Fungal spores are released from the fuzzy remains of parasitzed cloverworms - Photo: Kansas State University
Natural controls are numerous
Beneficial insects and diseases commonly regulate green cloverworm populations below economic injury levels in most areas where soybeans are grown.
The green cloverworm is considered a good food source for a number of predatory insects. In parts of the deep South, green cloverworms are ignored or are sometimes considered beneficial because low populations are thought to serve as a food source for predators that later help hold more serious defoliators in check.
Dense cloverworm populations and moist conditions help encourage epidemics of a fungal pathogen that can eliminate even threatening population in a few days, without the use of insecticides. The epidemic spreads from mummified remains of parasitized larvae on the ground or up in the foliage. Large numbers of spores are released from the bodies and infect other nearly larvae, frequently leading to a total population crash in a period of days.
How soybeans compensate for leaf loss
Most entomologists recommend using leaf defoliation as the best gauge for making management decisions, especially in fields with multiple insect species feeding on the leaves. Leon Higley, editor of the Handbook of Soybean Insect Pests , wrote the following description of soybean and leaf loss:
"Unlike many crop species, soybean has a remarkable capacity to withstand much insect injury without significant yield loss. It accomplishes this by both tolerating and compensating for injury. For example, soybean plants can tolerate large levels of leaf feeding without ill effect. Yield losses are prevented because soybean plants typically produce excess leaves."
"In addition, when leaf loss becomes too great, plants can help compensate for losses by retaining older leaves and maintaining high levels of photosynthesis. Soybean also can compensate for stand losses. Usually, gaps in soybean stands are filled by additional growth and branching of the remaining plants. In this way, soybean yields are maintained despite substantial reductions in plant population. These tolerance and compensatory traits of soybean reduce the need for pesticides or other management tactics in many situations."
Reductions in yield from insect feeding can occur during any crop stage, but the pod-forming and pod-filling stages are the most sensitive. In general, consider treatment of soybean only if leaf-feeding insects are present and defoliation reaches 20 percent in the pod-forming and pod-filling stages.
The economic threshold during the vegetative stage is much higher, generally 35-40% or in some cases even 50-60%. According to Marlin Rice, at Iowa State University, a 40 percent leaf loss during any vegetative stage will result in only a 3-7 percent yield reduction. Defoliation of 20 percent during the pod-forming and pod-filling stages will result in similar yield reductions.