The threecornered alfalfa hopper (Spissistilus festinus)—male (A) and female (B) shown above—causes damage to alfalfa, soybean, and peanut in the southern United States, but economic thresholds for management of the insect—and its status as a pest—are unclear.

In fields of alfalfa, soybean, and peanut in the southern United States, a small, green, teardrop-shaped insect is a common sight. But despite its observed propensity for gouging scars in plant stalks, no one is 100 percent sure of how much damage the threecornered alfalfa hopper (Spissistilus festinus) actually causes and whether deploying insecticides to control it is worthwhile.

“It is commonly present in very high numbers in several important agronomic crops, yet very little is known for certain about its impact on those crops,” says Mark Abney, Ph.D., assistant professor of entomology at the University of Georgia. “The adult stage is highly mobile, the nymphs are cryptic, and the damage is indirect. This is a formula for difficult entomology research.”

Abney and colleagues have received funding from the U.S. Department of Agriculture’s National Institute of Food and Agriculture to study and quantify economic thresholds for threecornered alfalfa hopper in peanut. A preliminary result of that work is a new, open-access profile on the biology and management of threecornered alfalfa hopper, published this week in the Journal of Integrated Pest Management.

“As a natural starting point to this project, we reviewed the literature and felt that synthesizing biology, pest status, and management information into a single, widely accessible and understandable publication would be very useful for researchers and pest management professionals,” Abney says.

When viewed from the front (A), adult Spissistilus festinus appears to have three corners, one at each “shoulder” and one at the apex of its pronotum—hence its common name, threecornered alfalfa hopper. From a ventral view (B), the female (left) and male (right) show a somewhat triangular form, as well.

The profile of S. festinus outlines the basic biology, life cycles, and behavior of the insect and the damage it causes via feeding. Threecornered alfalfa hopper feeds on phloem with its piercing-sucking mouthparts, and it regularly creates a ring of punctures around the circumference of a stem, commonly called a girdle. This injury to the plant interrupts nutrient flow and can weaken the structure of the plant, leaving stems prone to breaking.

Here, though, is where existing research does not widely agree on the insect’s impact. While some studies have shown that threecornered alfalfa hopper can negatively impact yield or quality in alfalfa, soybean, and peanut, others say the damage it causes via girdling does not result in economically significant losses in the crops. That’s created a situation in which growers that observe S. festinus often treat their fields with insecticides, even if it may not be ultimately necessary.

The most noticeable damage to crops caused by threecornered alfalfa hopper is “girdling” of plant stems, in which nymphs create a ring of punctures around the circumference of a stem (B). The damage can lead to gall formation and adventitious root growth (A).

The JIPM profile suggests sampling methods for growers to monitor for threecornered alfalfa hopper, particularly its nymphs, which are smaller but responsible for most of the damage the species causes.

“There is no question that scouting is underutilized in peanut and soybean in the southeastern U.S. This is somewhat understandable when one considers that economic thresholds do not exist for threecornered alfalfa hopper or several other plant-feeding insects that are commonly found in peanut,” says Brendan Beyer, lead author of the JIPM profile and a former masters’ student under Abney’s direction (Beyer is now a research assistant at the NSF Center for Integrated Pest Management at North Carolina State University). “Nevertheless, even a basic real-time knowledge of insect abundance would give producers the information they need to make more informed management decisions. We are hopeful that, as thresholds are developed and promoted, growers will see the value in monitoring pest populations and that the prevalence of scouting will increase.”