June beetles (white grubs)

(The full version of the article is posted on professional letter writer service)

In British Columbia, seedlings in newly established bareroot nurseries have occasionally been damaged by white grubs, which are June beetle larvae. Several June beetles occur in British Columbia. Polyphylla decemlineata is probably the most common and destructive in forest nurseries.

Host and damage

Adult P. decemlineata feed on coniferous foliage without doing much harm, but larvae feed on - and severely damage - roots of many kinds of plants, including several conifers. Bareroot seedlings can be affected at any age, but especially as 1+0 and 2+1 stock. To date, container-grown seedlings have not been affected. Damage normally occurs in late spring through summer, when white grubs voraciously feed on roots, causing shoots to turn brown and dry out. Frequently, the main stem of seedlings or transplants is cut off slightly below the soil surface (Figure 83), so that affected seedlings can easily be pulled from seedbeds.

White grubs occur around the roots of damaged seedlings or can be found at the time of seedbed preparation, sowing, and planting. Grubs often move from one seedling to another, affecting several adjacent drill or transplant seedlings. Grasses are excellent white grub hosts. Thus, recently established nurseries on former sod areas frequently suffer damage, and distribution coincides with that of the plowed-under sod. Depending upon environmental and other factors affecting the insect's development, white grub damage may peak at 3- or 4-year intervals. Because of climate, insect life history, and previous cropping history factors, damage may be acute, with seedling losses of up to 30% being common in small areas.

Life history (Figure 84)

Depending mainly upon climatic factors, the life cycle requires 3 or 4 years for completion. Grubs and adults overwinter in the soil, and in late June and early July adults emerge on warm evenings, mate, and return to the soil. These flights are repeated daily for 2-3 weeks. Eggs are 3.4 x 2 mm, slightly oval, and creamy-white. They are deposited in the soil and hatch after 2-6 weeks. For the remainder of this growing season, and for the following two, larvae feed on organic matter and roots near the soil surface. Pupation occurs early to mid-summer in the 3rd year of development, in cells about 10 cm below the soil surface.

White grubs have curved, milky-white, thick bodies (0.3-3.1 cm long, depending upon age) with three pairs of prominent legs and darker, chitinized heads and mouths. The hind part of the body is smooth and shiny, the body contents showing through the skin (Figure 85). There are two rows of minute hairs on the underside of the last segment that distinguish white grubs from similar looking larvae.

Management

Nursery managers should familiarize themselves with the local 3- or 4-year peak population cycle, in anticipation of possible grub damage. Light traps can be used to monitor adult population levels. Areas that have been in sod for 2 or more years and are to be converted to forest nursery, and infested fallow soils in long-established nurseries, should be shallow-plowed and frequently disked in early summer. Sometimes, pre-plant soil fumigation may be needed to rid an area of grubs. Insecticide drenches are sometimes applied to infested seedbeds, but they are expensive and may be ineffective because of the difficulty of distributing the material throughout the soil. Also, the short-lived efficacy of many of today's insecticides necessitates repeated applications.

Selected References

Banham, F.L. and J.C. Arrand. 1970. Recognition and life history of the major insect and applied pests of vegetables in British Columbia. B.C. Dep. Agric., Bull. 70-9, Victoria, B.C.

Downes, W. and H. Andison. 1940. Notes on the life history of the June beetle Polyphylla perversa Casey. Proc. Entomol. Soc. Brit. Col. 37: 5-8.

Summary

June beetles (white grubs)

Figures

Click on any image to see the full size version. Press "Back" on your browser to return to this screen.

    Figure 83. Douglas-fir damaged by June beetle larvae (white grubs).

     Figure 84. Life history of June beetles (3-year life cycle).

 Figure 85. June beetle larva, white grub (left) and adult of the ten-lined June beetle.