Billbug and White Grub Management in Lawns

Introduction

White grubs and billbugs can be important pests of turfgrass in parts of Colorado. Both groups of insects feed below ground and damage roots or feed within the growing crown area of the plant.

White Grubs

White grubs feed on the roots of grasses and usually can be found within the top couple inches of soil. The body is creamy white with a reddish-brown head and they have three pairs of legs on the thorax. Normally they will be seen to curve their bodies into a distinctive C-shape and grubs of the larger Colorado species may reach nearly 1 inch long.

White grubs are the most damaging insects that occur in Colorado, and native species have long been particularly common in the TriRiver counties and in towns along the Arkansas and South Platte River valleys. Injury to plants occurs as the white grubs feed on roots, which causes drought stress due to root loss. In severe infestations, turf roots are so severely pruned that areas of the lawn can be lifted or peeled back easily, causing extensive loss of turf. The presence of white grubs in lawns is also attractive to raccoons and skunks, which will dig up lawns while searching for grubs, often causing more damage. 

Adult stages of white grubs are known as scarab beetles (Coleoptera: Scarabaeidae). Common names for some of the specific types of white grubs include May or June beetles, chafers, and dung beetles. Only a very few species damage turfgrass or other plants and most white grubs feed on decaying plant material and animal manure, providing a very helpful role in helping to recycle the nutrients of these materials. For example, a commonly encountered scarab beetle is Euphoria inda, known as the bumble flower beetle, which is often found developing in compost. These scavenger species are not damaging to lawns and only under unusual conditions ever feed on live roots of plants. However, a few species do feed on plant roots, particularly of grasses, and discussion of these follows.  

Southwestern masked chafer (Cyclocephala hirta)

These are the white grubs that most often  damage turf in communities along the Arkansas Valley, South Platte, and West Slope areas. They are a type of annual white grub, which requires one year to complete a life cycle. Flights of egg-laying adult beetles occur in June and continue for about one and a half months. 

Larvae feed on grass roots through summer and early fall. Peak injury occurs in late summer and early fall but some feeding occurs in spring when larvae return to feed on roots as they complete development. On drought-stressed turf, populations of nine or more grubs per square foot can produce visible injury. Higher grub populations can be tolerated on more vigorously growing turfgrass. Some grub feeding occurs in spring, but favorable growing conditions in spring usually allow grass to outgrow visible damage during that time. When soils cool in autumn and approach 60^oF, white grubs move more deeply into the soil where they remain in a dormant condition until soils warm in spring. 

May and June beetles (Phyllophaga spp., Polyphylla spp.) 

These are the largest of the white grubs and damage by them is restricted to Eastern Colorado. Most injury by these insects occurs along the Eastern Plains, particularly in the southeast area of the state. 

Most May and June beetles have a three-year life cycle. Adult beetles emerge during May and June and lay eggs in the soil. Grubs feed during the summer and move deep in the soil to overwinter. Grubs return to the root zone and feed throughout the following summer. May and June beetle grubs cause most injury during this second season of their life cycle. During spring and early summer of the next year, the grubs complete development, cease feeding, and turn into pupae and adults that remain inactive in the soil. Adult beetles emerge next season. Because of their large size, lawn injury by May or June beetle grubs can occur from populations of five or fewer grubs per square foot.

Although roots of grasses are the primary food used by larvae of May/June beetles, they will sometimes feed on roots of trees and shrubs. This habit is particularly true for the very large grubs of the genus Polyphylla, whose best known representative is the ten-lined June beetle (Polyphylla decemlineata). 

Japanese Beetle (Popillia japonica)

The Japanese beetle is a new arrival to Colorado and presently (2018) found in several areas of the Front Range, including most of the Denver Metro area, Boulder, and Pueblo. Unlike other white grub species present in the state, Japanese beetle is also important as a pest in the adult stage, which chew on leaves and flowers of a great many garden plants.  

Japanese beetles begin to lay eggs in mid to late June. Egg laying is largely concluded by late July but may continue through late summer. Peak damage to turfgrass occurs in late summer. Larvae that feed on lawns in spring do little injury and the actively growing plants at this time of year are well able to recover from root-feeding grubs.

Identifying different types of white grubs. 

All of the different kinds of white grubs found in lawns have a generally similar appearance.  However, on close examination, they can be distinguished by looking at the pattern of hairs and folds on the hind end of the abdomen.  This is known as the rastral pattern. A comparison of the rastral patterns of a masked chafer, May/June beetle, and the Japanese beetle are illustrated in Figures 13a-c. 

