for gardeners growing plants in the cucurbit family, including squash, pumpkins, and melons. These insects can quickly devastate a garden, causing plant damage and significant yield loss. Many gardeners turn to natural solutions, and planting marigolds near vulnerable crops is widely advised. This article investigates the validity of this companion planting technique, examining the pest, the chemical theory behind the claim, and the scientific evidence for its effectiveness.
Identifying the Squash Bug Threat
The squash bug (order Hemiptera) feeds exclusively on cucurbit plants, preferring squash and pumpkins. Adults are large, flattened, dark gray to dark brown insects, measuring about 5/8 inch long. They have alternating orange and brown stripes visible along the edges of their abdomen. They overwinter in garden debris and emerge in the spring to mate and lay eggs.
Females deposit clusters of approximately 20 elliptical, bronze-to-yellowish eggs, typically found on the undersides of leaves in a V-shape along the veins. Nymphs hatch and progress through five developmental stages (instars), transitioning from pale green to grayish-brown as they mature. Both nymphs and adults use piercing-sucking mouthparts to draw sap from the plant tissue. This feeding destroys cells and disrupts the vascular system, causing leaves to develop yellow spots that eventually wilt, a condition sometimes called “squash bug wilt.” Young plants are particularly susceptible to this damage and may quickly die from heavy infestations.
The Marigold Repellent Hypothesis
The belief that marigolds repel squash bugs is rooted in the concept of companion planting. Marigolds (Tagetes species), particularly French marigolds (Tagetes patula), produce strong volatile organic compounds that give them a distinctive, pungent odor. Gardeners theorize this intense aroma acts as a natural deterrent. It is thought to either confuse the squash bug by masking the host plant’s scent or simply repel the insect directly.
The repelling action is attributed to various chemical compounds produced by the plant, including terpenes and thiophene derivatives such as alpha-terthienyl. Alpha-terthienyl is a sulfur-containing compound released by marigold roots that is toxic to soil-dwelling pests. Other compounds, like pyrethrins, are natural insecticides present in some marigold varieties that disrupt the nervous system of certain insects. This chemical profile provides the theoretical basis for marigolds acting as a biological control against above-ground pests.
Separating Fact from Garden Lore
Despite anecdotal support and the chemical theory, scientific evidence does not support the claim that marigolds effectively repel or control squash bugs. Research and university extension publications find that marigolds offer little protection against this specific pest in real-world garden conditions. The compounds marigolds produce, while toxic in concentrated form, are not potent enough in their naturally emitted state to deter large, hard-bodied insects like the squash bug.
Marigolds are scientifically proven to be highly effective in controlling root-knot nematodes, microscopic worms that attack plant roots. The compound alpha-terthienyl, released by the roots, is particularly toxic to these soil-borne pests. This benefit is often mistakenly extended to above-ground insect control. Marigolds also attract beneficial insects like hover flies and parasitic wasps, which prey on other pests like aphids and whiteflies. This indirect improvement in garden health may be misattributed to direct squash bug repulsion, as their reputation against the squash bug appears to be garden lore.
Effective Strategies for Squash Bug Management
Since marigolds are not a reliable defense, gardeners must implement proven, integrated pest management strategies to protect cucurbit crops. Cultural controls begin with sanitation, as adult squash bugs overwinter in garden debris, under rocks, or in mulch. Removing all plant residue and tilling the soil at the end of the season significantly reduces the overwintering population.
Physical barriers are highly effective early in the season. Using floating row covers over young plants prevents adult squash bugs from reaching the seedlings to feed and lay eggs. These covers must be removed when flowering begins to allow for necessary pollination. Mechanical control involves regular scouting and destruction of the pests and their eggs.
Mechanical Control Methods
- Look for the bronze egg clusters on the undersides of leaves and crush them or use duct tape to peel them off.
- Handpicking and dropping adults and nymphs into a bucket of soapy water is effective for small gardens.
- Place wooden boards or cardboard near the base of plants; squash bugs congregate underneath them overnight.
- Lift these traps early in the morning and destroy the collected pests.
For chemical control, organic options like neem oil or insecticidal soap are most effective when applied directly to the vulnerable, young nymph stages. Treatment should be concentrated at the base of the plant. Apply insecticides in the early morning or late evening to protect beneficial pollinators.