Author Topic: White Grubs  (Read 1737 times)

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White Grubs
« on: May 04, 2011, 04:05:06 PM »
White grubs are the most important insect pests of lawns and turf in our region. There are 11 species of white grubs in the Northeast. Some are limited to small geographic areas and others occur only in certain turf conditions. The most common white grubs are foreign imports such as Oriental beetle, Japanese beetle, Asiatic garden beetle, green June beetle and European chafer.

General Identification
Adults of white grubs are scarab beetles, and they have several characteristics in common: They have hard wing covers, chewing mouthparts and antennae ending in a club made of flat plates held tightly together. They range in size from 1/4-inch (Asiatic garden beetle) to nearly one inch (green June beetles). Specific descriptions of the adults of the more common species are provided below. The adults do not feed on turfgrass, but those of some species, such as Asiatic garden beetle and Japanese beetle, feed on a wide range of other plants and can be significant pests of ornamental plants in the landscape and in fruit and vegetable gardens.

All white grub species have three larval stages. The larval body consists of a brown head capsule with chewing-type mandibles; a thorax with three pairs of short, jointed legs; and the abdomen. Thorax and abdomen are gray-white to cream colored, but the hind part of the abdomen often appears darker because of ingested soil and plant material in the gut. The wrinkled skin is covered with scattered, short brown hairs.

Feeding or resting grubs assume a characteristic C shape. Mature larvae range in length from about 3/4 inch (Asiatic garden beetle) to two inches (green June beetle). White grubs can be identified by examining the pattern of spines, hairs and bare spaces on the underside of the abdomen just in front of the anus. The shape of the anal slit also varies among species. A 10- to 15- power hand lens is adequate for viewing these species-specific features.

Damage Signs and Symptoms
White grubs damage turf by chewing off roots close to the soil surface. The voracious feeding of late second- and third- stage grubs, when combined with hot and dry conditions, can result in quick and extensive loss of turf from late August through mid-October. In spring, damage is less common; it usually occurs only under exceptionally warm and dry conditions. All cool-season and many warm-season grasses are susceptible to white grubs, but tolerance to damage varies.

Being alert to the symptoms of white grub infestations helps avoid extensive damage. First signs of infestation include gradual thinning, yellowing and wilting in spite of adequate soil moisture, and the appearance of scattered, irregular dead patches. The patches can grow together until large turf areas are affected. Infested turf feels spongy underfoot due to the larval tunneling and can be pulled up like a carpet, exposing the C-shaped larvae. Secondary, often more severe damage can be caused by vertebrate predators (e.g., crows, skunks, raccoons, moles) that tear up or tunnel under the turf to feed on the grubs, sometimes even at grub densities that donít cause turf damage on their own.

Seasonal History and Habits
Adult beetles emerge between June and August. Adults either donít feed (masked chafers), feed very little (Oriental beetle, European chafer) or feed extensively on many different plants (Asiatic garden beetle, Japanese beetle) and fruit (green June beetle). After mating, the females return to the soil to lay eggs individually or in small batches (total of 20 to 60) over a period of two to four weeks, typically at a depth of one to four inches.

The egg stage, first larval stage, and second larval stage each last about two to three weeks. At the latitude of New Jersey most of the grubs will molt to the third larval stage during September, but further north many grubs will not reach the third larval stage until October or even spring, and some larvae may take two years to complete development.

When the soil is warm and moist, grubs may be found feeding throughout the root-zone. (Green June beetle larvae feed on dead plant matter, not live roots.) The majority are no more than one or two inches below the thatch. As the soil cools in October, the grubs move to deeper soil layers, where they overwinter in an inactive state. However, European chafer grubs continue feeding later into fall and resume feeding earlier in spring than other species. As the soil warms in spring, the grubs return to the root zone to feed for another four to six weeks in April and May (and into early June in more northern areas) before pupating in the soil at a depth of two to eight inches. After one to two weeks, the new beetles emerge to restart the life cycle.

Monitoring
Monitor for white grub populations in mid-August. Adult beetles generally prefer to lay their eggs in sunny locations on well-managed and well-watered turfgrass, so focus sampling on those areas. In areas of suspected infestation, low tolerance areas or areas with a history of grub infestation, take sod/soil samples with a golf hole cup cutter or a flat blade spade to a depth of three inches and look for grubs. Record the number and species (check raster pattern with a hand lens) found in each sampling spot on a data sheet or map. Place the soil and sod cap back and irrigate, if conditions are dry, to promote turf recovery.

Take several samples in a grid pattern. If no historical records exist, lay out a grid throughout the turf area (six- to 10-foot squares on lawns, 10- to 20- foot squares on sports fields), take a sample at each spot and record the number, species and stage of grubs found. Transform numbers into grubs per square foot values.

Damage thresholds lie in the range of six to 10 (Japanese beetle, Oriental beetle, European chafer, green June beetle) and 15 to 20 (Asiatic garden beetle) grubs per square foot. However, well maintained turf with an extensive root system can tolerate higher grub densities. Only treat areas in which grub densities exceed the damage threshold.

