Got Grubs?
By the time the 4th of July comes around, your lawn is probably the last place you want to be during Virginia’s hot summers. Quality lawns in Virginia can be challenging, especially during the hot, humid summers. Virginia is located in a transition zone for turfgrasses, with climates that can be harsh in both winter and summer. Cool-season grasses most commonly used here, such as Kentucky bluegrass, rye and tall fescue, grow strong enough and deep enough to cope with our hot summers and cold winters. Warm- season grasses such as Zoysia and Bermuda grass are sun-tough, plus drought and humidity tolerant, but they do go dormant in the winter, making for a brown lawn until spring. There is actually a green spray paint that supposedly conquers this problem, but why bother?
In recent lawn care articles for The Garden Shed, I have emphasized the importance of mowing high and watering long and infrequently. As a reminder, mowing height is 3-3.5 inches for fescue type grass or even 4 inches in the stressful heat of July and August. The warm season grasses such as Zoysia and Bermuda can be about 1.5-2 inches. A lawn prefers 1-1.5 inches of water per week and possibly more in July and August. It is the natural cycle of cool season grasses to go dormant with the heat, and then to green up in the fall — and that’s fine if you can stand to look at brown grass for the summer months.
White grubs
Grubs
Besides proper mowing and watering, there is actually very little else to do for your lawn in July. However, making some observations now may lead you to consider whether grubs are a problem in your lawn. The mere presence of grubs is not always a problem and may not require treatment.
Not sure you know a grub when you see one? When you first turned over the garden soil in the spring, you may have spotted some of the C-shaped white grubs. I know I did and instinctively, pulled them out of the way when planting, thinking they would eat my flowers or vegetables. However, grubs prefer eating the roots of your lawn! Their favorite place is grass in the sun with high moisture — such as beautiful green lawns that have an irrigation system. If this lawn is surrounded by dry non-irrigated lawns, it is a prime target for egg-laying grubs. Grubs usually do not like shady lawns. Thus, highly maintained lawns, golf courses and athletic fields are the most susceptible to grub damage.
Grub Life Cycle
Grubs are the larval stage of scarab beetles. Japanese beetles and northern masked chafer grubs are the predominant white grubs associated with damaging home lawns. Grubs are dirty white in color with a soft bodies, brown head and 6 legs. The size varies with the species and age, but full grown Japanese beetle grubs average about an inch. Japanese adult beetles are a brilliant metallic green, 3/8 inch to 1/2 inch long, with coppery brown wing covers, five lateral spots with white hairs on each side of the abdomen and short gray hairs covering the underside of the insect. I must say I have never examined a Japanese beetle this intensely to know for sure. What I do know is that these insects can sure eat their way through an ornamental garden, consuming foliage voraciously. The adults can cause more damage than the grubs themselves in a home landscape.
The life cycle of most of these beetles is 12 months. May or June beetles have a three year life cycle. The adult beetle lays its eggs in the ground during the summer. As soon as they hatch, they start feeding on the grass roots until cold weather drives them down two to eight inches deeper into the soil where they overwinter. When warm weather arrives in the spring, the grubs (now at their largest size) move up from the lower soil regions and begin feeding again near the surface until they become mature and pupate, usually in June and July. This is when you begin to see Japanese beetles in your yard. These adults again lay thousands of eggs throughout the summer.
Damage
Grub damage
How do I know if I have grub damage in my lawn? Look for patchy, wilting and browning of irregular shaped areas of the lawn. Also, think back to late last spring or fall, when grub damage is easier to spot. If the turf can be easily pulled from the soil, this suggests grub damage because of the lack of a root system. The other sign of grub damage is from skunks and raccoons digging up lawns in search of grubs to eat. The only way to confirm that this damage is from the grubs is to actually see them. In midsummer, though, they will be too small to identify. Most of the damage done by the skunks and raccoons is far worse than the grub damage itself. Late spring and mid-fall are the easiest time to see the damage rather than midsummer.
So in late August to October, turn the turf back in any damaged areas by taking a shovel and digging up a square foot of turf samples around the suspicious area, down to a depth of 2 inches. Look for the 3/4 inch white grubs. Some experts say if there are more than 6-10 grubs/square foot, treatment may be justified, but there are many variables and the number itself should not always lead to treatment. Be sure to lay the sample back down, pack it firmly and water the area to prevent it from drying out and completely dying.
