Hosts or Prey
Identification
The most conspicuous evidence that C. medicaginis is present is its golden to yellowish or white cocoon about 1/6 inch (4 mm) long that occurs near the leaf-chewing damage of the alfalfa caterpillar. A dead caterpillar with an emergence hole and a crooked posture may be found near the parasitoid cocoon.
The adult C. medicaginis wasp is about 1/8 inch (3 mm) long. It has a black head and body and black and brown to dark yellowish legs. The adult's beadlike antennae are black and about 1/8 inch long.
Larvae are white maggots up to 1/4 inch (6 mm) long and occur inside host caterpillars. A caterpillar parasitized by C. medicaginis may be recognized by having a portion of its surface more pale green than normal and somewhat shiny, unlike the darker green, velvety covering of an unparasitized alfalfa caterpillar. Parasitized caterpillars also stop feeding and can be swollen toward the rear end. Pulling the head off a caterpillar then gently pressing and squeezing the body starting from the rear and working forward can confirm parasitism by pushing out a white larva of C. medicaginis or a related parasitoid.
Life Cycle
Cotesia medicaginis develops through 4 life stages: egg, larva, pupa, and adult. Each female can parasitize about 100 caterpillars. When a female's antennae contact an alfalfa caterpillar, she jumps on top of the prey, inserts a single egg, then jumps off the host, all within seconds. First instar alfalfa caterpillars are the preferred stage for oviposition, but the wasp will also lay eggs in second and third instar hosts. After hatching, the wasp larva develops through 3 increasingly larger instars as it feeds inside the caterpillar.
The mature third instar C. medicaginis exits the caterpillar, during the host's third or fourth instar. It spins a silken cocoon in which it pupates attached to an alfalfa leaflet or stem. The adult wasp emerges by chewing a circular cap out of one end of its cocoon. The circular cap may remain attached to the cocoon by loose outer threads of silk.
Adult wasps feed on honeydew and nectar and live for 2 to 3 weeks. Overwintering may be as a larva inside host caterpillars. Egg to adult development time is about 2 weeks during the summer. Cotesia medicaginis has several generations per year.
Habitat
Cotesia medicaginis occurs throughout the state where alfalfa is grown. It is one of numerous natural enemies important in alfalfa. To increase the biological control of caterpillars and other alfalfa pests
- Control ants because they attack natural enemies of various pests.
- Grow flowering insectary plants to provide nectar to nourish adult wasps.
- Reduce dustiness that disrupts the activities of natural enemies. For example, irrigate overhead and wet nearby dirt roads before driving on them.
- Avoid the application of broad-spectrum, persistent insecticides for all pests. For example, apply Bacillus thuringiensis to control caterpillars without poisoning natural enemies.
See Protecting Natural Enemies and Pollinators and Relative Toxicities of Insecticides and Miticides Used in Alfalfa to Natural Enemies and Honey Bees to learn which practices and pesticides are most compatible with biological control.
Species
Over 1,000 species of Braconidae occur in the United States. Varying by the species, their hosts include larvae of beetles, caterpillars, flies, and sawflies. Alfalfa caterpillar is the principal host of C. medicaginis.
More Information
- Alfalfa Caterpillar Scientific Name: Colias eurytheme, UC IPM Pest Management Guidelines: Alfalfa
- The Biology of Apanteles medicaginis Muesebeck (Hymenoptera: Braconidae), UC Berkeley, Hilgardia
- Some Factors Influencing the Efficiency of Apanteles medicaginis Muesebeck (Hymenoptera: Braconidae) as a Parasitoid of the Alfalfa Caterpillar, Colias philodice eurytheme Boisduval, UC Berkeley, Hilgardia
Scientific classification:
Phylum: ArthropodaClass: Insecta
Order: Hymenoptera
Family: Braconidae