Asiatic Garden Beetle
Adults: 1/4-inch, velvety brown.
  • Voracious nighttime feeder on flowers and leaves of 100 host plants
  • Hides under weeds during the day, flies to lights at night
  • Larvae: Dominant in mossy lawns
  • Crescent raster

European Chafer
     Adults: 1/2-inch, light reddish-brown with slightly darker head and on the segment just behind head and distinct longitudinal grooves on the wing covers.
  • Feeds very little as an adult
  • Active during muggy evenings
  • Gets into houses, particularly through chimneys, during mating flight
  • Larvae: Causes more damage to turf than any other type of white grub
  • Y-shaped raster and anal slit
  • Most difficult to control with nematodes

Oriental Beetle
     Adults: 1/2-inch, straw-colored with variable black markings on the thorax and wing covers
  • Feeds to limited extent on flowers
  • Adults often found floating in swimming pools
  • Larvae: Found in high-organic potting media and in other ornamental and crop plants, in addition to turf
  • Parallel raster

Japanese Beetle
     Adults: 1/2-inch, metallic green head and thorax, copper colored wing covers, tufts of white hairs around abdomen
  • Feeds extensively on foliage and flowers of hundreds of host species
  • Pheromone/floral lures are available, although effectiveness at preventing damage is debatable
  • Larvae: V-shaped raster
  • Larvae are grass specialists - not found eating the roots of the many adult host plants

White Grub Management

Cultural control
Good turf management (proper irrigation, fertilization, mowing) will result in vigorous turf with a deep, extensive root system that can tolerate relatively high grub densities. Turfgrass species that have a deeper root system and higher heat and drought tolerance are generally more tolerant of grub feeding. Among the cool-season grasses, tall fescue is the most grub-tolerant species, followed by Kentucky bluegrass, the fine fescues and perennial ryegrass. Be aware that compost and many organic fertilizers may attract egg-laying females of green June beetle and encourage higher larval densities of this species.

Watering during adult beetle activity in summer attracts egg-laying females (especially when the soil in surrounding areas is dry), and increases survival of eggs and young larvae. However, later in the summer and in fall, irrigation makes the grass more tolerant of the increasing feeding activity of the larger (and more drought-resistant) grubs.

Mechanical control
No effective mechanical grub controls are available at this time. Aeration with spikes or tines to alleviate soil compaction will kill any grubs that sustain a direct hit, but normal aeration activity will kill only a small proportion of the grubs. However, any control agents that are applied at the same time will more easily penetrate through the thatch layer, if present, and may improve the control agentís efficacy.

Removal of adult beetles with traps has not proven to be very feasible. Japanese beetle traps attract large numbers of male and female beetles from long distances, thus increasing beetle populations in the vicinity of the traps. But only around 50 percent of the attracted beetles end up in the traps, while the remainder may start feeding on surrounding woody ornamental plants and perennials and eventually lay eggs close by. Thus the traps can actually increase problems with Japanese beetle grubs. Only if high densities of traps are used throughout a large area (e.g., a neighborhood) can this method be effective.

The same traps, but with a different lure, are available for oriental beetles. However, the traps only attract male oriental beetles (the lure contains only the sex pheromone), while the females continue to lay eggs with impunity. No data exist (yet) on reducing oriental beetle grub populations through mass-trapping of males. It appears rather unlikely that enough males (probably more than 90 percent) could be trapped out of a given area to significantly reduce the mating success of female oriental beetles.

Natural enemies
In addition to the various vertebrate natural enemies (crows, skunks, etc.) that unfortunately tend to cause turf damage when preying on white grubs, there are numerous more subtle invertebrate natural enemies. Ground beetles, rove beetles, ants and other beneficial insects prey on eggs and young grubs. Ants are probably the most effective predators of white grub eggs, as has been shown in several studies. Various parasitic wasps  and flies parasitize the older grubs or the adults was introduced for Japanese beetle control). Providing nectar sources (various flowering plants) for the adult wasps near areas with a history of white grub infestation should encourage their parasitic activity. Various naturally occurring pathogens (insect parasitic nematodes, fungi, bacteria, protozoa) kill or weaken white grubs. In the absence of regular applications of broad-spectrum insecticides, practitioners of organic lawn care should strive to preserve these beneficials and thereby decrease the number and severity of white grub outbreaks.

Milky disease
Since the commercially available strain of milky disease (Milky Spore) is ineffective against white grubs other than those of the Japanese beetle, applicators who are interested in using milky disease products should first make sure that their grubs are actually Japanese beetle grubs. (There are other, naturally present strains that infect other grub species, which explains why you may observe milky disease in grubs other than Japanese beetle grubs when sampling white grubs in the field.)

