It is tempting to simply label all insect-eating insects as “beneficial,” but black-and-white judgments on the human value of mantids are problematic. Mantids eat many pest insects, but they also eat insects that humans tend to appreciate, such as other insect predators, pollinators, butterflies, and so on.
Egg cases of nonnative mantids, including the Chinese mantis, are still bought and sold in many places in the hope of controlling agricultural and garden pests. Because of their indiscriminate diet (eating beneficial as well as pest insects) their usefulness is questionable. If you want to control pests biologically, by encouraging a variety of insect predators, avoid using broad-spectrum insecticides.
Because of the Chinese mantid’s possible role in the decline of some native North American mantids, the use of that species seems ill-advised. If you want to experiment with mantids as pest control agents, try using the egg cases of our native Carolina mantis. Many people kill Chinese mantids and destroy Chinese mantid egg cases, in hopes that their removal will improve ecological balance.
Mantids are often described not just as “predators” but as “deadly predators.” A moment’s reflection reveals the emotional and redundant charge in the latter phrase. We humans feel satisfied when mantids consume invasive stinkbugs, annoying houseflies, or moths whose caterpillars chomp our tomato plants, but it disturbs us when they capture honeybees, pretty butterflies, or other seemingly innocent insects.
Nonnative or invasive? Beneficial or pest? The distinction between nonnative and invasive usually hinges on whether or not the nonnative organism takes over or disrupts healthy ecosystems, or clearly causes serious declines of native organisms. The distinction between beneficial and pest usually hinges on the organism’s role in human economic (usually agricultural) interests — weighing its total impact, including both pros and cons. The case of the Chinese mantis seems complicated, apparently, by some combination of the following possible factors:
- The amount of unequivocal data on the Chinese mantis’s impact on native ecosystems and/or populations of native organisms
- The longstanding, already widespread North American distribution of the Chinese mantis, which complicates a researcher's ability to gauge its environmental impact and hampers any attempts to contain or control its numbers
- The costs, and likelihood of success, of trying to control its numbers
- The continuing desire of people to be free to buy, sell, and use them as biocontrol agents
- The issue’s position relative to a lengthy list of conservation and agricultural priorities.