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.

Turfgrass Dead Spots. Insects?

Brown spots or dead areas in lawns can be common during the summer months. It might be a disease, but then again, it could be something else. Could it be insects?

The three primary reasons that lawns develop brown spots or dead areas are due to irrigation problems, the development of diseases and insect damage.

 Insect damage. Probably the least likely of the three, and the most easy to detect, is insect damage. Most of the lawns in the Las Vegas Valley are turf-type tall fescues. Tall fescue is not a sod forming grass like Kentucky bluegrass. Instead, each seed produces a single plant. These individual plants are not linked together like they are with Kentucky bluegrass. Kentucky bluegrass sends out rhizomes, or underground stems, that pop up a distance away from the mother plant. When these rhizomes grow together, they form carpet-like sod that holds together. Because tall fescue produces only one plant for each seed that was planted, individual plants are not linked together.

White grubs feeding on lawn roots of sod

            This is important when diagnosing damage to a lawn. Insects that feed on the roots of lawn grasses, like white grubs, will cause areas of dead or brown grass. Because the mother insect lays a lot of eggs in one area, insect damage is usually localized in one or two areas. The eggs hatch, the young grubs begin to feed on grass roots and the lawn develops brown patches that correspond to where the grubs are feasting. The roots are severed by the grubs and the grass cannot get enough water so it dies in patches during warm or hot weather.

One type of an adult of the white grub

            Insect damage by grubs is usually in spots that are fairly well defined. Because the roots are severed, the grass can be lifted from the soil quite easily. Because tall fescue are separate, individual plants the root-severed, brown grass lifts easily from the soil. In the case of tall fescue, the grasses lift from the soil in the independently and separately from each other. In the case of a sod-forming grass like Kentucky bluegrass, they do not. Because they are linked together, the damage area lifts like a carpet.

            When inspecting a lawn to determine if the damage is from insects or not, go to the edge of the damaged area, not the center of the dead spot, and pull on the grass plants lightly with a closed hand. If grubs causes the damage and it is tall fescue, many of the plants will lift easily in your hands; dead ones and green ones at the same time that were recently severed.

Is This Brown Beetle a Pest in My Garden?

Joe's Bug

Q. What type of pest is this?  They seem to be on and around several of my fruit trees (apple, pomegranate, plum) but don’t appear to be causing any leaf or bark damage.  I did notice that there were several worm holes in the soil around my grape vines, but didn’t see any of these bugs on my grape leaves.  Are these pests harmful to my vines/trees? If harmful, how can they be controlled and/or eliminated?

-Joe

A. Well Joe this is one of the June beetles. They can also be called scarab beetles. Generally they all look the same except for their color. They are obviously beetles with that hard outer shell and range from ½ to 1 inch in length generally speaking. Colors range from your light brown one to dark brown and shiny metallic colors like the Green June beetle common here.

Green June Beetles attacking Kadota fig at the orchard

            There are so many different types that it is hard to narrow it down unless you were to take it to an entomologist. But some of them, like the Japanese beetle which this is not and does not live in southern Nevada, can cause a lot of damage to plants.

            Another scarab beetle is the Green June Beetle which we will start seeing now can also attack fresh fruits like figs and peaches. Yours is not the Green June beetle either since that pest is a very distinctive metallic green color. I will post in on my blog and it does live in southern Nevada.

White grub larvae attacking the roots of a lawn grass

            The immature form, grubs, of some of these June beetles can cause a lot of damage like white grubs which attack the roots of lawns causing wilting and extensive brown patches. The best I can tell you about this one is that the immature form (the beetle is the adult) could have possibly been attacking the roots of some plants in your garden.

            They could also have been in a compost pile since these grubs do like rotting and decaying plants. Some are actually dung beetles.

            If you don’t see a lot of them and don’t see any damage then don’t worry about it. They are part of some life cycle out in the garden and as long as the numbers are low they are not causing much damage.

Grubs in Container of Garden Soil Killing Plants

Grubs found in our compost piles

Q. I have discovered over 200 huge grubs in a 15 x 24 inch container that is about 2 ft deep. I had filled this container with a bag of garden soil from a garden center and planted strawberry plants. Of course the plants all died, so I decided to plant some seeds and discovered all these horrible grubs!!!