White Grub Control

Cultural Controls 

How lawns are managed can affect the damage done by white grubs. Lawns that are mowed at higher levels produce plants with larger root systems, which are better able to tolerate root pruning injuries. 

Watering practices can have variable effects. Irrigation can allow lawns to better tolerate grub injuries and maintaining good soil moisture in late summer and early autumn can help lawns recover from damage that has occurred. Conversely, lawns where soil moisture is kept high during the period when eggs are laid will tend to be more favorable to white grubs. Since the eggs and young grubs are sensitive to drying lawns that have some periodic drying will reduce their survival. This is particularly true for Japanese beetle. 

Some kinds of turfgrasses may be less favored by white grubs. Although all the commonly grown cool season grasses used in lawns (Kentucky bluegrass, ryegrasses, fescues) are susceptible, some warm season grass, such as bermudagrass, are less likely to be damaged by grubs. 

Chemical Controls 

Presently there are several insecticides that can provide very good control of white grubs when applied at the proper time (Table 1). Since younger stages of grubs are much more effectively controlled than are large, older grubs, insecticides are best used in a way that allows them to be in high concentration in the root zone at the same time eggs are hatching and young white grubs are present (Table 2). 

The two insecticides commonly used for white grub control (chlorantraniliprole and chlothianidan) have long persistence and can kill grubs for a few months after application. These are best used in a preventive application, applied so that they will be present when eggs are being laid and hatching. With all preventively applied insecticides, there will be lag (about two weeks) between insecticide application to the soil surface and when the insecticide moves into the root zone where grubs feed. 

Watering is important to insecticide performance. Some irrigation is needed to initially move the insecticide into the root zone. In the weeks immediately after application, some watering may be needed to maintain good soil moisture so grubs will feed nearer to the surface and thus be more exposed to the insecticide. When the upper soil dries, white grubs will dig more deeply to find areas of adequate moisture. 

Another insecticide available to control white grubs, trichlorfon, is applied differently. Trichlorfon does not have persistence after application, degrading very rapidly in soils. However, it can move rapidly into the soil and kill grubs shortly after application. Trichlorfon is best used as a rescue treatment after eggs have hatched and large numbers of grubs are actively feeding and damaging lawns. 

Insecticides applied to lawn areas can be a hazard to pollinating insects if there are dandelions, clovers, or flowering plants mixed with the turfgrasses in the treated area. To reduce this risk, lawns should be mowed to remove all blooms before applying insecticides. 

Biological Controls 

White grubs can be killed by use of commercially available insect parasitic nematodes. Variously described as “insect parasitic,” “insect predator,” or “entomopathogenic” nematodes, these are tiny roundworms that develop within and kill susceptible insects. Several kinds of these nematodes are sold and those in the genus Heterorhabditis are effective for control of white grubs; Heterorhabditis bacteriophora is the primary nematode species used for white grub control. Nematodes in the genus Steinernema, such as Steinernema carpocapsae, are not effective for control of white grubs. 

Insect parasitic nematodes are available from several mail order suppliers. They are designed to be mixed with water and drenched into the soil when grubs are present and feeding. Grubs that are successfully attacked by these organisms turn a reddish-brown color and die within a few days. The nematodes that develop within the grub will reproduce and newly produced nematodes may move into the soil and infect other grubs. However, numbers of the nematodes will normally greatly decline over time and will likely produce little, if any, observable effects on grub numbers in the year after application. Insect parasitic nematodes will also be unable to infect insects if soils are not warm enough (50^oF minimum) for them to be active. 

A new (2018) product being marketed for grub control is the galleriae strain of the bacterium Bacillus thuringiensis (Btg), presently sold under the trade name grubGONE! Optimal application timing is around the time of peak egg hatch of the white grubs. For southwestern masked chafer and Japanese beetle, this will occur sometime between mid-July to mid-August. Bacillus thuringiensis affects susceptible insects that consume the bacteria, which then paralyzes the midgut of the insect. Affected insects stop feeding very soon after infection, but it may take several days before insects expire. 

Another bacterium that has a different, more limited use for grub control is milky spore. This is a bacterium (Paenibacillus popilliae) that produces “milky disease” in Japanese beetle grubs, which is indicated by the infected insects turning a milky color. However, it does not affect other species of grubs.

