ELA presentation by Dr. Patricia J. Vittum, University of Massachusetts, Department of Entomology. Summary by Bruce Wenning, Horticulturist and Entomologist at The Country Club in Chestnut Hill, MA and Land’s Sake Consultant. Bruce also serves on the Ecological Landscape Alliance Board of Directors.
The term “grub” generally refers to the immature or larval stage of the beetle (Order, Coleoptera). White grubs are commonly referred to as pests of turfgrass. White grubs live in soil, are C-shaped, have six legs, chewing mouthparts, and feed on turfgrass roots and the roots of other plants. Contrary to popular belief not all turf-damaging white grubs are Japanese beetles.
The four white grub species that Dr. Vittum covered in her lecture are major pests of cool season turfgrasses in southern New England and are all in the family Scarabaeidae. They are Japanese beetle, JB, Popillia japonica, oriental beetle, OB, Anomala orientalis, European chafer, EC, Rhizotrogus majalis, and Asiatic garden beetle, AGB, Maladera castanea. All of these pests were introduced into the landscape and came to the United States without their natural enemies. That is why they are problematic in home lawns, athletic fields, parks, gardens, and anywhere their preferred hosts grow.
White Grub Biology
A review of the life cycle of a white grub clarifies specifics before going into depth on the species of concern. Dr. Vittum noted that it is easier to start with the adult stage, and she proceeded with a life cycle description using European chafer (EC) as the model. EC adults start flying in early evening by mid-June. They fly to tree and shrub branches and other objects such as light poles and chimneys in large numbers. This swarming activity can go unnoticed because it generally occurs around 9 p.m.
By early July, adult females produce a sex pheromone to attract males for mating. Once males find females, mating can take place for up to two weeks. After mating occurs, females carve out or excavate a small cell in soil or protected area and deposit just one egg per cell. Females initially search for, and are attracted to, the proper soil moisture conditions conducive to egg maturation and lay three to five eggs per night. Eggs can take between seven to 10 days to mature in this soil cell depending upon soil temperature and moisture conditions.
When eggs hatch, a small or first instar grub emerges and feeds for about two weeks on grass roots; the grub then molts to a second instar and feeds for about three weeks; then molts to the third instar and continues to feed from mid-August through October. ECs are not as cold sensitive as JB grubs so they can feed into December. EC is known as a cold weather feeder and observations have revealed them root feeding under snow cover. Eventually, as cold temperatures dominate, grubs stop feeding and dig down below the frost line. They return to the root zone by early spring, resume feeding for a few months then remain in the soil and enter the pupal stage (resting/transitional stage) around the first two weeks of June. The pupa (pre-adult) is a stage where the grub changes from a larval “feeding machine” to a reproducing adult.
Adults emerge by mid-June and are white colored until they “harden up” and become brown. When ready to fly, they feed primarily on locust and willow trees but not to damaging levels. Therefore, if you use the EC life cycle as the basic “grub model,” then the life-cycle events of the other three grub species occur about two weeks later.
Important Summary Points of a White Grub Life Cycle
(1) one generation per year.
(2) adults are active in summer (emerging around July 4th).
(3) grub stages are actively feeding in the fall (August through October) and again in the spring (April through May).
(4) not all turf and garden damaging white grubs are Japanese beetles!
Species Complex (distribution, identification, and behavior)
1. Japanese beetle (JB). Adults are also serious pests and feed on nearly 300 species of plants that include trees, shrubs, and vines. Adults have a green metallic colored thorax, coppery colored elytra (wing covers), chewing mouthparts and feed between the veins causing a characteristic skeletonizing appearance on leaves. Adults are strong day flyers and feeders. The JB grub has a small distinctive V-shaped rastral (spines) pattern and a transverse anal slit on the 10th abdominal segment discerning this species from the other three. JBs are widely distributed in all of southern New England and are more susceptible to insecticides and non-chemical controls than the other three species. According to Dr. Vittum, “Japanese beetle grubs are the easiest to control of the white grub species that we are covering today.” Keep that in mind.
2. Oriental beetle (OB). Adults look similar in size and shape to JBs but have a blotchy black and gray coloration to their elytra and a brown body with no metallic green thorax. Adults are usually night flyers and do not feed as voraciously as JB adults. Like the JB, this species has a transverse anal slit, but exhibits a distinct straight and parallel rastral pattern unique to this species. Dr. Vittum noted that this species used to be primarily a coastal problem but has now spread inland. OBs are found in Rhode Island, Connecticut, Cape Cod, eastern Mass., and around Amherst and the towns along the Connecticut River valley. The life cycle of an OB is similar to that of a JB.
OB grubs are less susceptible to many of the commonly used insecticides and are quick to move down in the soil profile during hot weather. Because OBs have the ability to detect and avoid hot, dry conditions, they become more difficult to control when insecticides are applied at this time. You can influence their behavior to your advantage by watering your lawn before you apply an insecticide. This action increases root zone moisture and cools soil temperatures causing the beetles to return to the root zone to feed. The OB is then more likely to come into contact with an insecticide if you choose to apply one.
