Your garden is full of amazing predators, easily overlooked among the foliage and flowers. No matter where you live in the U.S. and southern Canada, save perhaps for the northern Rocky Mountains, you probably have sundew assassin bugs of the genus Zelus lurking in the yard. These leggy insects move slowly about, or wait in ambush for potential prey.
There are currently five recognized species of Zelus north of Mexico, most with wide geographic distributions. Few are readily identifiable to species just by looking at them, but common ones include: the lovely green Zelus luridus with a sharp spine on either side of its thorax; the bright red and black Milkweed Assassin Bug, Z. longipes found in the southeast U.S.; and the non-descript Z. tetracanthus with its four small "knobs" across the top of its thorax. The Leafhopper Assassin Bug, Z. renardii, is common in the southwest U.S. and has been entertained as a potential biocontrol agent for pest reduction in agricultural settings.
Sundew assassins get their common name from the unique physiology that allows them to catch prey. While ambush bugs have extremely muscular front legs that snap shut on prey with stunning force, sundew assassins look like the 90-pound weakling by comparison. Their appendages are thin and seemingly delicate or flimsy. The tibiae ("shins") of the front legs (and to some degree the middle legs) are densely covered in short hairs, and this is part of their secret weapon.
Special glands in the exoskeleton of the legs secrete a glue-like material that that the insect intentionally smears over those hairs. This creates a sticky layer that small prey cannot escape from once the assassin grabs them. The prey-catching scenario is analogous to the insect-eating plant known as the sundew, which inspires the name "sundew assassin bugs."
When they hatch from the egg, nymphs do not have the ability to produce the glue they need. No matter, mom has covered the egg cluster in sticky goo to help repel egg parasites like tiny wasps, and the nymphs simply wipe their "arms" along the base of the egg mass to gather some glue (Weirauch, 2006).
Nymphs go through five instars (an instar is the interval between molts) on their way to adulthood. They are noticeably larger after each molt, with wing pads evident in the last couple of stages.
The egg mass reminds me of a tiny crème brûlée, but perhaps I need to eat lunch and return to finish this post later....I was delighted to stumble upon a couple of egg masses in the act of hatching at the Cheyenne Mountain Zoo here in Colorado Springs back on July 21. It seems remarkable that such long-legged creatures could be packaged in such small vessels.
There are at least two generations annually here along the Front Range in Colorado, probably three or more generations in southerly latitudes. I notice nymphs of Z. luridus in spring, so it is likely that winter is passed in the egg stage.
Zelus species range from 14-21 millimeters in body length as adults, and are relatively easy to recognize with their slender build, long legs and antennae, and diurnal period of activity. They are largely arboreal, so you will find them mostly on foliage and flowers of trees, shrubs, and herbs.
Assassin bugs in general are indiscriminate predators of other insects, and Zelus is no exception. So, while they will dine on leafhoppers, small caterpillars, and other pests, they will also capture beneficial insects such as small bees.
Despite our tendency to classify insects as "good" or "bad" for our own selfish reasons, sundew assassin bugs should be welcomed for the benefits they provide in occasional pest control, and their unique behavior and "personality," if I dare use that word for an invertebrate.
Weirauch, C. 2006. "Observations on the sticky trap predator Zelus luridus STÅL (Heteroptera, Reduviidae, Harpactorinae), with the description of a novel gland associated with the female genitalia," Denisia 19, zugleich Kataloge der OÖ. Landesmuseen Neue Serie 50 (2006), 1169-1180.
Wolf, Klaus Werner and Walton Reid. 2001. "Surface Morphology of Legs in the Assassin Bug Zelus longipes (Hemiptera: Reduviidae): A Scanning Electron Microscopy Study With an Emphasis on Hairs and Pores," Ann. Entomol. Soc. Am. 94(3): 457-461.