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White Grub Biology and Management1
Eileen A. Buss2White grubs have been a well-kept secret in Florida turfgrass until recently. The major pests of turfgrass used to just be mole crickets and chinch bugs, but grubs are getting harder to control.
What's the problem? Well, a lot of the grub control recommendations that are given on the internet, in books and extension fact sheets come from studies in the northern and midwestern U.S. We have several different species in Florida and they develop faster here and have different hosts. Thus, the timing may be different for preventive and curative insecticide applications.
White grubs are immature scarab beetles. They hatch from eggs laid in the soil, have three larval stages or instars, and also pupate in the soil. The third instar is usually the most damaging. The adults are rarely turf pests, but some may feed on tree leaves or make mounds in the soil.
Signs of Infestation
When white grubs feed on grass roots, the grass gradually thins, yellows, and dies, which leaves the turf feeling soft and spongy. Scattered, irregular, brown patches of grass appear, which increase in size over time. The root injury reduces the turf's ability to take up water and nutrients and withstand drought stress. Heavily infested grass pulls up easily because the root system is gone.In addition, white grubs attract moles, raccoons, armadillos, and birds, which can make an already damaged area look worse. However, these animals may be interested in earthworms or other insects besides grubs. Large numbers of dark-colored, parasitic wasps hovering over the lawn on sunny days in the summer or fall may also be a sign of infestation. Sample the area to confirm that a white grub problem really exists.
Sampling
Proper monitoring and identification can prevent turf loss and costly renovation. To confirm a grub infestation, get a shovel and sift through the top 3 to 4 inches of soil, roots, and thatch. Look for creamy-white, C-shaped beetle larvae, with tan to rusty-brown heads and six legs. Larvae that look like grubs but lack legs are probably billbugs. Mature grubs vary in length from 1/4 to 2 inches, depending on species and age. The pattern of hairs, or raster ( Figure 1 ), on the tip of the grub's abdomen helps in identification, but adults are needed for an accurate species identification. After examining the soil, tamp the grass back into place and water it. It is normal to find an occasional grub and is not cause for alarm. Healthy turf can usually outgrow the root loss caused by a couple of grubs.
Figure 1. Raster (hair) pattern on the inner tip of a grub's abdomen. Life Cycles
The most abundant species of masked chafers ( Figure 2 ) in Florida are Cyclocephala lurida and C. parallela. They have 2 generations each year throughout Florida, and the first generation is the most damaging. Adults are tan, about 5/8 inches long, and slightly smaller than May/June beetles. Adults fly from April to June and again in August and September. They are attracted to lights at night, but do not feed. Adults lay their eggs in the top inch or two of soil, often in small clusters. Small grubs hatch from the eggs and feed on grass roots. Most damage occurs from the first generation in July. The second generation rarely needs to be controlled. The raster pattern is indistinct and does not have any rows of short, thick hairs ( Figure 3 ). They pupate in earthen cells.
CREDITS: L. Buss, University of Florida
Figure 2. Masked chafer adult. Sugarcane grubs (Tomarus subtropicus) have a 1 year life cycle. The grubs attack sugarcane, most warm season turfgrasses, and some ornamental plant roots. Most adults emerge from late April to August, but are most abundant in June. Eggs are white and round, and are laid about 3-4 inches deep, singly. Small grubs are dark gray and whiten as they grow. Third instar (mature) grubs are present and feeding from September to late spring, and are nearly 2 inches long. The larvae have rusty-red colored head capsules, and their raster pattern looks similar to that of masked chafers ( Figure 3 ). Pupae are tan-colored. Adult beetles are about 1 inch long, and are nearly black, with tiny horns behind the head ( Figure 4 ). This species is most damaging along the Gulf and Atlantic Coasts. A sister species, T. cuniculus, is increasing in abundance in southeastern Florida. The larvae look nearly identical to sugarcane grubs. The adults are present every month of the year in Fort Lauderdale, with greater activity during the summer. The adults are more hairy than sugarcane grub adults.
Figure 3. Raster patterns of some white grubs in Florida; May/June beetle (left), Masked chafer (center), Green June beetle (right). (Adapted from Ohio State University Cooperative Extension Service.)
CREDITS: L. Buss, University of Florida
Figure 4. Sugarcane grub adult and larva. Most of the May/June beetle (Phyllophaga spp.) grubs have a one-year life cycle in Florida, but Cuban May beetle, P. bruneri, adults have 2 generations in southern Florida and can defoliate some ornamentals. Florida has about 54 species of May/June beetles. Adults ( Figure 5 ) of different species have the same general shape, but may vary somewhat in color, size (1/2 to 1 inch long), and hairiness. They often feed at night on tree and shrub leaves, from late April to early July. Eggs are laid singly in the soil. Mature grubs reach up to 1 1/4 inch long. Grubs have a zipper-like raster pattern ( Figure 3 ). Grubs can be very damaging to the roots of ornamental plants grown in pots or nurseries. These grubs are less damaging on turfgrass than some of the other scarabs.