3. European chafer (EC). Adults are night flyers and feed to such a limited extent that they are not considered a pest in this stage. EC grub populations are found in eastern Massachusetts; at the base of Cape Cod; in Rhode Island; in the Berkshires close to the New York border; along the Connecticut River valley; across southern New Hampshire; and up to the mid-coast of Maine. Eastern Massachusetts populations are within route 495 and 15 miles outside of this highway. Dr. Vittum stressed that the grub stage of this species is “an eating machine on turfgrass roots and are the most damaging of the grub species to turfgrass managers.” She also noted that “this particular species are cold weather feeders and cause turf damage earlier in the spring and later in the fall than the other three grub species [described here].” Reports of EC grub activity have revealed that they can feed under snow as early as February.
This species usually does four to five years of heavy damage before it declines to tolerable levels. Dr. Vittum stated that damage usually declines, but not always. The assumption about the “time pattern of damage” is that some natural enemies of this grub begin to reduce damage caused by the EC population. EC grubs are hard to control using labeled insecticides due to their physical size EC grubs are larger than JB and OB grubs. Also, EC grubs possess genetic characteristics allowing them to avoid and/or metabolize insecticides efficiently enough to escape being killed. EC grubs also are less susceptible to being controlled by some biological control agents than JB grubs.
To identify this species, look for a rastral pattern that is somewhat Y-shaped; rows of rastral spines look like an opening zipper near the anal slit. The life cycle is two weeks earlier and they feed longer than the other three species identified here.
4. Asiatic garden beetle. It is common to find the adults of this species in daylily beds; however, adults are nocturnal. AGBs are attracted to lights and are seldom seen feeding on host plants located near their daytime hiding places. Adult flight activity does not occur below 70 degrees Fahrenheit. These beetles are widely distributed in Massachusetts.
According to Dr. Vittum, chemical control using labeled insecticides has been inconsistent. Merit has not been effective against AGB, and it is suspected that the spread of this species has been the result of overuse of Merit which has killed the other grub species, allowing expansion of this one. To identify this grub look for a rastral pattern of spines in a reduced semi-circle arrangement. This grub is the smallest in size, as compared to the other three species, but it is the most aggressive. After digging these four grub species out of the ground and placing them in a sampling pan, AGB will “chase” after and bite the other grub species.
Approximate thresholds to white grub damage
White grubs damage turf by destroying the roots, making it easy to dislodge the turf from the soil. Birds, skunks, and raccoons provide an effective biological control by feeding on the grubs. Their feeding results in secondary damage and also alerts homeowners and turf managers to the presence of grubs.
|Insect||Approximate threshold per square foot||Size in comparison to each other|
|EC||4 to 6 larvae||large|
|JB||6 to 12 larvae||medium|
|OB||6 to 12 larvae||medium|
|ABG||10 to 20 larvae||small|
Thresholds indicated in the preceding table are based on skunk and raccoon activity. There seems to be a minimum grub population number that they can detect for each grub species. However, there is no special number of grubs that determine the need for control for all turfgrass situations. Thresholds vary by species. Soil, site, turf use, and management practices also vary for most turfgrass managers, so thresholds are approximate for non-irrigated turf. Irrigated turf can withstand greater numbers of white grubs than indicated in the table. Watering and good cultural practices ensure good turfgrass vigor throughout the growing season and can help safeguard against insecticide dependency for white grub control.
To monitor for white grubs, take a shovel and cut three sides of a one-square-foot sized turf plot. Pulling back on this dig so that the uncut side acts as a hinge, you will see the soil and root mass. Tap this mass periodically with a trowel to dislodge the soil from the roots and count the falling grubs as they are dislodged from the mass. Do this three times at different sites on your lawn and average the grub count. If the count for the species in question falls within the approximate threshold range, control may be warranted. Dr. Vittum indicated that researchers now use a golf course cup cutter (1/10 of a square foot) with conversion factors to estimate grubs per square foot.
Pheromone traps and mating disruption
Japanese beetle pheromone traps are not good for JB control; however, they are good as a monitoring tool to detect JB activity. If you decide to use such devices, do not place them in your roses. Instead, place traps at least 30 feet from the plants that you want to protect. The JB pheromone traps contain two lures; one is a flower scent, the other is a female sex pheromone that attracts males. Most of the available traps have yellow colored baffles that are attractive to JBs and help direct incoming flying males to a collection bag for later disposal.
Researchers at Rutgers University have conducted experiments using OB sex pheromone as a mating disruption technique that confuses males as to the location of females. Basically, OB female sex pheromone is broadly applied to the surface of turfgrass to confuse searching males. Males attempt to mate with the turfgrass/weed that is covered with pheromone. Consequently, mating between pairs is disrupted and greatly reduced.
