Phyllophaga and others Coleoptera Scarabaeidae

Natural History

Distribution. The whitish, soil-dwelling larvae of scarab beetle species are called white grubs. Most are in the genus, Phyllophaga, but sometimes included are some members of the genera, Anomala, Cyclocephala, and Polyphylla. The adults are called June beetles, May beetles, chafers, or cockchafers. They are found throughout the United States and southern Canada, but are most abundant east of the Rocky Mountains. Nearly all species are native. Among the most damaging species are common June beetle, Phyllophaga anxia (LeConte); common cockchafer, P. fervida (Fabricius); northern June beetle, P. fusca (Froelich); fever June beetle, P. futilis (LeConte); southern June beetle, P. inversa (Horn); wheat white grub, P. lanceolata (Say); and ten-lined June beetle, Polyphylla decemlineata (Say).

Host Plants. Both larvae and adults can be damaging, but they usually have markedly different feeding preferences. Larvae are found in the soil, and normally feed on the roots of grasses. Adults, however, usually feed on the leaves and flowers of deciduous trees and shrubs, particularly ash, elm, hackberry, hickory, locust, oak, poplar, walnut, and willow, but sometimes pine. Root crops such as beet, potato, and turnip are among the crops most susceptible to injury by larvae, but corn and strawberry are sometimes damaged, and the seedlings of virtually all vegetables are susceptible if grub density is high. Vegetables crops are not preferred oviposition sites, so damage usually occurs only when crops are planted into land that recently has been grass sod, pasture, or sometimes grass crops such as timothy, small grains, or sorghum. Tree seedlings, especially conifers, are damaged in both northern and southern states when planted into grass sod or pasture. Alfalfa and clover seem to be avoided even though both crops often contain some grass. Adults disperse widely, and trees adjacent to grassland, especially when few in number, are sometimes injured by defoliation.

Natural Enemies. A large number of predators, parasitoids, and pathogens of white grubs is known. Undoubtedly, many soil-inhabiting predators, especially beetles (Coleoptera), kill white grubs, but there is little documentation of this. Larvae of robber flies (Diptera: Asilidae) and beeflies (Diptera: Bombyliidae)

Larvae Robberflies
White grub, Phyllophaga sp.

are frequently found in association with grubs. Among the parasitoids are such tachinids as Cryptomeigenia theutis (Walker), Eutrixa exilis (Coquillett), Eutrixoides jonesii Walton, and Microphthalma spp. (all Diptera Tachinidae); the light flies, Pyrgota spp. (Diptera: Pyr-gotidae); and the wasps Pelecinus polyturator (Drury) (Hymenoptera: Pelecinidae); and Tiphia spp. (Hyme-noptera: Tiphiidae). Of the parasitoids, only Tiphia spp. are documented to be of considerable importance, sometimes accounting for up to 50% mortality among grubs. Mites (Acari: several families) are commonly associated with grubs and beetles; eggs and pupae generally are free of mites. However, few of these mite associates thought to be are parasitic or detrimental to white grubs (Poprawski and Yule, 1992). All the common groups of insect pathogens have been found associated with white grubs, especially Metarhizium anisopliae and Beauveria bassiana fungi. There is no evidence that these pathogens, and many of the other associates of white grubs, have significant impact of grub density. A review of many natural enemies of white grubs was published by Lim et al. (1981b).

Life Cycle and Description. The life cycle of the various species ranges from 1 to 4 years, though in the Phyllophaga species it is 2-4 years, with the three-year life cycle most frequent. Duration of the life cycle is often related to latitude. In the southern United States two year life cycles are common, whereas in the north central states three-year cycles predominate, and in northern states and Canada 3-4 year cycles occur.

The general three-year life cycle starts with oviposi-tion in the spring or early summer, followed by feeding and growth until autumn when cold weather induces a period of inactivity, and overwintering by second instar larvae. During the second year, feeding is resumed in the spring, the third instar is attained, and feeding continues until cold weather when another period of inactivity occurs. In the spring of the third year, larval growth is completed and pupa tion occurs during the summer. The adult remains in the soil until the following spring. Thus, the three-year life cycle occurs during a four-year calendar period.

