CTN - 08A
Jill R. Sidebottom,
Extension Forestry, 1993 and
Jim McGraw, Forest Resources
Updated November 1, 1995
Introduction. White grubs are the immature stage of beetles. Depending on the species, they can live in the soil for up to three years before maturing into the adult beetle. Grubs have a white body with six legs and a dark head. Not all grubs will eat Fraser fir roots. Some, such as the Japanese beetle grubs, only eat grass roots. Others, such as the Green June beetle grubs, only eat dead organic matter on the surface of the soil at night.
The grubs that will eat Fraser fir roots are the brown May and June beetle grubs (Phyllophaga and Polyphylla spp.). Even though these grubs prefer to eat the roots of grasses or wild strawberries, when they are present in pastures where herbicides have completely killed grass sod, the grubs are forced to eat the only remaining fresh food -- the recently planted conifer roots. Though grubs are primarily a pest of young trees, older trees can also be damaged by grubs if grass growing between rows is suddenly killed because of herbicide applications or drought.
Life cycle. The adult beetles feed on the leaves of hardwood trees, especially oaks. They mate and lay their eggs in short grass from May through July. Grubs are common in old pastures or where grass is kept short through mowing. During the winter, the grubs burrow several feet deep in the soil to keep from being frozen. When the soil warms up in the spring, the grubs gradually return to the root zone, feeding throughout the growing season. During the winter of their second year, they begin molting to the adult form. They will emerge as an adult beetle the following spring to begin the life cycle again.
Grub damage. White grubs damage trees by eating the roots. The needles of damaged trees will yellow, and the trees will eventually die. These symptoms can have many causes besides white grubs, especially on trees set one or two years in the field. Phytophthora root rot, drought damage, or improper planting produce identical above-ground symptoms. To distinguish white grub damage from other problems, some affected plants should be pulled up and the roots examined. Grubs will eat the feeder roots and bark from primary roots, leaving little of the root system though what is left will appear healthy. You should also look for the grubs in the soil if you see these symptoms (see below).
An IPM approach to grub control. Integrated pest management (IPM) provides the best approach to managing grub problems. Tree damage due to grubs can be avoided by scouting for grubs before planting and treating with a pesticide if necessary. Proper vegetation management is critical to grub control by decreasing grub damage and discouraging grubs from coming back. If trees are already dying from grub damage, pesticide applications will not save trees.
Scouting for grubs. Fields should be scouted for white grubs before trees are set. Grubs are difficult to kill because the insecticide is bound to the clay and organic matter in the soil and will not move deep enough in the soil to affect the grubs. Grub control is improved if you can incorporate an insecticide into the soil which can only be done before trees are set. It is especially important to scout old pastures that are to be set in trees since grubs will almost always be present at such locations.
Scout for grubs from June through October. It is not possible to adequately assess the prevalence of grubs in the spring since they may not have returned to the root zone. Fields to be planted in the spring should be scouted for grubs the summer or fall before planting.
Select five locations in a one to two acre field. Select locations where grubs are suspected because of poor stands of grass (possibly due to grub feeding), wild strawberries (a favorite food of grubs) or where moles, foxes, crows, or other animals have been feeding on grubs. Otherwise, choose locations at random.
At each location, dig a square foot hole that is 6-8 inches deep. Sift the soil through your fingers, looking for grubs and keeping count of the number found. Grub size will vary from the size of your thumb to some no longer than 1/4 of an inch. These small grubs can be easily missed so look carefully.
If trees are turning yellow-green and dying and grubs are suspected, scout for grubs in the same way, only dig holes near affected trees.
It may be necessary to have the grubs identified to make certain they are May and June beetle grubs, and not some other species. Since it is difficult to distinguish between the different types of grubs, ask your county Extension agent to examine any grubs you may find and send them to the Plant Disease and Insect Clinic for positive identi- fication. Grubs will survive for several days in a plastic bag with moist soil that is kept cool. Large grubs that wiggle and crawl on their backs are Green June beetle grubs (different from the June beetle) and will not damage tree roots.
Economic threshold. Record the number of grubs found in each of the five holes. Add up the total number of grubs found in all the holes and divide by five. This is the average number of grubs. If this number is greater than one, chemical control is recommended.
Pre-plant chemical control. The best time to treat for grubs is in September. Incorporate an appropriate insecticide in the soil along the strip where trees will be set. Only kill the ground cover in this strip. Leaving ground cover between the rows will provide a food source for grubs so they will be less likely to eat the roots of newly set Fraser fir transplants.
Chemical treatment after trees are set. To get the best possible kill after trees are set, treat in September with an insecticide such as Triumph 4E which will move through the soil. Spray the material on the ground where tree roots are located. Treat before a soaking rain, but do not treat if heavy rains are expected that might cause run-off. Do not use Triumph close to streams or ponds since it is very toxic to fish. Triumph 4E has a 24 (c) Special Local Needs permit in North Carolina. You must either have a pesticide applicators licence or be working under the direct supervision of someone with a licence to use this material, as well as a copy of this North Carolina label.
Scouting after chemical control. Two to three weeks after chemical control, scout the fields in the same way as before grub treatment to see if grubs can still be found. If you see dead grubs on the surface of the soil after applying an insecticide, you have only killed the green June beetle grubs which feed on the surface of the soil on dead plant material at night. You may or may not have killed the grubs that are damaging your trees.
Ground cover management. Proper ground cover management is more important than treating with an insecticide to control grubs and to keep them from returning. Maintain a ground cover with suppressive rates of post-emergent herbicides. Grubs that are already in the ground will have something to eat besides Fraser fir roots. All grubs will mature and leave within two growing seasons. By not mowing from May through August, no more eggs will be laid in the field by the adult beetle to produce more grubs. Your county Extension agent can help you develop a ground cover management program for your trees.
Recommendations for use of agricultural chemical brand names and any mention of commercial products or services does not imply endorsement by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned in this publication.