What You Should Know About Grubs

My question and answer blog, Xtremehorticulture of the Desert, receives over 30,000 visits each month. Much to my surprise, one of the most popular topics among my readership is grubs. Readers find them feeding on the roots of ornamentals, vegetables, fruit trees, grasses and in their compost piles.

These are common grubs, specifically white grubs. They can be a common past feeding on the small, immature roots of many plants including lawn grasses, vegetables, flowers, perennial flowers and even shrubs.

White grubs are the immature forms of beetles. The category or order of beetles, Coleoptera, is by far the largest group of insects in the world. The common names for adult forms of the white grub, which occupy a much smaller subset of beetles, are recognizable to many; scarab beetles, June or May beetles, dung beetles or the word “beetle” tagged on to some other descriptive moniker such as “Japanese” beetle.

This is the adult of a white grub called a scarab beetle.This particular beetle is the green metallic June beetle but others more commonly can be brown. These adults, even though they are called June beetles don't have to appear in June but usually in the late spring months. The adults mate and deposit their eggs near the soil surface.

If you talk to a turfgrass managers or golf course superintendents they automatically think of C-shaped, upside down white grubs found feeding on turfgrass roots. In sod forming grasses that produce rhizomes and stolons, damaged turfgrass can be rolled back like a carpet. In bunch grasses like tall fescue, damaged grass is easily pulled from the soil in clumps.

The chemical industry and educators have done a good job of directing professionals and homeowners to pesticides intended to control white grubs in turfgrass. You could walk into any garden retail outlet and see bottles or packages of pesticides with a large picture of a white grub or the words “Controls White Grubs” printed on the label.

White grubs can be found in compost piles, soil amended with compost or manure and along the roots of plants where they feed when they are young.

Lawns are disappearing in many parts of the country with water restrictions. Damage from white grubs feeding in places other than lawns is more noticeable than it used to be and not as recognizable. The number one place, according to readers of my blog, for finding white grubs are in compost piles or where compost has been applied to the landscape.

Unlike plant names, where two or three plants can share the same common name, entomologists back in 1903 agreed upon a list of common names for insects that is updated on a regular basis. This makes discussing white grubs a lot easier for us non-entomologists.

I first learned about white grubs studying turfgrass in college. In turfgrass they can cause severe damage feeding on the roots of the grasses just an inch or so below the surface. Lawns that grow together like a carpet, Kentucky bluegrass for instance, can be rolled back like a carpet where these insects have chewed off the roots.

Since grubs are immature forms of insects and not the adult insect itself, the word “white grub” doesn’t communicate very well until we talk about what grubs are feeding on. Grubs feeding on the roots of grass plants usually narrow down the insect possibilities to three or four different kinds.

We can narrow it down further if we know the plant damage is located in Florida, California or New York. Different geographical regions have different types of “white grub” problems. Knowing all this is important but approaches to controlling this pest needs to be focused their stage of development. This means that the timing for applications of control products is extremely important.

            Grubs feeding on plant roots are difficult to control if the control measure isn’t timed right. These immature forms are voracious feeders when young and when they are most susceptible, but as they get older and begin the transition to winged adult, control becomes increasingly more difficult. As this transition occurs, the major pathway for controlling this insect, its voracious appetite, slows and eventually closes.

When they are nearing maturity, they stop feeding in preparation for pupation or turning into adults with wings, the June beetles.Laying on their backs with their feet upward so there mouthpart can feet on the grassroots which are growing down.
From University of California – Riverside

The life cycle of these winged insects is like many others that lay eggs; the winged adult emerges from the soil, locates a mate, after mating she flies off and lays her eggs in a location with plenty of food and protection for her young. In the case of white grubs, she flies to the nearest food supply such as tender roots, rotting vegetation, a dung heap or an immature compost pile.