Predicting grub damage
Trying to predict grub damage is difficult as insects go in cycles. They may have been in your lawn previously and now they are at your neighbor’s. But if you see lots of adult beetles, specifically Japanese beetles, in July, that is an indication they are laying eggs there. The masked chafers are more active after sundown, so it will be difficult to identify them. They are chestnut brown and covered with fine hairs and are about 1/2 inch long.
Can I prevent grub damage?
One option is allowing the lawn to go into dormancy as conditions dry in July (assuming there is little rainfall), reducing the odds of grub damage. Also, monitor the lawn closely as we advance into summer and fall. Then if you see damaged areas, you must determine for sure if grubs caused it, and whether it is worth treating.
To treat or not to treat
If you’re sure you have grub damage, but the turf is reasonably healthy and summer drought and heat stress are minimal, it is likely that treatment is not necessary, even on cool season grasses, which can withstand a bit of grub feeding. Warm season grasses have an even greater inherent tolerance to grubs since they are actively growing during the summer months. This is an individual decision, but treatment should not be done if the damage is just minimal and spotty. If treatment is necessary, try the least toxic methods first.
Preserve Invertebrate Natural Enemies of Grubs: In addition to the various vertebrate natural enemies that unfortunately tend to cause turf damage when preying on white grubs, there are numerous more subtle invertebrate natural enemies. Ground beetles, ants, and other beneficial insects prey on eggs and young grubs. Various parasitic wasps and flies parasitize the older grubs. Various naturally occurring pathogens (insect-parasitic nematodes, fungi, bacteria, protozoa) kill or weaken the grubs. Preserve these natural enemies as important buffers against grub outbreaks by using insecticides only when and where necessary to avoid intolerable damage.
Nonchemical-Cultural Controls: Good turf management (proper irrigation, fertilization, mowing) results in vigorous turf with a deep, extensive root system that can tolerate higher grub densities without showing damage. While no grub-resistant turfgrasses exist, species with a deeper root-system and higher heat/drought tolerance are generally more tolerant of grub feeding. Among the cool-season grasses, tall fescue is the most tolerant species and perennial ryegrass the least tolerant.
Your watering practices can also control grubs. Watering during peak beetle activity in summer tends to attract egg-laying females, especially when the soil in surrounding areas is dry. Watering also increases survival of eggs and young larvae. In late summer and fall, however, irrigation makes the grass more grub-tolerant. Thus, if homeowners can keep their lawns dry during July and early August, beetle eggs may dry up and die. Obviously, the down side is your lawn will be brown until water is applied.
Nonchemical-Curative Controls:
Milky spore can be applied in late August. However, it is only effective on the Japanese beetle grub population and may take 3-5 years to become sufficiently established in your lawn to suppress the Japanese beetle. Also, insect parasitic nematodes can suppress several white grub species. Nematodes are very small, live unsegmented worms. They search out the white grubs and after entering the grub, release a bacteria that kills the grub. This product should be applied late in the day in moist soil and watered in afterwards.
Chemical-Curative Controls:
We encourage the cultural practices listed above. If the damage is serious and large, contact a professional in regards to the use of any curative products. Curative grub control has to be done at the appropriate time when they are small and actively feeding. For Virginia, this is July and August. Treatment in the spring when the grubs are large is not as effective. The need for preventative applications can be based on historical monitoring and current season adult Japanese beetle population. .ALWAYS FOLLOW THE LABEL INSTRUCTIONS TO MAXIMIZE CONTROL AND MINIMIZE NON-TARGET EFFECTS. THE LABEL IS THE LAW.
Sources:
“Beetlemania-White Grub Control in Lawns,” Virginia Extension Office Publication, http://www.ext.vt.edu/topics/lawn-garden/turfgrass/turfandgardentips/tips/beetlemania.html
“How to choose when to apply grub control products for your lawn,” Michigan State University Extension Office Publication, http://msue.anr.msu.edu/news/how_to_choose_and_when_to_apply_grub_control_products_for_your_lawn
“FAQ’s on White Grubs in Lawn,” University of Illinois Extension Office Publication, extension.illinois.edu/lawnfaqs/grubs.cfm
Paul Heller, “White Grubs in Home Lawns,”Penn State Extension Office Publication, psu.edu/extension/factsheets/white-grubs-lawns