Natural Enemies of Aphids

Binodoxys communis Photo credit: Camila Botero, University of Wisconsin

Contributed by Camila Botero, University of Wisconsin

Binodoxys communis is a tiny non-stinging parasitoid wasp that parasitizes and ultimately kills soybean aphids. It belongs to the insect order Hymenoptera (ants, wasps, bees and sawflies), and is in the family Braconidae. Many braconid wasps are considered beneficial because they help control agricultural pests, and they are one of the most important groups of natural enemies of aphids.

Why B. communis?

There are almost always parasitoids that have coevolved with aphids in their native home. In this case, the soybean aphid was accidentally introduced to North America in 2000, and into a completely new environment. Parasitoids and other natural enemies were left behind.

Midwestern researchers have traveled to China, the native home of the soybean aphid, and other parts of Asia to search for natural enemies with potential for biological control. A number of species were identified and brought back to the U.S. where they have been in quarantine. Several years of research have been conducted on the biology, safety, and impacts on non-target aphid species of potential candidates.

So far, B. communis is the most promising species for release. It appears to be very well adapted to soybean aphid and exhibits fairly high levels of host specificity in the laboratory. This means that the chance of this parasitoid disrupting other native species is low and that soybean aphid is its number one target. It was approved for release by the US Department of Agriculture in 2007, and releases were made in summer 2007 and 2008.

Life Cycle of B. communis
Like all parasitoids, B. communis uses an egg-laying organ called the ovipositor to lay a single egg inside each aphid host. When the egg hatches, the legless cream-colored larva starts feeding on the aphid’s blood and organs. As it consumes the aphid from the inside out, the larva grows and develops through several stages ultimately killing its host. Because it feeds internally within the aphid, it is called an endoparasitoid.

The “mummy” is the shell of the dead parasitized aphid. Photo credit: Dan Mahr, University of Wisconsin

B. communis emerging from a parasitized aphid. Photo credit: University of Minnesota

When the parasitoid larva finishes feeding, the only part remaining of the aphid is its empty skin, which appears puffed up, brown and hardened. The “mummy”, as the shell of the dead parasitized aphid is called, is usually formed 6 to 8 days after parasitization, and this is where the larva pupates and transforms into an adult. In the pupating process the parasitoid larva glues the mummy to the plant surface and spins a cocoon inside where it remains protected. Four to six days later, the adult wasp chews a round hole in the aphid shell and emerges from it. After emergence, males seek females for mating and females start seeking suitable aphid hosts to attack.

Females have the potential to lay up to 200 eggs, depending on the temperature, humidity, parasitoid population density, and soybean aphid numbers and quality.  Adults feed only on water, flower nectar, honey dew, or other sugar sources. Many parasitoid species have been observed on floral nectar in fields, and research suggests that crops with nearby flowering vegetation have higher parasitism rates.

The whole process from egg to adult emergence takes between 10 and 14 days, and adults live about 10 days. Since females start reproducing almost immediately after emergence, it takes about 10 to 14 days for an entire generation to be completed. The number of generations per year is not yet known.

B. communis adult Photo credit: KelleyTilmon, South Dakota State University

The overwintering habits of B. communis are also not yet known, and this is currently an area of active research. Scientists think the parasitoid either disperses with the soybean aphid to buckthorn or it uses an alternate aphid host, otherwise it would not survive after soybean is harvested or when it is not available.

Control of soybean aphids by B. communis
Research suggests that B. communis is most effective as a natural enemy under low soybean aphid densities, and as aphid numbers increase, the amount of parasitization decreases. This may be because dense aphid colonies tend to attract predators (particularly Asian lady beetles) which not only prey on soybean aphids but on parasitized aphids and mummies as well. Therefore, B. communis might be most effective in early season when soybean aphid numbers are low, preventing high density aphid colonies from building up in soybean fields.

How to scout for B. communis in the field

B. communis is smaller than a pinhead and usually walks or makes small flights throughout soybean plants searching for suitable hosts. It is difficult to see in the field, so the best and more reliable way to determine its presence is by searching for it's mummies. The advantage of searching for mummies rather than for the adult is that they are easy to see, they conserve their shape, and they stay attached to the plant even after the adult parasitoid has already emerged from it.