Aphid parasitoids are beneficial insects that serve as natural pest control in gardens. Unlike a parasite, which lives on its host without causing death, a parasitoid’s life cycle is linked to the demise of its host. The adult parasitoid, a small wasp, lays its eggs inside an aphid. The resulting larva consumes the aphid from the inside, a process that kills the host and makes these insects effective for managing aphid populations.
The Parasitoid Life Cycle
The life of an aphid parasitoid begins when a female wasp selects an aphid host. She uses her ovipositor, a specialized organ, to deposit a single egg inside the aphid’s body. The aphid continues to live and feed for a short period. The parasitoid egg hatches in two to three days, and the larva begins to feed on the aphid’s internal tissues and fluids.
As the larva grows, it consumes the host’s organs until the aphid is killed. After about a week, the larva glues the aphid’s empty exoskeleton to a leaf or stem. Inside this protective casing, the larva pupates, transforming the dead aphid into a hardened, bronze or tan-colored shell known as a “mummy.”
The aphid mummy is the most conspicuous sign of parasitoid activity. After nine to 23 days of development, depending on temperature, the adult wasp chews a circular hole in the mummy and emerges. The wasp is then ready to mate and continue the cycle by finding new aphids to host its offspring. This exit hole is a clear sign that a new beneficial wasp has joined the garden’s ecosystem.
Common Types of Aphid Parasitoids
Many wasps act as aphid parasitoids, with many having a preference for specific aphid species. Among the most common are the braconid wasps, particularly those from the Aphidius genus. These are tiny, slender wasps that are specialized aphid hunters. Aphidius colemani is frequently used to control species like the green peach aphid and the cotton aphid.
Another species, Aphidius ervi, targets larger aphids, such as the potato aphid and pea aphid. This host specificity is an important aspect of their biology, as the success of a parasitoid is tied to the availability of its preferred aphid. This specialization means identifying the type of aphid in a garden can help determine which natural enemies might be most effective.
Chalcid wasps from the Aphelinus genus are also common aphid parasitoids. A primary difference in their activity is the appearance of the mummies they create. While braconid wasps produce tan or golden-brown mummies, aphids parasitized by Aphelinus wasps turn black. This visual cue can help gardeners identify the types of parasitoids at work.
Supporting Parasitoids in the Garden
Gardeners can encourage populations of naturally occurring aphid parasitoids by creating a welcoming environment. Adult wasps, unlike their larval stage, do not feed on aphids. Instead, they require nectar and pollen as an energy source to fly, mate, and locate new hosts. Planting flowers with small, shallow blossoms provides an accessible food source.
Plants in the carrot and aster families are particularly effective because their composite flower heads offer numerous tiny, nectar-rich flowers. A continuous bloom of such flowers throughout the season ensures a reliable food supply, which can increase the wasps’ longevity and reproductive success. Good choices include:
- Dill
- Fennel
- Cilantro
- Parsley
- Yarrow
- Cosmos
- Chamomile
An important practice for supporting parasitoids is avoiding broad-spectrum chemical pesticides. These insecticides are indiscriminate, killing beneficial insects just as readily as pests. Tolerating a small population of aphids is also necessary. Without aphids to serve as hosts, the parasitoids cannot complete their life cycle and will either leave the area or die out.
Using Aphid Parasitoids for Pest Control
For a significant aphid problem, purchasing and releasing commercial aphid parasitoids can be an effective strategy. These beneficial insects are available from online suppliers and garden centers. They are shipped as a collection of aphid mummies, which are in the pupal stage. This ensures the wasps emerge as adults ready to begin their work upon arrival.
The release process is straightforward. The container of mummies should be placed in a shaded location, protected from direct sun and harsh weather, near the plants with the heaviest aphid infestations. Simply open the container, allowing the adult wasps to emerge over several days and disperse to find hosts.
Timing the release is an important factor for success. It is best to introduce parasitoids early in an infestation, before aphid numbers become overwhelming. Releasing them when the pest population is low allows the parasitoid population to establish itself and keep pace with the aphids’ reproductive rate. A single female wasp can lay eggs in about 100 aphids, making a well-timed release an effective preventative measure.