Doppelgangers: Giant water bug vs Cockroach

Years ago, I was walking home from the University of Alberta campus on a September evening, watching the ground, as entomologists are wont to do, when I saw a huge insect on its back on the sidewalk. It was just off of a major crosswalk under a streetlight. Upon a quick glance, I knew it was one of two things: a giant water bug or a cockroach. Either way, I needed it for my collection, so I carefully collected it using the only container I had on hand. I say that I “carefully collected” it, because water bugs are known for their painful bite and I did not want to take any risks. From inside the container, it was clear that I had scooped up a water bug.

Giant water bugs (Lethocerus americanus) are true bugs (Hemiptera) that belong to the family Belostomatidae. There are over 150 species of water bugs in the Belostomatidae, but most are quite large (> 2 cm). Belostomatids are usually found in ponds, lakes, or slow moving rivers and streams. They spend most of their time in the water, but disperse between bodies of water by flying (except in species that have reduced wings and are flightless), at which time they may be found around streetlights or porchlights. They are predators of other insects, small fish, snails, amphibians, and other animals that they encounter in the water. Giant water bugs use their forelegs to capture prey and then use their long beak-like proboscis to feed on their prey. First, they inject enzymes into their prey that breakdown prey tissues into a liquid. Then they feast on a liquid lunch by sucking their victim dry. Their bite can be very painful. These ‘toe-biters’ are best avoided, but they are important beneficial insects in aquatic ecosystems.

Giant water bugs can be easily mistaken for adult cockroaches (especially the American cockroach, Periplaneta americana, Blattodea: Blattidae). Both water bugs and cockroaches have large, oval shaped bodies that are usually brown coloured. A casual glance or quick encounter with either may lead to a case of mistaken identity. But, upon closer examination, some key differences are easy to see:

  1. Cockroaches have long, obvious antennae. Water bugs do not appear to have antennae unless closely examined.
  2. The head and eyes of water bugs are visible from above. The head and eyes of cockroaches are hidden underneath the pronotum.
  3. Cockroaches have spiny legs evolved for running and quick movement. All six of their legs look the same. Water bugs have forelegs adapted to grabbing prey (raptorial legs). Their legs are also adapted for swimming and have no obvious spines.

Cockroaches are usually classified as pests by humans, but some provide important ecosystem services (i.e. they are decomposers) and others are television and movie stars (i.e. Madagascar hissing cockroaches).

Giant water bug
cc-by-nc 4.0 Christian Schwartz
American cockroach
cc-by-nc 2.0 K. Schneider

Doppelgangers may be related (e.g. same genus) or may not be related, as in the case of monarch butterflies (Danaus plexippus) and viceroys (Limenitis achrippus).  Doppelgangers are usually relatively harmless but sometimes the doppelganger is a pest yet their behaviour, lifecycle or hosts may be different.

Correctly identifying a pest enables selection of the most accurate scouting or monitoring protocol. Identification and monitoring enables the application of economic thresholds. It also enables a producer to select and apply the most effective control option(s) including method and timing of application.  For the rest of the growing season, the Insect of the Week will feature insect crop pests and their doppelgangers.

The case of the innocuous versus the evil twin
When making pest management decisions, be sure that the suspect is actually a pest. This can be challenge since insects often mimic each other or look very similar. An insect that looks, moves and acts like a pest may in fact be a look-alike or doppelganger.

Review previously featured insects by visiting the Insect of the Week page.