A Parasitoid Wasp Cotesia congregata (Say) (Insecta: Hymenoptera: Braconidae)1
Cotesia congregata (Say) is a gregarious endoparasitoid that attacks more than a dozen species of sphingid caterpillars and a few known semi-permissive noctuid hosts. Semi-permissive hosts are not fully susceptible to the immunosuppressant polydnavirus associated with Cotesia congregata. Thus, they exhibit an immune response to some of the parasitoid eggs that are oviposited into the hemocoel, and may not always become parasitoid hosts. Its most notable host, especially in the southeastern US, is the tobacco hornworm, Manduca sexta (Linnaeus). This species of Cotesia has been widely used as a model system in insect physiology. It has also been used to examine insect learning in host-parasitoid-plant interactions.
Apanteles (Protapanteles) augustus Viereck, 1917
Apanteles congregatus (Say, 1836)
Cotesia augusta (Viereck, 1917)
Cotesia utilis (French, 1880)
Microgaster congregatus Say, 1836
Microgaster utilis French, 1880
Cotesia congregata Mason 1981
Currently, there are around 400 described species of the genus Cotesia (Shaw and Huddleston 1991). It is estimated that the entire genus comprises 1500-2000 species worldwide (Mason 1981).
Cotesia congregata is widely distributed throughout North America, Central America, South America, and the West Indies (Krombein 1979).
Description and Life Cycle
The eggs are white and wedge-shaped, measuring 0.12–0.16 mm in length and approximately 0.04 mm in diameter towards the larger end (Fulton 1940). During a single oviposition, around 65 eggs are oviposited into the hemolymph of the larval host (range: 5–162) (Lawrence 2010). It has been shown that some larvae can emerge from the egg into the hemolymph in as little as two to three days after oviposition at 29°–30°C (Fulton 1940) (Figure 3).
Larvae are pale yellow-white and grub-like in appearance. The first instar larvae are free moving in the body cavity close to the epidermal layer of the cuticle. They are around 0.4–0.5 mm in size at the time of hatching, and will continue to grow up to 2.4 mm in length by the end of the first instar (8 days). The first instar larvae also have a short caudal appendage (tail), small curved mandibles (0.04 mm), and eleven body segments each with a row of curved bristles. These features become less apparent from the early first instar to the late first instar. During the second instar, the larvae will grow from around 2.4 mm to 4.5–5.0 mm in length. The larvae will lose the curved bristles that were found in the previous instar and gain an additional posterior body segment. It is at this stage that the larva will begin to tear through the host's cuticle, using its mandibles in a back and forth motion, and emerge to continue its development.
Emergence of wasp larvae from the host cuticle is largely synchronous and subsequent emergence of wasp larvae is rare. The number of wasp larvae emerging from the host can vary from a few to well over 300 depending on the number of females that oviposit into the larval host (Fulton 1940). In total, the time from oviposition to emergence from the host cuticle is around 12–16 days, though the fastest time observed from oviposition to emergence was only five days (de Buron and Beckage 1992; Gilmore 1938). Upon emergence, the wasp larvae will undergo another larval molt (3rd instar) before spinning conspicuous white cocoons where they will develop into pupae and adults.
The pupae are formed after cocoon construction is completed. The period spent in the cocoon can range from 3–8 days depending on the temperature (de Buron and Beckage 1992; Fulton 1940; Gilmore 1938). Larvae that build their cocoons in late winter will overwinter as larvae and pupate in the spring of the following year (Fulton 1940). The cocoons will stay on the host for a while, but will eventually fall down to the soil.
Body length is around 2–3 mm for both adult males and females with antennae length nearly equal to body length. The body and eyes are black and the wings are translucent, with an opaque dark spot on the outer edge of each wing called the pterostigma. The shield-shaped plates where the wings meet the body (also called the tegulae) are pale yellow. The ovipositor of female Cotesia congregata is about 0.5 mm long and tapers from a wide base to a very narrow middle and tip that are uniform in thickness.
Females of Cotesia congregata display both inherent and learned responses to chemical cues that are released from plant leaves during the feeding of their caterpillar hosts (Kester and Barbosa, 1994; Turlings et al. 1990; Lentz-Ronning and Kester 2013). These cues allow the females to locate adequate hosts due to the high level of specificity between the chemicals released by the plants and the host species. Upon recognition of the chemical cues, females will fly, upwind, to the location of the host plant. Once on the host plant, females will engage in a host searching behavior which involves moving their antennae back and forth rapidly to further sense additional host cues such as cuticular semiochemicals (Kester and Barbosa 1991). Once the female has located a suitable host she will insert her ovipositor through the cuticle into the hemocoel and deposit her eggs inside of the caterpillar. Wasp larvae develop inside of the caterpillar host, feeding on the hemolymph, and eventually emerge to spin small white cocoons on the cuticle of the host. Larvae complete their development into adults and emerge from the cocoons to mate and find new hosts.
Adult males of Cotesia congregata engage in courtship behavior prior to mating (Bredlau et al. 2013). Males attract females with an auditory display that is produced by the wings beating rapidly, and is characterized by a buzz and a series of distinct high amplitude 'boing' noises. Multiple males will perform this courtship ritual in the presence of a single female wasp.
When female Cotesia congregata oviposit into the hemocoel of their larval host they simultaneously inject venom and a symbiotic virus called a polydnavirus (one of the bracoviruses) into the hemocoel. The injected virus prevents the host's immune system from attacking and killing the wasp eggs through encapsulation. The symbiotic polydnavirus also prevents the formation of host storage proteins, leaving more available resources in the hemolymph for the wasp larvae as they develop (Espagne 2004). The injected venom and special hormone-releasing cells from the wasp eggs (teratocytes) work in conjunction with the polydnavirus to arrest the development of the larval host (Beckage 1994).
Cotesia congregata attacks members of the family Sphingidae (the sphinx moths), including but not limited to the following species (Sharkey et al. 2005; Fulton 1940; Molina-Ochoa et al. 2003):
Ceratomia catalpae (Boisduval) (the catalpa sphinx)
Darapsa myron (Cramer) (the Virginia creeper sphinx)
Darapsa versicolor (Harris) (the hydrangea sphinx)
Dolba hyloeus (Drury) (the paw paw sphinx)
Eumorpha achemon (Drury) (the achemon sphinx)
Eumorpha pandorus (Hübner) (the Pandora sphinx)
Hemaris diffinis (Boisduval) (the snowberry clearwing)
Lapara bombycoides Walker (the northern pine spnix)
Manduca quinquemaculata (Haworth) (the five-spotted hawkmoth)
Manduca sexta (Linnaeus) (the tobacco hornworm)
Paratrea plebeja (Fabricius) (the plebeian sphinx)
Sphecodina abbottii (Swainson) (Abbott's sphinx)
Sphinx chersis (Hübner) (the great ash sphinx or the northern ash sphinx)
Sphinx kalmiae Smith (the laurel sphinx)
Spodoptera frugiperda (Smith)(fall armyworm)
Trichoplusia ni (Hübner) (the cabbage looper) (semi-permissive host)
Cotesia congregata is an important natural enemy of the tobacco hornworm, Manduca sexta, a detrimental pest species that feeds on many plants in the Solanaceae (tobacco, pepper, tomato, etc.) family.
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