Biological control alternatives to synthetic insecticides
1. Nematodes. Nematodes are microscopic worms that occur naturally in soil. Some species of nematodes attack white grub species. When those specific nematodes are applied as a bio-control agent to soil/turfgrass for white grub control they are referred to and sold as beneficial or entomopathogenic nematodes. These “commercialized” organisms are especially appealing to organic lawn care practitioners for locations where synthetic pesticides are not wanted or recommended. Not all landscape pests can be controlled using these bio-control agents. Nematodes that are commercially available are specific to pests stated on the product label. For best results, be certain that the beneficial nematode matches the biology of the pest in question. READ AND FOLLOW ALL LABELED INSTRUCTIONS BEFORE PROCEEDING.
The beneficial nematode Heterorhabditis bacteriophora (HB) is readily available commercially for white grub control on turf. Dr. Vittum stated that her research using this nematode (limited in scope) found a 70 to 90 percent control, in late summer trials, for all four white grub species. However, Dr. Albrecht Koppenhofer, Rutgers University, found that HB nematodes work on only the JB grubs and give 40 to 50 percent control of the remaining grub species. Dr. Vittum deferred to Dr. Koppenhofer’s research results. Steinernema scarabaei, a new species from New Jersey, is not yet commercially available. Research at Rutgers University found 80 to 90 percent control using this species against the four grub species described above. Dr. Vittum stated that continuing research on these organisms would certainly reveal new nematode species for turf pest control.
Dr. Vittum noted that there are handling concerns when using beneficial nematodes. They are sensitive to desiccation and must be watered in immediately after application. Nematodes for grub control are soil dwellers and live in the water films between soil particles. They are best applied before 10 a.m. or after 4 p.m. Dr. Vittum also indicated that the nematode Steinernema carpocapsae is not effective at controlling white grubs and should not be used for that purpose.
How do these nematodes work at controlling grubs? Bacteria that are pathogenic to white grubs are located in the gut of the nematodes. The nematode penetrates the grub through its natural openings such as the mouth, spiracles, and anus, and releases its gut inhabiting bacteria while feeding on the grub. The bacteria gradually kill the grub. Nematodes reproduce in the dying grub, move out into the soil, and penetrate and feed on more grubs, thereby infecting them and continuing the cycle.
What is the best time of year for application? Application from mid to late August targets second and third stage instars because grubs at these stages have larger openings to allow for nematode entry. The key to success at using these organisms is to follow labeled instructions scrupulously and pay special attention to watering!!
Nematodes move in soil water and are sensitive to dry conditions. Cool soil temperatures slow nematode activity. Note that the E.P.A. exempts nematodes from chemical registration and that protective equipment for application is not needed.
2. Bacteria. Milky disease or “milky spore” is caused by the bacterium, Paenibacillus popilliae (Bacillus popilliae) and kills only JB grubs. Dr. Vittum found it difficult to recommend this product because most grub damaging species are not JB. This bacterium naturally occurs in our soils but not at levels great enough to significantly control JB grubs. This product is available commercially and can survive low winter temperatures; however, it does not provide consistent JB grub control in western Massachusetts because our soils do not stay warm enough during the summer for long enough to ensure significant spore survival.
For unknown reasons, spore survival increases when this product is used in southeastern Massachusetts, near coastal zones (i.e., Cape Cod) or on sandy soils. All bacterial products have an E.P.A. registration and users should follow all guidelines for safety.
3. Fungi. Many soil fungi are associated with arthropods, primarily insects. Fungi that are necrotrophic (i.e., kill host cells in advance of its hyphae, then lives on the dead and dying cells) are naturally occurring biological control agents. Metarhizium anisopliae is one such fungus. It attacks white grubs naturally (when environmental conditions are right), but sparingly, thus it would not be an effective control agent in most turf situations. M. anisopliae is not commercially available. Dr. Vittum tested M. anisopliae against white grubs, using a slice seeder machine, and got “good control.”
4. Biorational compounds. MACH 2 is a biorational insecticide. It is less toxic than the petrochemical insecticide. The active ingredient is halofenozide. What does it do? When grubs ingest MACH 2 it interferes with their natural molting process by accelerating the instar growth stages beyond normal time progressio thus causing death.
MACH stands for Molt Accelerating Compound Halofenozide. This insecticide only works on first and second instar larvae. It does not work on third instars. Therefore, monitor your grub populations for proper instar stage BEFORE you decide to apply this product. Apply this product from hypens mid-July to mid-August. Dr. Vittum stated that MACH 2 is very effective for JB control, and quite effective for EC control. However, Dr. Vittum noted that, for unknown reasons, it is less effective for OB control and control for AGB is inconsistent. Dylox, a petrochemical insecticide, works on AGB, but it cannot be used on school fields. MACH 2 is available to homeowners. Check the label for halofenozide.
(1) Identify the species of grubs to select best control options.
(2) The best non-toxic control option for white grubs is the nematode, H. bacteriophora.
(3) For any bio or chemical product you must water turf immediately after application to ensure good control. Use at least 1/10 inch water for MACH 2, and 1/4 inch water for nematodes.
Bruce Wenning suggests I.P.M. Labs, Locke, New York as a reputable supplier of beneficial organisms. Phone: 315-497-2063. Web: www.ipmlabs.com. They supply Heterorhabditis bacteriophora.