The infection process begins when spores are ingested along with soil as a white grub feeds on plant roots. The spores germinate in the grubís midgut, and the bacteria penetrate through the midgut wall into the grubís body cavity, where they multiply and eventually sporulate. The high concentration of spores during the final stages of infection gives the grubís body fluid the distinctively milky-white color. When the infected grub dies, typically after several weeks or even months, several billion spores may be released into the soil from the disintegrating cadaver.

Milky Sporeģ is applied in a four-foot grid pattern and leaches into the root zone with irrigation and rainwater. Japanese beetle grubs feeding under those spots may become infected with the disease. When they die several weeks later, billions of new spores are released.

Under the right conditions (soil temperatures of 70 degrees Fahrenheit for several months of the year and high larval densities), milky disease slowly spreads through an entire lawn and can suppress Japanese beetle grubs for many years.

Insect Parasitic Nematodes
The information provided here pertains specifically to the use of insect parasitic nematodes against white grubs. See the information on insect parasitic nematodes presented earlier in this chapter for general recommendations on application of nematodes (rates, adaptation of sprayers, irrigation, etc.).

Susceptibility of White Grubs to Nematodes
The efficacy of different nematode species varies with white grub species. Among the common white grub species that damage turfgrass in the Northeast, the Japanese beetle is the most susceptible to the most nematode species. Among the nematodes that are presently commercially available, Heterorhabditis bacteriophora has generally provided the best control of Japanese beetle grubs, with around 70 percent control at a rate of one billion infective juveniles per acre applied around September.

Other white grub species such as Oriental beetle, European chafer and masked chafers appear to be less susceptible to Heterorhabditis species and to Steinernema glaseri. For example, mid- September applications of H. bacteriophora against the oriental beetle have provided 0 to 50 percent control. Heterorhabditis zealandica, a species already commercially available in Australia and recently isolated in Florida, could be very effective against masked chafer and somewhat more effective than H. bacteriophora against Oriental beetle, should it become available. Another species that is not presently available, Steinernema scarabaei (discussed in more detail below), has shown exceptionally high activity against all white grub species except masked chafers.

Larval Stage and Nematode Efficacy
Laboratory studies have shown that the efficacy of H. bacteriophora declines against Japanese beetle and oriental beetle larvae as larval development proceeds. Second instars may be somewhat less susceptible than first instars, but third instars are clearly less susceptible than second instars. It has also been observed that young third instars of Oriental beetle are significantly more susceptible than older third instars. Based on larval stage alone, this would mean that H. bacteriophora should be applied while the larvae are still mostly second instars and early third instars (mid-August through early September at the latitude of New Jersey) to increase efficacy. The effectiveness of nematodes on the different larval stages, however, seems to vary with nematode species and white grub species. For example, susceptibility of Oriental beetle second instars vs. third instars was similar for both S. scarabaei and S. glaseri.

Optimal Timing for Nematode Applications
Based on optimal soil temperatures and white grub larval stage susceptibility, the best time to apply most nematode species against white grubs should be from mid-August into early September. Application after mid-September or in spring will generally be ineffective. Late summer applications will not only provide better soil temperatures and, at least for H. bacteriophora vs. Japanese beetle and Oriental beetle, more susceptible targets, but also a longer period of time for the nematodes to be active. Because nematodes are susceptible to UV radiation and extreme temperatures, it is generally recommended that they be applied either early or late in the day. Time of day is less critical on cloudy days or with high spray volumes, if treatments are quickly followed by sufficient irrigation.

Soil Type
Insect parasitic nematodes can move better through coarse
(sandy) soils than they can through fine (higher clay content) soils.  
However, in the field many other factors affect soil type, including soil compaction, organic matter, roots, tunnels of soil organisms, etc. In addition, finer soils hold soil moisture better. In a summary of dozens of field trials using Heterorhabditis bacteriophora against Japanese beetle grubs, the nematode tended to perform better in the heavier, loamy soils. Insect parasitic nematodes work best in moderately moist soil.

Soil Temperature
While optimum temperature ranges vary somewhat by species, nematodes generally work better at soil temperatures of 60 to 93 degrees Fahrenheit, with the optimum being 70 to 85 degrees Fahrenheit. For the presently available nematode species that attack white grubs, particularly Heterorhabditis bacteriophora, that means that optimum activity against grubs can be expected from August into late September (depending on local conditions). After September and in spring, these nematodes will not be effective.

Organic Control Options Under Development As mentioned above, the nematode species Heterorhabditis zealandica, which is already commercially available in Australia, could be very effective against masked chafer and somewhat more effective than H. bacteriophora against Oriental beetle, should it become available in the United States.

The recently discovered species Steinernema scarabaei has exceptionally high activity against most white grub species (except masked chafers) and may provide several years of grub control, but it cannot be mass-produced at this time.

Mating disruption using sex pheromones deployed from waxed disks to disrupt communication between males and females has already been shown to be highly effective for Oriental beetle control in blueberries and ornamental nurseries. However, in turfgrass more persistent formulations need to be developed.
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