Tenlined June beetle

Tenlined June beetle is widely found in sandy soils west of the Rocky Mountains. Larvae feed on plant roots and can weaken or kill the plant. Adults feed on foliage but do not cause economic damage to fruit trees. Infestations spread slowly because of a lack of movement by mated females and the long time span of each generation, which can be up to four years in the Northwest.

The adult tenlined June beetle and the related Polyphylla species are the largest scarab beetles in Colorado, ranging from 22-30 mm in length. Larvae 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.

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 Cycle: The larval period can last 2 to 4 years in the Northwest, depending on the site and the length of the growing season. Most older grubs are found in the top foot of the soil, where they feed on woody roots, while younger grubs live deeper in the soil and eat the finer and more tender roots. Most of the damage to the tree is done by the older grubs. The grubs begin to pupate in May and June in pupal cells a few inches below the soil surface.

The cells are about 2 inches 50 mm long and 3/4 inch 18 mm wide pupal period lasts about 5 weeks. Adult activity begins in June or July and continues until fall. The adult bores an emergence hole from the pupal cell to the soil surface but may not emerge immediately. Adults stay under cover during the day, hiding in weeds or grass in the orchard. They make a peculiar wheezy, hissing noise when disturbed.

They become active around dusk and are active longer on warm nights. There is little activity when temperatures are below 60. Males are attracted to females by a sex pheromone. They mate at or near the female’s emergence hole, and she often lays eggs in the same hole. Dispersal of females may be very limited. Females lay 60 to 70 eggs in the soil. The eggs hatch in about 3 to 4 weeks. Young larvae feed on decaying vegetable matter or fine roots. They take 3 to 4 years to develop fully. In cold-winter climates, larvae may move deeper in the soil to avoid frost and move closer to the surface again in the spring to continue feeding.

Hosts: Hosts of the tenlined June beetle larvae probably include all deciduous tree fruits grown in the Pacific Northwest. Infestations in Washington tree fruits have mostly been associated with apple. The tenlined June beetle has also been well studied as a pest of almonds in California. Other hosts include strawberries, cane fruits, roses, potatoes, corn, and possibly willow and poplar.

Damage: The grubs’ feeding on the roots can weaken or kill the tree. Adults feed on plant foliage but cause little damage. Larvae feed on roots, causing severe injury and death to mature trees. Initial damage to root systems may not be immediately evident in above ground tree growth. Adults cause no damage.

Controls: Tenlined June beetle infestations are localized within orchards and are often first noticed when a clump of trees start to decline and die. Infestations usually spread slowly from the initial sites where they are first identified in orchards, killing neighboring trees. Control requires the removal of infested trees and soil fumigation before replanting. When removing dead trees, inspect roots for the presence of larvae or larval feeding. Remove all trees in the infested area plus one or two uninfested trees on all sides of the infested area to stop the spread within the orchard.

Male tenlined June beetles are attracted to light sources example tungsten lights, black lights and generally first appear in early June. Although there are no proven methods for controlling tenlined June beetle grubs other than removing infested trees and neighboring trees including roots and fumigating the soil before replanting, it has been observed that soil drenches of an organophosphate insecticide as soon as first adults emerge can reduce populations. Because the insect has a 2-year life cycle in the soil, this approach must be repeated in consecutive years. Grub control is difficult because no effective methods exist to move insecticides downward in the soil to where the larger second and third instar grubs are common more than 8 inches below the soil surface. Natural enemies that parasitize the grubs and adults tachinid flies do exist, but these biocontrol agents do not exert high levels of mortality to the beetle. Impacts of predators are also limited.

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