Under the leaf of a milkweed plant lurks a cleverly disguised brilliant red and black predator, patiently waiting for a juicy caterpillar to come close enough to assassinate. Slowly the caterpillar crosses an invisible line and the predator strikes, jabbing its prey with its secret weapon and paralyzing it with poison. Then, the hunter sucks up the larva’s life juices greedily, just as you or I would enjoy a thick chocolate malt.
Meet the colorful (and handsome) James Bond of the insect world: the horned assassin bug (Repipta taurus). This species belongs to a very large family of true bugs called assassin bugs. It is well suited to its role as a silent, stealthy killer.
Who would ever think of bright red and black as camouflage? But, for Repipta taurus, its coloring is perfect. These hues mimic that of the milkweed bug (Oncopeltus fasciatus). Milkweed bugs are harmless herbivores that taste bad to predators. Because the horned assassin so closely resembles the milkweed bug, it doesn’t alarm its prey and bigger predators avoid it.
Of course, this particular form of camouflage is so useful that a close cousin of the horned assassin, the Milkweed Assassin Bug (Zelus longipes) uses it too. The resemblance among the three insects can cause considerable confusion.
One difference is in size. The horned assassin bug is small, about 1/2 inch in length (11-13 mm), while the Milkweed Assassin Bug measures about ¾ inch in length (20 mm). The horned assassin also has black spines projecting from its head and its thorax. These spines are absent in both the milkweed bug and the Milkweed Assassin Bug.
The “secret weapon” that helps define a horned assassin bug as a beneficial insect is its curved proboscis. This proboscis is unique to all assassin bugs because it curves outwards from the head and swings forward to attack such pests as cucumber beetles, spiders, caterpillars and snails.
Like other members of its family, the horned assassin bug uses the sharp stylets in its proboscis, first like a syringe and then like a straw. One stylet punctures its prey and injects saliva to paralyze its meal; then another stylet sucks up the bodily fluids of its hapless victim. Once the assassin bug completes its dinner, the tip of the proboscis tucks neatly into a groove on its post sternum (roughly the equivalent of a human breastbone).
Other members of the assassin bug family are not quite so benevolent in their use of this secret weapon. They can, and do, use it to bite us. Wheel bugs (Arilus cristalus) can inflict a painful bite that can be dangerous to allergic individuals. In South America, a cousin to the horned assassin bug spreads Chagas disease, a debilitating and often fatal disease among humans. In the Texas Gulf Coast area, however, most members of the assassin bug family do not actively seek humans as prey. Encounters with us are usually accidental.
How common are the horned assassin bug and the Milkweed Assassin Bug? Milkweed Assassin Bugs are very common in our area (Master Gardeners regularly see them patrolling the plants in the Demonstration Garden adjacent to the County Extension Office even during winter days when temperatures are mild). While the horned assassin bug is only moderately common, it is a very effective insect predator.
So, here, the overall effect of the horned assassin bug, and its cousins, is to help keep some of our common garden pests under control. In order to take advantage of this positive trait, all that is required of us is to be aware of, and respectful of, any potentially negative qualities.
Beneficials in the Garden & Landscape is an Earth-KindTM program coordinated through Extension Horticulture at Texas A&M University. Earth-Kind uses research-proven techniques to provide maximum gardening and landscape enjoyment while preserving and protecting our environment.
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