Eupithecia is a large, worldwide genus of inchworms (moths in the family Geometridae). The Eupithecia in Hawaii are unique because of the particular ecological niche they fill – they are predators, while nearly all other known caterpillars are plant feeders only.
Here is the abstract of the original paper describing carnivorous Eupithecia (Montgomery, S. L., 1983. Carnivorous caterpillars: the behavior, biogeography and conservation of Eupithecia (Lepidoptera: Geometridae) in the Hawaiian Islands. GeoJournal 7:549-556.):
A completely new feeding pattern has been found among caterpillars native to Hawaii: certain geometrid larvae (commonly called inchworms) consume no leaves or other plant matter. Instead, they perch inconspicuously along leaf edges and stems to seize insects that touch their posterior body section. By bending the front of their body backwards in a very rapid strike, the caterpillars opportunistically capture their prey with elongated, spiny legs and 900 larvae and eggs of these moths have been collected from native forests of all the main islands and reared in the laboratory. All are species of Eupithecia, a worldwide group of over 1000 members that had been reported to feed only on plant matter such as flowers, leaves or seeds. At least 6 of Hawaii’s described Eupithecia species are raptorially carnivorous, only 2 are known to feed predominantly on plant material, especially Metrosideros flowers. A diet including protein-rich flower pollen and a defensive behavior of snapping may have preadapted Hawaii’s ancestral Eupithecia for a shift to predation. Severe barriers to dispersal of mantids and other continental insect predators into Hawaii resulted in an environment favoring behavioral and consequent morphological adaptations that produced these singular insects, which can be commonly called the grappling inchworms.
When insects colonized the remote Hawaiian Islands over millions of years, the results were remarkable because the chances of them getting there were so slim. This meant that a species that managed to be blown out over vast distances of ocean and land on one of the islands faced little competition or predation pressure. They then diversified into ecological niches that would not have been possible for their mainland relatives. Because there were fewer predators in the islands than the mainland from which the colonizing Eupithecia originated, there was wide-open opportunity in predation and Eupithecia had the right biological tools to grab it, despite its evolutionary history as an eater of plant parts. Interestingly, people collected these caterpillars for years without recognizing them as predators, because it was assumed that all caterpillars must be plant feeders. The story is that they always died in the lab until a fly inadvertantly got into a rearing cage, and the caterpillar was observed eating it. In the clarity of hindsight, it is interesting that it did not occur to people that these were predators, because they have quite distinct morphology and behavior. They have raptorial claws adapted for grabbing struggling prey, and long thin appendages on the tip of their abdomens which probably work somewhat like the trigger hairs in venus fly-traps. They are sit-and-wait predators, disguising their long bodies along the edge of a leaf — behavior that makes no sense for an herbivore caterpillar which must move around a lot to feed on the plant. When an insect touches the Eupithecia while crawling up a leaf edge, it whips its head around and captures it.
Nearly all the 22 known Hawaiian species of Eupithecia behave as described in the paper. Interestingly, some are host plant specific, even though they do not eat the plant, because they look so much like that particular plant and it gives them a great advantage in disguise. Some are even specific to the part of the plant on which they rest. One Eupithecia is specific to the green, living fronds of the native Hawaiian fern known as uluhe, and it is the perfect green to match their color. Another dark brown species is always found on the mats of dead fronds underlying the green, living part of the plant.
One exception to the sit-and-wait behavior of most Hawaiian Eupithecia is the species E. monticolans (above), which appears to behave much more like a plant-feeding inchworm. It does not have the raptorial claws or the trigger hairs on the abdomen, and does not sit along leaf edges as its congeners do. When I first collected it, its food was not known, but assumed to be flowers on the `ohi`a tree (as mentioned in the above abstract). I suspected this was not necessarily the case because I collected many from trees that were not in flower. I was not successful in rearing E. monticolans caterpillars for over a year, until I inadvertantly provided one with leaves that were loaded with galls made by small insects related to aphids, in the family Psyllidae. Within a day the leaves looked like swiss cheese, with all the galls eaten out of the leaves. From then on, I always provided this species with leaves covered with galls and I never again had problems rearing them to adulthood. So while E. monticolans was thought to be a “missing link” from pollen-eating to predatory behavior, they are in fact predators, but without the morphology to make a quick strike, they must rely on eating sessile insects that cannot escape or defend themselves from a slow-moving caterpillar.
Almost no parasitoids have been reared from Eupithecia caterpillars. It certainly seems likely that Eupithecia may be rarely parasitized because it is difficult for a parasitic wasp to sneak up on it and lay an egg without being caught. Here is a brief video showing lightning-fast E. orichloris capturing a parasitic wasp (and releasing it – apparently they are not very tasty).
Listen to the jazz tune “Inchworm” here…