I have been told that a product called Grub-Away is safe to use and that nematodes are even safer. But my main concern is all the earthworms that are in my gardens. I put in the worms last year and am seeing tons of babies. I really do not want to harm them. So far, I have been digging up sections (all raised beds) and destroying the grubs.

If the grubs were in the bags of soil I bought, do I need to treat future purchases?

A. Most of the grub control information is focused on lawns. However there are other grub problems such as those you find in the compost and even bagged compost. In most of the United States the problems in compost are considered minor compared to lawns.

White grubs feeding on the roots of a lawn grass, killing it

Here in our desert Southwest where we have fewer lawns the importance of grub control in compost piles and even bagged compost is usually just or even more important than lawns. The nonchemical control of the white grubs in lawns will also work for the most part in compost or bagged compost. Technically, to be legal the recommendation or control should include the site of application. I have to give you that warning to be in compliance with the law.

Adults of one of the compost grubs, masked chafer

For instance if the product is being sold for pest control in lawns, then it should also include your application on the label. If you choose to use it for this purpose you are doing it at your own risk of success or failure. Having given you the Party line, those pest-control products that are biological or natural should be perfectly safe to use.

There are some very expensive bagged composts that are actually very good but those producers believe in NOT sterilizing the compost. And this is a great idea in theory. The reasoning is that they want all of that biological activity added to a garden soil. You go through the expense of quality composting that produces all of this wonderful biological activity for the soil and then you kill it.

Another adult form of compost grub, green june beetle

This biological activity contributes to the breakdown of organic material in the compost and releasing plant nutrients and a wonderful chemical activity that can take years to develop into normal Desert soil. Many organic growers would be dumbfounded in the logic of killing biological activity in compost. This biological activity is precisely one of the reasons why you compost.

On the other hand, not sterilizing a bag of compost will introduce all of this biological activity to whatever you add this compost to. So if you buy a bag of compost that has not been sterilized and you use it with your houseplants there is going to be a very good chance your houseplants will be infested with dusky winged fungus gnats for example. I get many questions submitted to me on how to control this pest in houseplants. This is seldom a problem outside in the garden in our climate.

Another adult of a grub, ten-lined june beetle

So you have to be careful which type of compost you use for which application. In my opinion I would never use an unsterilized compost for houseplants for precisely this reason. Inside the house or greenhouse all of your products should be sterilized if you don’t want these problems. These are closed environmental systems and must be managed as such. You must scrutinize everything you bring into a closed environmental system because there are few checks and balances to help keep introduced pests under control.

I like these newer bagged composts very much and there is a developing market for them primarily due to the development of local markets for food. They are expensive, running $20 per bag or more. One product line that is popular right now is Fox Farm products. If they can keep their quality control as the demand for products like this climb, it will be a good product but check the bag and see if it was sterilized or not.

Worm for composting

If not and you want it sterilized then put the bags in full sun during the summer months for four days and flip the bags over every day. This should heat sterilize the contents provided the temperatures reach 160F for at least 30 minutes. That should be no problem in our environment but if you want to make sure then buy an inexpensive compost thermometer online (I have not seen any available in Las Vegas) and stick it in the compost bag, burying the probe to the hilt. In mid-day you want the temperature to hit at least 160F.

Secondly, you should inspect any compost and get rid of any grubs. These are decomposers and are working at helping to decompose plant materials but are first level decomposers and will attack the roots of healthy plants as well. So screen your bagged compost before using it if you want to maintain its biological activity.

There are natural pest control products for grubs such as beneficial nematodes and bacteria like Milky Spore but they will focus on some grubs and may not others. They are somewhat selective and may be a good alternative treatment that will leave earthworms alone.

Remember that earthworms can move pretty fast. If they don’t like an environment they usually flee. Grubs have a harder time doing this. But the earthworms must be able to flee somewhere and if they are in a bag that is getting progressively hotter they will not escape.

I hope this helps.