By the wwai Editorial Team
Just when everyone expected the cool, pleasant days of autumn to bring fewer bugs, Beijing and other cities suddenly found themselves overrun with caterpillars — white, fuzzy, and frankly creepy. These caterpillars not only chew through leaves, they drop from the trees in droves, leaving a lasting psychological scar on anyone who walks underneath.
But this caterpillar is no ordinary pest. Beyond terrifying residents, it is a headache for forestry and agriculture workers alike. Its name: the notorious fall webworm (Hyphantria cunea).
A Troublemaker on the Move — and How to Subdue It
The fall webworm is a malignant invasive insect native to North America. It eats almost anything, has a huge appetite, reproduces rapidly, and wreaks havoc wherever it goes. In China, it feeds on more than 100 plant species, including crops, garden plants, flowers, and vegetables — a serious blow to agricultural and forestry production as well as everyday life. The damage it leaves behind is so destructive that it is known as the "smokeless fire."
Since arriving in China in 1979, the fall webworm has kept expanding its range. Forestry researchers have fought it for decades, trying to slow its spread or wipe it out entirely. Years of control efforts have slightly eased the trend, yet in some autumns the population still rebounds in many regions.
Faced with another outbreak, local authorities have stepped up countermeasures — one of which is spraying large amounts of traditional chemical pesticides such as emamectin benzoate, cypermethrin, and deltamethrin. Chemical control is fast and relatively cheap, but it inevitably brings problems of its own: pesticide resistance, residues, and more. Biological control may be a stronger weapon.
Simply put, biological control means using one organism to control another harmful organism. The former is called a biological agent. Common agents include natural-enemy insects and pathogenic microorganisms. When it comes to the fall webworm, the natural enemy to know is the parasitoid wasp Chouioia cunea (white webworm Chouioia wasp).
No Natural Enemies at Home, So the Moth Runs Rampant
In its native North America, the fall webworm has a long list of natural enemies — at least 86 species of predators and parasitoids have been recorded. In Louisiana alone, 16 insect species and 19 spider species feed on fall webworm larvae. Surveys in Illinois found that 34–39% of fall webworm populations were parasitized by native enemies, with slightly more (43%) infected by pathogenic microorganisms.
These native enemies share a long co-evolutionary history with the fall webworm, forming a relatively tight and balanced food chain and food web. Under this natural regulation, North American populations rarely erupt widely and repeatedly over long periods.
But when the fall webworm arrives in a new habitat, the natural enemies it co-evolved with are missing, so its population dynamics are no longer constrained. The balance is broken, and the moth expands and erupts far more easily in its "new home." This phenomenon — known as enemy release — is one of the leading hypotheses in ecology for explaining invasive species outbreaks.
A "Three-High" Wasp: The Fighter Among Fighters
China has spent over 20 years searching for and screening biological agents against the fall webworm. Chinese entomologists have found 37 species of insects that parasitize or prey on it. Among them, the most famous is Chouioia cunea.
Chouioia cunea belongs to the family Eulophidae (order Hymenoptera). Of the 37 candidate natural enemies, only this one stood out, gained wide application, and entered industrial production. The reason comes down to three striking traits, nicknamed the "three highs": high parasitism rate, high offspring output, and a high female-to-male sex ratio.
The earliest study reporting this wasp found that of more than 400 fall webworms collected from the field, over 340 were parasitized — a parasitism rate above 80%. In laboratory rearing, each parasitized pupa hatched more than 200 wasps on average, and the emerging wasps had a sex ratio of 68 females to 1 male.
The higher these three indicators, the better Chouioia cunea works as a control agent. Parasitism rate and offspring output are easy to grasp — they win on probability and numbers. The sex ratio refers to the proportion of female to male offspring, and it may be one of the most important metrics for evaluating a parasitoid's effectiveness. Here is why:
Chouioia cunea is a pupal parasitoid. Males only mate; it is the females that quickly locate mature fall webworm larvae and pupae and lay eggs inside them to continue the cycle. Once those eggs hatch, the larvae feed on the webworm pupa. To destroy more webworms, the more females in the next generation, the better.
Further rearing data shows the average sex ratio of Chouioia cunea is 44:1, reaching as high as 96:1. Such an extreme ratio is rare among parasitoid wasps — a trait that benefits pest control itself and also makes