CREDITS: P. Choate, University of Florida
Figure 5. May/June beetle (Phyllophaga) adult. Green June beetle larvae feed on decaying vegetation and turfgrass roots. They may be damaging in over-fertilized turfgrass or where organic fertilizers are used. Grubs feed in the soil during the day and feed aboveground on decaying organic matter at night. Their frass pellets on the soil surface are a sign of infestation. Mature grubs (ca. 2 inches long) can cause considerable turf damage through February during a warm winter. This is one of two damaging grub species in Florida that can walk on their backs with their legs in the air ( Figure 6 ). Its raster pattern has a lot of very short hairs ( Figure 3 ). The coppery-green adults (3/4 to 1 inch long; Figure 7 ) fly during the day and feed on grapes and other thin-skinned fruit during the summer. They have a one-year life cycle.
CREDITS: L. Buss, University of Florida
Figure 6. Green June beetle grub walking on its back.
CREDITS: L. Buss, University of Florida
Figure 7. Adult green June beetle. The other grub that walks on its back is the flower beetle, Euphoria sepulcralis. This day-flying beetle may have one or two generations each year, but adults occur nearly year-round throughout Florida. The adults are about 1/2 inch long, dark brown to black, with metallic bronze or green reflections ( Figure 8 ). Adults feed on several plants (corn, roses, mangos, avocados, thistle, mock orange, milkweed, dogwood, sumac, yarrow, daisies, and goldenrod), as well as fruit tree blooms, sap, and decaying fruit. Grubs live in the soil beneath dead sod or manure, in sandy soil containing oak roots and decaying oak leaves, and in seashore paspalum and bermudagrass. Grubs may develop for about two months, and pupate in earthen cells for about two weeks. The raster pattern is V-shaped. An infestation of 8-10 grubs per square foot may not cause obvious turf damage, but is enough to attract birds.
CREDITS: L. Buss, University of Florida
Figure 8. Adult Euphoria sepulcralis, a flower beetle. White Grub Management
Water
Moist soil is critical for eggs to hatch and grubs to survive. Frequent irrigation during adult flights may attract egg-laying females, especially if surrounding areas are dry. But, proper irrigation and fertility may help the turf to tolerate or outgrow moderate infestations. This latter strategy is risky, though, because animals may dig up the turf or watering restrictions may occur.Natural Enemies
Several predators (e.g., ground beetles and ants) and parasitoids (e.g., tiphiids or scoliids) attack white grubs. Parasitic wasp larvae usually feed externally on the grub, kill it, and then spin a fuzzy, brown cocoon in the soil. Few of the natural enemies of grubs are known in Florida, so their effectiveness is also unknown.Pathogens
Currently, no biopesticides exist that provide reliable control of white grubs. Although several strains of the milky disease, Bacillus popilliae, attack white grubs, most current commercial products are considered ineffective by researchers.Insect Parasitic Nematodes
Commercial preparations of insect parasitic nematodes (Steinernema and Heterorhabditis species) can suppress white grub populations. Field trials of S. carpocapsae strains have generally resulted in less than 50% control, though H. heliothidis strains have achieved 80% control or better in the Midwest. Some nematodes may work synergistically with imidacloprid (Merit®) to provide greater grub control.Curative Applications
Curative treatments like trichlorfon (Dylox® or Bayer Advanced® 24-Hour Grub Control) are applied after grubs have been feeding and damaging the turf. Application timing depends on when the grubs or damage are discovered, but usually occurs when grubs are second or third instars. The best timing may be in July for masked chafers, but may be in August or September for sugarcane grubs. Curative products have short residuals, so a second application may be needed. Most grubs die in the soil, but Green June beetle and flower beetle grubs die on the soil surface, where they rot and make a mess.Preventive Applications
In areas where grubs are repeatedly a problem, preventive applications may be helpful. Preventive treatments give turfgrass managers more flexibility in application timing than curative treatments. They also require less sampling and monitoring of grub populations. Preventive control requires the use of long residual insecticides, such as imidacloprid (Merit®, Season-Long Grub Control®), halofenozide (Mach2®, Ortho Grub-B-Gon®), and clothianidin (Arena®). These products give good control of newly hatched grubs. The best application period is before egg hatch until the time when very young grubs are present. Preventive products provide poor curative control of older grubs.Ensuring Successful Treatments
To get the best results with any grub insecticide, mow and rake out dead grass and thatch (if more than ½ inch thick) before treatment. This allows the insecticide to penetrate better and reduces the amount of insecticide bound in the thatch and organic matter. Also, water the grass immediately after treatment (1/4 to 1/2 inch of water) to leach the insecticide into the root zone where the grubs are feeding. Irrigating also brings grubs closer to the soil surface, which increases their contact with the insecticide residues. If irrigation is not possible, apply the insecticide just before a good rain. Prompt, post-treatment irrigation is especially important for sprays; once spray residue dries on foliage, it cannot be washed into the root zone by later drenching. Granular formulations may work better if you can't irrigate immediately.For Additional Information
Sugarcane grub: http://creatures.ifas.ufl.edu/ orn/turf/sugarcane_grub.htmRitcher, P. O. 1966. White grubs and their allies. Oregon State University Press, Corvalis, OR. 219 pp.
Footnotes
1. This document is ENY-321 (LH037), one of a series of the Department of Entomology and Nematology, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Date first published October 1993. Revised: January 2006. Please visit the EDIS Website at http://edis.ifas.ufl.edu.2. Eileen A. Buss, assistant professor, Department of Entomology and Nematology, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.
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