Diaeretiella rapae–parasitoid wasp of aphids

Diaeretiella rapae is a solitary endoparasite of aphids. Although females lay multiple eggs within each aphid host, only one larva lives to maturity. Females emerge from aphid hosts with fully developed eggs, and can lay fertilized or unfertilized eggs within new hosts (Kant et al. 2008). One study found that D. rapae females fed with buckwheat nectar had significantly higher egg loads than those fed on nectar from other flower species or water alone (Araj & Wratten 2015). New females will develop from fertilized eggs and males from unfertilized eggs, as is common with Hymenoptera (Kant et al. 2012a). 

Nectar is the primary food source for adult D. rapae. Other parasitoid species often feed on honeydew in addition to or instead of nectar, but cabbage aphid honeydew is coated in wax, making it inaccessible to D. rapae as a food source (Jamont et al. 2014). Without a nectar source, adults can survive for up to a few days, but access to nectar increases D. rapae longevity, fecundity, and host searching time (Varennes et al. 2015).

Females are attracted to host plants by volatiles given off by plants in the brassica family, and then find aphid hosts by searching randomly on the plant (Ayal 1987, Read et al. 2012). Males are attracted to host plants by an odor given off by female D. rapae (Reed et al. 2012). Research has found that D. rapae are more successful at finding larger (and therefore older) cabbage aphids than smaller/younger aphids, and prefer ovipositing into larger/older nymphs despite the older hosts having better defenses and requiring more handling time (Kant et al. 2012a). D. rapae preferentially lay eggs in host abdomens (Kant et al. 2008). A single female can produce up to several hundred offspring. The size of the host that a female emerges from has been shown to impact the number of offspring that female will produce, with females that developed from larger hosts producing 62% more offspring than those that developed in smaller hosts (Kant et al. 2012a).

D. rapae is a koinobiont species, meaning that their aphid hosts may continue to develop and reproduce even after being parasitized. However, if D. rapae parasitizes an aphid when the host is still small enough, the host may not contain enough resources for the developing wasp. The developing wasp may, in that case, consume aphid nymphs that are also developing within the host and the host may not produce any nymphs before being killed (Kant et al. 2012b). The D. rapae larva kills the aphid host within 2-3 days, leaving only a bronze-colored, hard aphid exoskeleton, now called a mummy. The developing wasp then pupates within the aphid mummy before emerging as a new adult. In a lab study, the entire life cycle from egg to adult took 9-15 days. Adult females live 10-15 days, males 7-10 days (Baer et al. 2004).

In temperate climates, D. rapae overwinters within aphid mummies on host plants (Geiger et al. 2005). Adults emerge in the spring to find aphid hosts.

D. rapae has been reported parasitizing over 60 species of aphids worldwide. It is perhaps the most well-known for parasitizing the cabbage aphid (Brevicoryne brassicae). Researchers in the Czech Republic found cabbage aphids to be the population diversity center for D. rapae, meaning that the population of D. rapae found attacking cabbage aphids was the most diverse of the populations found attacking a given host. D. rapae is also the most significant parasitoid of cabbage aphids. Researchers in Washington state found that 92.5% of parasitized aphids they collected were parasitized by D. rapae (Pike et al. 1999).

Cabbage aphid is a pest of brassica crops, including cabbage, broccoli, cauliflower, Brussels sprouts, mustards, radishes, and turnips. They are light-green but adults often appear gray because of the dusty/waxy coating that covers their exoskeleton. Adults are 2-2.5 mm long. Cabbage aphids have characteristically short cornicles (appendages that extend backwards from the rear of the abdomen), which distinguishes them from most other aphid species. Cabbage aphid colonies form on the undersides of host leaves. They preferentially infest new growth and are often found in growing points and in Brussels sprout buds. Feeding damage can cause stunting and deformation of crops, but the primary form of damage is the physical presence of the aphids themselves on marketable crop parts. In warm climates, or during warm seasons, females give birth to live nymphs. These nymphs are produced asexually and are genetically identical to the mother. Cold temperatures or reduction in photoperiod trigger the production of male aphids. Mating occurs and females lay eggs, which overwinter on host plants.

Green peach aphid (Myzus persicae), Russian wheat aphid (Diuraphis noxia), and asparagus aphid (Brachycorynella asparagi) are also significant hosts for D. rapae in the U.S. Green peach aphid feeds on plants in more than 40 families, including tree fruits, vegetables, and ornamentals. They overwinter in the egg state on Prunus spp. and in the spring, nymphs feed on Prunus flowers. When populations become too large, winged green peach aphids move to summer hosts. Feeding damage can result in leaf curling and yellowing, but green peach aphids also transmit many plant viruses, which is an important form of damage.

Asparagus aphids attack only asparagus and feed in ferns. A toxin injected into the plants during feeding causes shortening of internodes, causing ferns to develop a bush-like/witches-broom-like formation. Heavy infestations deplete sugars and cause reduced root growth, which can kill plants over a few years. Asparagus aphids can also vector viruses, including asparagus viruses I and II and tobacco streak virus. Asparagus aphids are oval-shaped and green-to-gray, and covered in a waxy secretion. They do not have cornicles but have openings at the rear of the abdomen in their place (UMass Fact Sheet). Russian wheat aphid feeds on cereal grains primarily wheat and barley, followed by rye and triticale. Oats sustain little to no damage from Russian wheat aphid, and corn, sorghum, and rice are not hosts. Toxins injected into the plants during feeding cause white or purple longitudinal streaks on the foliage, and infested leaves curl up longitudinally like drinking straws. The awn can become trapped in the curled flag leaf, giving the seed head a fishhook-like appearance.