Predators of the eggs and young of white grubs are numerous. In a Kentucky study, ants were the number one predator of white grubs feeding in turfgrass followed by spiders and other types of beetles. Residuals from soil applications of (active ingredients) carbaryl, cyfluthrin and isazofos to control other turfgrass pests significantly reduced these predator populations for up to 10 weeks, resulting in increased feeding damage because of a higher population of grubs.

Eggs that survive the initial onslaught from predators hatch in about two weeks and begin voraciously feeding when soil temperatures reach about 60° F. They quickly gain most of their eventual size and weight as soil temperatures steadily increase.

As white grubs approach the size they need for pupation to adult, they progressively decrease and eventually stop feeding. Since feeding is the primary pathway used for controlling them, it is very important to focus control efforts during their early stages of growth and heavy feeding. Focusing control efforts too early or too late in their life cycle decreases or mitigates their effectiveness.

Determining when to apply control measures to white grubs through scouting can be labor-intensive and hard to ascertain in the field. A method which might help identify the appropriate timing for control measures is the use of insect traps.

Shining a light on a white sheet at night is very effective at identifying which insects are flying at that time of year. Do this in mid spring to identify when the adults of white grubs are flying.

Three types of traps which can help determine the appropriate timing to begin control measures are light traps, flight interception traps and soil emergence traps. These traps are poised to capture adults that fly during the day, fly at night or emerge from the soil. Depending on the type of white grubs present, any or all of these three might be effective.

Casual observation, florescent lights shining at night attract insects  as well.Black lights are a little more expensive but they do a better job of attracting insects than white light and they are less obtrusive.

Traps can catch a wide range of insects that have recently emerged so identification of these adults is necessary. Adult stages are much easier to identify than the immature forms of white grubs. Once the emergence date has been pinpointed and recorded on the calendar, the application of soil applied control measures could be initiated about 4 to 8 weeks later.

I am featuring products on Amazon because they are easy for me to post. If you find the same active ingredient at the same concentration for a better price, want it, then buy it.

This can be found on the front of the label on the pesticide. Remember a pesticide is anything that kiss a pest. So, technically if you kill a pest with a hammer, the hammer is a "pesticide". But all chemical pesticides must be listed with the USEPA and labeled. Part of the legal label is a statement regarding its contents including active ingredients. The label must state the chemical name of the pesticide (active ingredients) , all other chemicals that total the contents to 100% AND the percentage of each. (From  Texas A and M)

What is an active ingredient?

Conventional Grub Control Insecticide at Lowes

Control measures applied to the soil vary from traditional pesticides to biological controls. If applied early enough in the season, nearly any registered soil applied insecticide will control white grubs provided it is moved into their feeding area with sufficient irrigation. Applications made when white grubs are more mature may have mixed results. Traditional pesticides are effective but, as noted earlier, they take their toll on the natural predator population.

Milky Spore Product on Amazon

Unlike milky spore bacterium which is only effective against the Japanese beetle, biological control using beneficial nematodes have been shown to be very effective against a number of soil dwelling insects including many different types of white grubs. Beneficial nematodes are living organisms so handling and application techniques are different from traditional pesticides and very important.

Beneficial Nematode Product on Amazon
Beneficial Nematodes at Grow Organic

There are basically two types of beneficial nematodes; Steinernema and Heterorhabditis. It is generally accepted that Heterorhabditis is more effective on white grubs under field conditions but it is more expensive and less available because it is harder to produce and keep in storage. These beneficial nematodes are not dangerous to humans, plants or other animals.

Applications can be made with traditional spray equipment or applied to the soil as a liquid drench. Beneficial nematodes are susceptible to drying conditions and ultraviolet light so it is important to apply them early in the morning or at dusk and move them into the soil profile with irrigation water as soon as possible after application and maintain moist soil conditions for several days.

Beneficial nematodes are quite tolerant of nitrogen fertilizers and most commonly applied insecticides but they are not tolerant of dry, hot or cold soils. They perform best in wet soils at temperatures between 60 and 90° F.