The Ten-Lined June Beetle, Polyphylla decemlineata, is a large insect found across Western North America, from the Pacific Northwest down into parts of the American Southwest. This beetle often attracts attention because of its substantial size, ranging from three-quarters of an inch to over an inch in length. They are commonly seen during the summer months, particularly in June and July, when they emerge as adults and are strongly drawn to porch and street lights at night. Their sudden appearance and heavy, clumsy flight can lead people to wonder if this impressive-looking beetle poses a threat or has the ability to bite a person.
The Direct Answer: Do They Bite?
The Ten-Lined June Beetle does not bite humans. This beetle, like all scarab beetles, possesses mandibles designed for processing food. The adult beetle’s mandibles are structured for its herbivorous diet, consisting of feeding on the foliage of various shrubs and trees. These mouthparts are not adapted for defense against vertebrates, nor are they structured to pierce human skin.
The beetle is not aggressive toward people, and any physical interaction is generally the result of the insect accidentally flying into someone or being picked up. There is no venom or poison associated with the Ten-Lined June Beetle, meaning it is completely harmless to humans and pets. The concern about biting is a common misunderstanding stemming from the beetle’s large size and sometimes startling presence during warm summer nights.
What Happens When They Are Threatened
While they cannot bite, Ten-Lined June Beetles do have a defensive behavior when they feel threatened. When disturbed, the beetle will produce a distinctive, loud hissing or wheezing sound. This noise is not made with its mouth but is a process called stridulation, where the beetle forcefully expels air from its spiracles, which are the openings to its respiratory system along the body. This loud sound is purely a scare tactic intended to startle potential predators, such as bats or birds.
Handling the beetle may also result in a sensation that feels like a pinch or scratch, which is caused by the beetle’s spiny legs, or tarsi, gripping tightly in an effort to gain purchase or escape. This gripping action is a mechanical reflex and does not cause any real injury.
Identifying the Ten-Lined June Beetle
Adults are characterized by a stout, brown body covered with a pattern of white or yellowish stripes that run lengthwise down the wing covers, or elytra. The name refers to the prominent white lines on the beetle’s back, which are actually formed by tiny overlapping scales. Males of the species are especially noticeable for their large, fan-like antennae, which they use to detect pheromones released by females.
The life cycle of this insect includes a long larval stage. The larvae, known as white grubs, live underground for two to four years, where they feed on the roots of various plants, including trees and grasses. While the adult beetles are harmless and feed on foliage without causing much damage, the grubs are sometimes considered agricultural pests because their root-feeding habits can occasionally lead to the death or stunting of young plants. The adult beetles emerge in early summer, often attracted to lights.