Assassin bugs, belonging to the insect order Hemiptera along with cicadas, aphids, and stinkbugs, are distinguished by their predatory behavior and unique feeding habits. With approximately 7,000 species known worldwide, these insects play a significant role in controlling invertebrate populations. However, they are not considered as important in biological control of pests as other predatory groups such as bigeyed bugs and minute pirate bugs. This article delves into the dietary preferences, feeding mechanisms, and ecological significance of assassin bugs.
Physical Characteristics and Identification
Assassin bugs exhibit distinct physical traits that aid in their identification. Both adults and nymphs have an elongated head and body, coupled with long legs. The narrow head features rounded, beady eyes and long, hinged, needle-like mouthparts. Their legs are often described as long and gangly, and their antennae can give them a spider-like appearance. The front legs of some species may be raptorial, or grasping, similar to those of praying mantids.
Adult assassin bugs range in size from approximately 1/5 to 1-1/4 inches (5-30 mm) long, varying by species. Coloration varies greatly by species; adults can be blackish, black and orange, a mix of colors (e.g., black, brown, green, and reddish), or brownish, grayish, or pale overall. The beak, which is at least twice as thick as their antennae, is shorter, stouter, and curved, with the beak tip fitting into a groove between the front legs. In comparison with the relatively parallel-sided abdomen of Zelus spp., the abdomen of Sinea spp.
Nymphs can be pale or dark colored overall or have coloration resembling adults of the species. Zelus nymphs have pale-colored legs and antennae, while nymphs of leaffooted bugs have dark-colored antennae and legs.
Life Cycle and Habitat
True bugs develop through three life stages: eggs, nymphs, and adults. Eggs hatch into wingless nymphs, which progress through five increasingly larger instars (immature stages). Assassin bugs typically have one or two generations per year, with overwintering habits varying by location and species. For instance, assassin bugs in the Zelus genus commonly overwinter as adults, often observed on walls and windows during fall as they seek shelter.
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Assassin bugs are versatile and can be found on almost any terrestrial plant, including row and tree crops, gardens, and landscapes. In California, there are at least 13 genera of assassin bugs, including multiple species of Zelus and Sinea. Zelus renardii is common throughout California and the Western Hemisphere, closely resembling Z. tetracanthus, which in California is found mostly in natural areas. At least 6 species of Sinea occur in California, including the similar-looking Sinea confusa and S. diadema.
Predatory Behavior and Diet
All species of assassin bugs are predators of invertebrates or true parasites of vertebrates. Most assassin bugs feed on insects including caterpillars, larvae of leaf beetles and sawflies, and adults and nymphs of other true bugs. Nymphs and adults ambush or stalk prey, impale them with their tubular mouthparts, inject venom, and suck the body contents. They stealthily approach their prey before piercing them with their sharp, beak-like mouthparts, known as a “rostrum,” injecting a potent venom that liquefies their insides.
Assassin bugs are general predators and also feed on bees, lacewings, lady beetles, and other beneficial species. Certain species feed on the blood of birds, mammals, or reptiles, including conenose bugs and kissing bugs (Reduviidae: Triatominae).
Examples of Assassin Bugs
There are many species of assassin bugs, but two of note are the North American wheel bug (Arilus cristatus), and the striking orange and black milkweed assassin bug (Zelus longipes).
Ecological Role and Pest Control
Assassin bugs contribute to the regulation of pest populations in gardens and agricultural settings. They feed on pests such as aphids, beetles, stinkbugs, flies, and caterpillars, helping to keep their numbers at manageable levels. While they may not completely eliminate all pests, their presence can reduce the harm caused to plants.
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Defense Mechanisms
To deter predators, assassin bugs can spray their venom up to a foot away! Be cautious of getting close to some of these bugs, like the wheel bug, as they can pierce your flesh!
Distinguishing Assassin Bugs from Similar Insects
Zelus nymphs can be confused with young instars of leaffooted bugs, Leptoglossus spp. (Coreidae). These bugs can occur together because assassin bugs prey on the plant-feeding leaffooted bugs. Zelus nymphs have pale-colored legs and antennae. Nymphs of leaffooted bugs have dark-colored antennae and legs. Thread-legged bugs (Reduviidae: Emesinae) and certain other groups have a distinctly different appearance. Thread-legged bugs are long and thin overall and can be confused with stilt bugs (Berytidae), such as the predatory spined stilt bug. Eggs of the leafhopper assassin bug, Zelus renardii, are oblong and dark brown. They have a white cap and occur in groups glued to plant surfaces. Eggs of the spined assassin bug, Sinea diadema, are also oblong and brown with a pale top, but after deposition each egg cap unfolds to form a fringed corolla or umbrellalike cover.
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