The soybean looper (Chrysodeixis includens) is a green caterpillar and one of the most destructive leaf-feeding pests of soybeans in the Americas. It gets its name from the distinctive looping motion it makes when crawling, arching its body upward because it lacks the middle set of legs that most caterpillars have. This pest is notoriously difficult to control because it has developed resistance to nearly every major class of insecticide used against it.
How to Identify the Soybean Looper
The caterpillar is light to dark green with white stripes running along its back and sides. Its body is thicker at the rear end and tapers toward the head, reaching up to 1.3 inches (about 3 cm) at full size. The key identifying feature is the number of fleshy abdominal prolegs: the soybean looper has only two pairs. This is what separates it from lookalike species. The velvetbean caterpillar, for example, which feeds on the same crops and looks similar, has three or more pairs of abdominal prolegs. If you flip the caterpillar over and count, that’s the fastest way to confirm what you’re dealing with. The thoracic legs near the head can be either black or green.
The looping movement itself is also a giveaway. Because those middle prolegs are missing, the caterpillar pulls its rear end forward to meet the front, forming an upside-down “U” shape before extending forward again.
Life Cycle and Development
Female moths lay eggs at night, placing them on the undersides of leaves in the lower half of the plant canopy. After hatching, the larvae pass through six instars (growth stages), though anywhere from five to seven has been observed. The first instar lasts 3 to 4 days. Most middle instars take 2 to 3 days each, while the final instar stretches to 5 or 6 days. In total, caterpillars feed for about two to three weeks before pupating.
The bulk of the damage happens at the end of larval development. The fourth through sixth instars consume the most foliage, doing the heaviest feeding in just the last 4 to 5 days before pupation. This means a field can look relatively clean one week and show severe defoliation the next as larvae reach their final growth stages.
Feeding Damage and Economic Thresholds
Soybean loopers are leaf feeders. They chew through leaf tissue from the underside, often leaving a thin layer of the upper leaf surface intact at first, which creates a windowpane effect. As larvae grow, they consume entire sections of leaf, and heavy infestations can strip a canopy down to stems and veins.
The economic thresholds for soybean defoliation, supported by replicated data across multiple growing regions, are 30% leaf loss during vegetative stages and 20% during reproductive stages. Those percentages sound high, but soybeans are surprisingly tolerant of leaf loss early in the season. Once the plant is flowering and filling pods, though, every bit of leaf area matters for yield, and that’s when the threshold drops. Estimating defoliation visually takes practice because people tend to overestimate damage. Pulling sample leaves from multiple spots in the field and comparing them to reference charts gives a more accurate read.
Why Soybean Loopers Are Hard to Kill
The soybean looper has developed resistance to almost every older class of insecticide, including organophosphates, carbamates, pyrethroids, and even DDT-era chemicals. Permethrin, a common pyrethroid, was initially effective but resistance was documented as early as 1987. One reason resistance spread so quickly is the cropping systems themselves. In areas where cotton and soybeans are grown together, heavy pyrethroid use on cotton unintentionally selected for resistant soybean looper populations that then moved into adjacent soybean fields.
The resistance mechanisms are biological. The insects produce elevated levels of detoxifying enzymes (particularly a family called cytochrome P450s) that break down pyrethroids before they can do lethal damage. This enzyme-based resistance is inheritable, so surviving moths pass the trait to the next generation.
More recently, there are growing concerns about resistance to diamide insecticides, which had been a reliable option. Populations in Puerto Rico have already shown considerable resistance to one widely used diamide. Research has also found that resistance to one chitin inhibitor (a class of insecticides that disrupts molting) confers high cross-resistance to other chitin inhibitors, further narrowing the chemical toolbox. The dependence on a shrinking number of effective insecticide groups across the southern United States is a serious challenge for growers.
Natural Enemies and Biological Control
Soybean loopers have a rich complex of natural enemies that can suppress populations significantly under the right conditions. Surveys in soybean fields have identified at least nine parasitoid species attacking this pest, with a tiny wasp called Copidosoma truncatellum being the most common. These parasitoids lay their eggs inside looper larvae or eggs, and the developing parasitoid eventually kills its host.
Diseases are even more impactful. In one field survey, a naturally occurring virus (a nucleopolyhedrovirus, or NPV) was found infecting 63.7% of soybean looper larvae. Two fungal pathogens were also present, infecting 27.3% and 9% of larvae respectively. When conditions are warm and humid, these pathogens can cause dramatic population crashes without any spraying.
A species-specific viral product based on this NPV has been developed as a biological insecticide. Known as ChinNPV, it works when caterpillars eat virus particles on treated leaves. Because the virus targets only soybean loopers, it leaves beneficial insects unharmed, an important advantage in integrated pest management programs where you want parasitoids and predators to keep working.
Bt Soybeans
Genetically modified soybeans that produce an insecticidal protein from the bacterium Bacillus thuringiensis (Bt) offer another layer of control. The Bt soybean variety MON 87701 × MON 89788, which produces the Cry1Ac protein, effectively controls soybean loopers and velvetbean caterpillars. It is not effective against armyworm species (Spodoptera), so it’s not a silver bullet for all caterpillar pests, but it does take two of the most damaging defoliators off the table. Bt soybeans are more widely grown in South America than in the United States, where their adoption has been slower.
Scouting and Practical Management
Because the heaviest feeding happens in the final days of larval development, early scouting is critical. By the time defoliation is obvious from the field edge, the damage is already done. Check the lower canopy, where eggs are laid and young larvae begin feeding. Shake plants over a drop cloth or beat sheet to dislodge caterpillars, and count them to gauge population density.
Given the widespread resistance to older chemistries, rotating between insecticide classes with different modes of action is essential for preserving whatever tools still work. Pairing chemical control with conservation of natural enemies, using selective products like ChinNPV when possible, and planting Bt varieties where available gives the broadest and most durable protection. The soybean looper’s long track record of evolving resistance means that any single tactic used in isolation will eventually fail.