Wednesday, June 1, 2022

Wasp Wednesday: Anastatus spp. Eupelmids

As is the case at least fifty percent of the time, we owe this post to my sharp-eyed partner, Heidi Eaton, who noticed a clutch of eggs laid by a true bug. Upon further inspection, she saw a tiny wasp meandering through the ova. The micro-wasp was indeed an egg parasitoid, in the family Eupelmidae, probably genus Anastatus. The drama took place in a glade habitat at Graham Cave State Park in north-central Missouri, USA, yesterday.

Measuring a host egg with her antennae.

Among the unique features of eupelmids in the subfamily Eupelminae are drastic sexual dimorphism, and stupendous leaping ability, as described in this edited passage by Gary Gibson on Bugguide:

Because of modifications to increase jumping ability the flight apparatus and consequently the ability of female eupelmines to fly has apparently been reduced. …One common name that was suggested for the group was "back-rolling wonders" because the jumps are so powerful [that the insects] tend to tumble on landing. Females prefer not to fly and will simply walk around until disturbed. Males, however, fly as readily as any other chalcidoid. It is possible that females were modified for jumping to enhance [their] ability to escape rapidly from predators such as ants and spiders….The jumping mechanism of females is….unlike other insects which use muscles inserted directly onto the legs to power jumping, Instead, very large dorso-longitudinal muscles in the thorax are used. When these muscles contract the thorax contorts, which changes a longitudinal force of action into a vertical force of action to power the middle legs for jumping. A result of this is that females often die in a U-like posture.

Dead female eupelmid in U-shaped posture after contraction of the jumping muscles

The energy for this instant athleticism is stored in large blacks of resilin, an elastic, rubber-like protein found in many jumping insects. What triggers the flood of energy is unknown, but it causes the muscles to contract, making the thorax suddenly shorter, which in turn pulls the basal segment of the middle leg inwards along a tendon-like muscle. A large spur on the tip of the tibia of the middle leg gives it purchase as it kicks off of the substrate (leaf, twig, or other surface).

There are approximately eighteen known species in the genus Anastatus in North America north of Mexico, and about 160 species globally. All have females with a distinct pattern of bands on the forewing, thought to enhance their mimicry of ants. The wings adhere close to the body when the insect is not flying, to the degree that at first glance the tiny wasps look to be wingless. The ovipositor is almost entirely concealed, which helps separate this genus from similar-looking genera.

Laying her egg inside the host egg.

At least two species have been introduced to this continent, one purposefully, the other accidentally. A. disparis was imported to aid in the control of the Spongy Moth (formerly known as the Gypsy Moth). A. tenuipes apparently arrived with its host, the Brown-banded Cockroach. The species we observed at Graham Cave was clearly one hosted by a true bug, likely in the family Coreidae (leaf-footed bugs). The female wasp inserts a single egg into the host egg, and her larval offspring will be an internal parasitoid of the developing host, ultimately killing it.

Wheel Bug eggs and newly-emerged nymphs.

Interestingly, earlier in the month, while at Fountain Creek Nature Center in Fountain, Colorado, I happened to notice an Anastatus female suspiciously close to a batch of hatching Wheel Bug eggs discovered by another member of our party. There is a eupelmid, Anastatus reduvii, that is a known parasite of Wheel Bug ova, so perhaps that was the unknown suspect.

Fleeing the scene of the crime? Female eupelmid suspiciously close to the Wheel Bug eggs.

Encountering parasitoid behavior in the field is a rather rare occurrence. Rearing the eggs of insects, especially those you can associate with an adult female insect, can add to our collective understanding of host-parasitoid relationships, and even result in the discovery of species new to science. In the case of Anastatus, many species are known only from females. In other cases, males are rare, and/or have not yet been associated with the conspecific females. We clearly have a lot to learn.

Sources: Burks, B.D. 1967. “The North American Species of Anastatus Motschulsky (Hymenoptera, Eupelmidae),” Trans. Amer. Ent. Soc. 93(4):423-432.
Universal Chalcidoidea Database
Eaton, Eric R. 2021. Wasps: The Astonishing Diversity of a Misunderstood Insect. Princeton, New Jersey: Princeton University Press. 256 pp.
Goulet, Henri and John T. Huber, editors. 1993. Hymenoptera of the World: An identification guide to families. Ottawa: Agriculture Canada. 668 pp.

1 comment:

  1. Great post! Love learning about the little Chalcid wasps.


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