Scouting and Control of Common Pests of Fraser Fir  

White grubs are the immature stage of scarab beetles. They live in the soil for up to 3 years before maturing into the adult beetle.
    Symptoms. White grubs eat Fraser fir roots, leaving few feeder roots and stripping the bark off of main roots. Occasionally, feeding marks can be found on larger roots. The loss of root causes aboveground symptoms of tree yellowing, wilting, and dying.
    Pest description. White grubs have a white body with six legs and a dark head and usually lie in a curled or C-shaped position. The grubs that feed on  roots of Fraser fir are the immature May and June beetles, (Phyllophaga and Polyphylla spp.). White grubs are identified by the arrangement of bristles and hairs on the underside of the tip of the abdomen, called the raster pattern, which can usually be seen with a hand lens.

 

    Similar appearing problems. Symptoms similar to grub damage can have many causes in the field including Phytophthora root rot, drought damage, and improper planting, especially on trees set 1 or 2 years. To distinguish between these problems, pull up some affected plants and examine roots, looking for the loss of roots due to grub feeding. Also, dig in the soil around these tress to look for grubs.
    Not all white grubs attack Fraser fir roots. One commonly found grub that eats organic matter on the surface of the soil is the green June beetle grub. This grub is large and wiggles on its back when laid on the surface of the soil. Since it is often difficult to distinguish between the different types of grubs, ask your county extension agent to examine any grubs you may find, or send them to the Plant Disease and Insect Clinic for positive identification. Grubs will survive for several days if kept cool in a plastic bag with moist soil. However, larger grubs may eat smaller ones, so it may be necessary to separate them.
    When to scout. Grubs bury deep in the soil during the winter to avoid freezing. Therefore, it is necessary to let soils warm before scouting for grubs.
    Scouting method. Select five sites in a 1- to 2-acre field. Select sites where grubs are suspected because of yellowing trees. Other indications of grubs include holes where foxes, crows, or other animals may have been digging for grubs, poor stands of grass, or patches of wild strawberries. Without such indicators, select sites at random.
At each site, dig a hole 1-square foot and 6- to 8- inches deep. Sift through the soil, and count any grubs found. Some grubs are only 1/4 inch long; others are 2 to 3 inches. Small grubs are easy to overlook so check the soil carefully. Have grubs identified so that you will know if they are the species that damages Fraser fir.
    Treatment threshold. If on the average, more than one grub per hole is found, chemical control is recommended.
Scouting frequency. Scout for grubs the fall before trees are set and any time throughout the growing season that yellowing or dying trees are observed.
    Effects of weather. Weather that puts trees under stress, such as hot, dry weather, will increase aboveground symptom expression of grub damage since the root system is not able to support the tree. Droughts can also kill grass ground covers leaving nothing else for grubs to eat except tree roots.
    Control. If trees have not been set, incorporate an appropriate insecticide in the ground along the strip where trees will be set. Kill the ground cover only in the strip to be planted. Leaving ground cover between rows provides a food source for grubs and makes them less likely to eat Fraser fir roots. If trees have already been planted, apply the insecticide in a band over the trees during or just before a soaking rain. Do not treat where runoff is likely to enter streams or ponds.
    Proper ground cover management is more important than treating with an insecticide to control grubs and to keep them from returning. Do not mow from May through early August since adult beetles prefer to lay eggs in short grass. Maintaining ground covers with low rates of post-emergent herbicides will keep adult beetles from laying eggs and hatching more grubs. Your county extension agent can help you develop a ground cover management program.
    Scouting after control. To make sure grubs have been killed, dig in the soil where grubs had been readily found 1 to 2 weeks after treatment. If live, healthy grubs are found, it may be necessary to retreat with another material.