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Tenlined June Beetles

Among the largest of the scarab beetles in Colorado, the adult tenlined June beetle ranges in size from 22-30mm in length.  Commonly called watermelon beetle and June bugs, this beetle is widely distributed in the state.  Most commonly found east of the Rockies, they tend to prefer soils that are lighter and sandier in composition.  Adults usually fly from late June through early August, which is where their June bug title comes from.  Males, which can be identified by their larger antennae, are highly attracted to lights.  Their antennae can be opened and spread in a fan-like manner, which is typically done in a defensive display.  Females rarely visit lights and have significantly smaller antennae.  Both genders feed on foliage of several species of trees and shrubs; however, feeding done by adults is rarely noticeable.  Due to their large size and markings, tenlined June beetles often attract attention.  When disturbed, adults tend to create a loud hissing sound by powerfully expelling air from their spiracles.

Eggs are laid in soil, where larvae emerge and begin their growth cycle. Larvae of the tenlined June beetles are a type of white grub, which feeds on plant roots.  A wide variety of host plants are known to be consumed by the larvae including grasses, perennials, trees and shrubs.  On occasion the can cause significant damage to roots of woody plants with pines being the most commonly damaged in this manner.  Tenlined June beetles grubs tend to be more tightly curled and stiffer bodied that their relatives.  When fully grown in the spring of their third season since hatching, pupation is completed and adults emerge later that summer.  Full grown larvae may have a circumference of a 50-cent piece.

Several look-alikes occur within the state, but lack the distinct markings of the tenlined June beetles with coloring that is more diffuse and/or with more predominant dark markings versus the more vibrant coloring of the tenlined species.