Milky spore will not produce any noticeable control of grubs in the year of application. Instead, it’s used as a one-time application to try and establish this disease for long-term suppression of Japanese beetle grubs. Once milky disease is established at a site, it can then reproduce and spread without further attention. In the eastern United States, where milky disease has long been present among Japanese beetle it will annually infect a small percentage of the grubs, typically in the range of 2-5% of the population. 

Billbugs

Billbugs (Sphenophorus species) are weevils (“snout beetles”) that develop in grasses. Adult weevils are gray to nearly black, about 3/8-1/2 inch long and have a pronounced snout tipped with chewing mouthparts. Adults are commonly observed as they cross sidewalks and driveways, and they will characteristically “play dead” when disturbed, drawing in their legs and remaining motionless. Adults chew on grasses but their feeding injuries cause minimal damage and is rarely noticeable. Females also cut small holes in the stems of plants where eggs are then inserted.

The young billbug “grubs” are the primary damaging stage. Billbug grubs are legless, generally white or cream, with a brown head. They may reach 1/3 to 1/2 inch long when full-grown. Young grubs feed within the crown area of the plant and kill it. The stems of infested plants are easily detached at the soil surface and the ends show evidence of ragged chewing. The larvae also produce a highly characteristic excrement around the base of the plants, which is generally similar to sawdust. Older billbug larvae feed in the lower crown and plant root zone in a manner similar to white grubs.  

Billbug injury is most common on new lawns, particularly those established with sod. Billbug injury appears as wilting and occasional death of grass, often in small, scattered patches. Extensive areas of a lawn may be killed during severe infestations. 

At least two species of billbugs can be found in Colorado lawns. The bluegrass billbug (Sphenophorus parvulus) overwinters as an adult in protected areas, such as under debris near building foundations or at the interface of turf and sidewalk. Eggs are produced and laid in late May, June, and early July. Larvae develop over the course of several months. Peak larval injury occurs in late June and July. When full-grown, the larvae pupate a few inches deep in the soil. Adults emerge in two to three weeks, feed briefly, and seek overwintering shelter. There is one generation per year. 

The Rocky Mountain billbug (Sphenophorus cicastristriatus), also known as the “Denver billbug”, is far more common throughout Colorado. The life cycle of this insect is more complicated than the bluegrass billbug. Some of the insects overwinter as adults, but most remain in the larval stage and feed throughout spring. Egg-laying occurs throughout most of the growing season, peaking in June and July. 

Billbug Control 

Cultural Controls. 

Increased resistance in Kentucky bluegrass occurs with varieties that have fine stems and leaves and tougher plant tissues.  Ryegrasses and fescues that contain endophytic fungi can have high levels of billbug resistance. Serious problems with billbugs should largely be avoidable where about a third of the plants or more in a lawn area have good billbug resistance features. 

Cultivars that spread more aggressively can allow damaged areas of lawns to recover more quickly. Adequate fertilization and watering will also mask injury symptoms and help lawns repair damage. 

Biological Controls. 

A few kinds of insect parasitic nematodes (Steinernema carpocapsae, Heterorhabditis bacteriophora) are effective against both larvae and adult stages and may be used as a biological control. These nematodes are available from many mail-order suppliers and some nurseries. 

Chemical Controls. 

The best control for bluegrass billbugs occurs when sprays are applied in early May to kill adult insects prior to egg laying. Larval control can be difficult due to the habit of young larvae to tunnel within crowns of plants. Information on the Rocky Mountain billbug indicates that a slightly later timing, in early to mid-June, is more appropriate for adult control. Apply adult sprays so insecticide residues remain as long as possible on foliage and in the crown area of the plant. Pyrethroid insecticides (bifenthrin, beta-cyhalothrin, cyhalothrin) are particularly good for persisting on turf surfaces. However, none of these will move into soil and cannot control larvae of either billbugs or white grubs. Insecticides recommended for billbug control are summarized in Table 1. 

Controlling billbugs with insecticides is more difficult during the larval (grub) stage. Young larvae are protected within the plant. Older larvae occur in the root zone, where insecticides fail to penetrate. However, some more recently marketed insecticides, such as chlorantraniprole, can provide good control of larvae and have some systemic activity within plants, allowing some control of crown feeding stages. Applications for larval control should be applied in late spring (bluegrass billbug) to early summer (Rocky Mountain billbug) to target early-stage larvae.