The term “potato bug” can be confusing, as it refers to two distinct types of organisms: a true insect pest and a group of beneficial crustaceans. Understanding these differences helps gardeners identify what might be in their soil or on their plants.
Identifying the “Potato Bug”
One creature commonly called a “potato bug” is the Colorado Potato Beetle (Leptinotarsa decemlineata). Adults are bright yellow or orange with ten black stripes down their wing covers, oval-shaped, and about 3/8 inch long.
The other “potato bugs” are sowbugs and pillbugs, which are terrestrial crustaceans, more closely related to shrimp and lobsters. Sowbugs (Porcellionides pruinosus or Porcellio laevis) and pillbugs (Armadillidium vulgare) are grayish, oval-shaped, and usually between 1/4 to 1/2 inch long. Pillbugs can roll into a tight ball when disturbed, earning them the nickname “roly-polies,” while sowbugs cannot. Both have seven pairs of legs and require moist environments.
The Colorado Potato Beetle’s Diet
The Colorado Potato Beetle is a pest of agricultural crops, particularly those in the nightshade family (Solanaceae). They primarily feed on potatoes, tomatoes, eggplants, and peppers. They also feed on wild nightshade plants, which can serve as alternative hosts when cultivated crops are unavailable.
Both adults and larvae are heavy feeders. Adults consume about 10 square centimeters of foliage daily. Larvae consume up to 40 square centimeters of potato leaves during their stage, causing the majority of feeding damage. This extensive feeding can lead to severe defoliation, potentially destroying entire plants if infestations are left unchecked.
The Sowbug and Pillbug Diet
Sowbugs and pillbugs are primarily decomposers. Their diet consists mainly of decaying organic matter, such as rotting wood, leaf litter, mulch, and fungi. They break down this material, recycling nutrients back into the soil and aiding in the composting process.
While these crustaceans prefer dead and decaying plant material, they can occasionally feed on live plants, especially when preferred food sources are scarce or populations are high. This incidental feeding typically targets tender seedlings, new roots, lower leaves, or soft fruits in contact with damp soil. However, such damage is generally minor and rarely harms healthy, established plants.
Impact on Gardens and Management
The differing diets of these “potato bugs” lead to very different impacts on gardens. Colorado Potato Beetles, with their preference for live nightshade plants, can cause substantial damage to crops. Their feeding, by both adults and larvae, results in significant defoliation, which can reduce yields or even kill plants if not managed. Managing Colorado Potato Beetles often involves strategies like hand-picking adults and larvae, crushing their yellowish-orange egg clusters found on the underside of leaves, and rotating crops to disrupt their life cycle. Row covers can physically exclude beetles, and biological controls, such as the bacterium Bacillus thuringiensis var. tenebrionis (Bt), target young larvae.
Sowbugs and pillbugs are generally beneficial to gardens due to their role as decomposers, improving soil quality. However, when populations become excessive or decaying matter is limited, they may consume tender plant parts. To manage sowbug and pillbug populations and prevent potential damage to live plants, reducing moisture around garden beds is effective, as they thrive in damp environments. This involves watering earlier in the day, improving drainage, and removing excessive organic debris like leaf piles or mulch where they hide. Traps using potato halves or melon rinds can also be used to collect and relocate them, often to a compost pile where they continue their beneficial work.