Tenlined June Beetle

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Compiled by Whitney Cranshaw, Colorado State University:

Colorado Insects of Interest

Tenlined June Beetle

Figure 1. Tenlined June beetle (Polyphylla decemlineata) male.
Scientific Name: Polyphylla decemlineata (Say)

Order: Coleoptera (Beetles)
Family: Scarabaeidae (Scarabs, chafers, May/June beetles, dung beetles)

Identification and Descriptive Features: The adult tenlined June beetle (Figure 1) and the related Polyphylla species are the largest scarab beetles in Colorado, ranging from 22-30 mm in length.

Larvae (Figure 3) are a type of white grub and occur in soil. Compared to other large root feeding white grubs, notably those of the various Phyllophaga species (May/June beetles), they tend to be more tightly curled and stiffer bodied. When full grown they may have the circumference of a 50-cent piece.

Figure 2. Tenlined June beetle male showing antennae. The antennae may open and spread in a fan-like manner.
Distribution in Colorado: The tenlined June beetle is widely distributed in the state. It is most commonly found east of the Rockies, including the San Luis Valley. It tends to be more common where soils are lighter and sandier.

Life History and Habits: Adults of this largest Colorado “June bug” usually fly from late June through early August. Males, which have large antennae (Figure 2), are strongly attracted to lights; females, which have substantially smaller antennae, rarely visit lights. Adults of both sexes feed on foliage of various trees and shrubs, but feeding injuries are rarely noticeable.

Figure 3. Full grown larva of the tenlined June beetle.
Eggs are laid in soil and larvae of the tenlined June beetle feed on plant roots. They have a wide host range and are known to chew on grasses, perennials, trees and shrubs. (On rare occasion they can cause significant damage to roots of woody plants, with pines being most often injured.) In fall, grubs preparing to overwinter move deeply into the soil, returning near the soil surface with returning warm soil temperatures in spring. In the spring of the third season after eggs are laid pupation is completed and the adults emerge.

Although the tenlined June beetle causes little plant injury it is an impressively large, well-marked insect that commonly attracts interest. Furthermore, adults when disturbed can produce an impressive defensive display, hissing loudly by forcefully expelling air from their spiracles. This may also be accompanied by male beetles spreading and fanning out their large clubbed antennae. However, the insects are harmless.

Figure 4. The three Polyphylla spp. present in Colorado: Left, P. decemlineata; center, P. diffracta; right, P. hammondi. All are males except P. decemlineata female in lower left.
Related Species: Two other large June beetles of similar size and appearance occur in Colorado (Figure 4). Polyphylla diffracta Casey has obviously lined wing covers but the striping is somewhat more diffuse and markings are less consistently distinct than those of the tenlined June beetle. It is the most common Polyphylla species found on the West Slope and also occurs in extreme southeastern Colorado (Baca County). Polyphylla hammondi LeConte occurs primarily in eastern and, particularly, southeastern Colorado. However, it is also known from Moffat County. It tends to have more brownish coloration and fainter striping than the tenlined June beetle.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.