In the typical two-year life cycle, oviposition occurs in the summer, the eggs hatch in late summer, and young larvae commence feeding, quickly attaining the third larval instar and overwinter. They feed again throughout the second summer and overwinter as mature third instars. The following spring they feed briefly, pupate, and adults emerge, mate, and oviposit. Thus, in the two-year life cycle larvae develop very quickly and the adult does not undergo a period of arrested development, as occurs in the three-year cycle. The two-year period of development occurs over three calendar years.

Following is a description of common June beetle, P. anxia, perhaps the most abundant and damaging Phyllophaga in northern areas, and known from nearly all of Canada and the United States. It serves well to illustrate the biology of white grubs and June beetles.

In Quebec, common June beetle displays the typical three-year life cycle. The adults begin to fly in May and oviposit in June. First instar grubs are abundant in July and molt into second instars in August. The second instars overwinter and molt into third instars during June of the second year. Third instars overwinter at the end of the second year, developing into prepupae during June of the third year, and pupating in July. They are fully formed by September, but remain in the soil until the following spring (Lim et al., 1981a). As noted earlier, the three-year life cycle is spread over four calendar years.

  1. About 10 days after mating the female deposits pearly-white and elongate-oval eggs about 2.4 mm long and 1.5 mm wide. The eggs absorb water, becoming enlarged in size, and eventually measure about 3 mm long and 2 mm wide. Mean fecundity is about 55 eggs per female. Eggs hatch after about 20-30 days.
  2. Young grubs often feed on decaying vegetation before turning to root feeding, but they are not associated with manure. The grubs are pearly-white, with a dark head and legs. They bear a thin covering of stout hairs, and when dug from the soil, they assume a curved-body posture or C-shape. Although the grubs feed at a depth of perhaps 5-15 cm during the summer months, they descend to about 30100 cm during the winter months. Depth of burrowing is markedly affected by soil and moisture conditions and the age of the grub. Survival of grubs is higher in light soils and in moderate moisture conditions; heavy rain is detrimental both to oviposition and survival of young grubs. Grubs pass through three instars over about a 24-month period that is spread over part of three calendar years (Lim et al., 1980a). (See color figure 122.)
  3. The larva prepares a cell in the soil for pupation. This cell also serves as the overwintering site for the adult, which does not emerge from the soil until the following spring.
  4. The body of adult P. anxia is oblong-ovate, dark brown in color, and measures 17-21 mm long. The elytra are marked with numerous fine and shallow punctures. The underside of the thorax bears a dense coat of long hairs. The antennae are 10-segmen-ted, elbowed, and expanded at the tip. The basal segments of the legs are generally stout, the tibiae are broadened and armed with teeth or stout spurs, but the tarsal segments are elongate and narrow. (See color figure 121.)

An excellent treatment of Phyllophaga biology, and a taxonomic treatment of adults, was presented by Luginbill and Painter (1953). Other reliable sources of general biology, damage and management information were found in Davis (1922), Luginbill (1938), and Ritcher (1940). Occurrence of white grub species in the north central states was given by Pike et al. (1977) and in the southeastern states by Forschler and Gardner (1990). The publication of Ritcher (1966) was an especially comprehensive treatment of grubs. Rearing techniques were reviewed by Lim et al. (1980a).

Damage

White grubs are only occasional pests in vegetable production. Grubs have limited mobility; their distri

Terminal abdominal segment (ventral view) of white grub.
Adult white grub, also called May beetle or June beetle.

bution is largely a function of oviposition preference by females. Thus, though the roots and tubers of vegetables may be consumed, this is often due to conversion of grassland bearing partially grown grubs to vegetable production areas, and a decrease in the availability of their preferred host plants, or grasses. Typically, larvae clip the roots of plants with fibrous-roots systems, especially during the second year of larval life, resulting in the wilting and death of young plants. However, they may chew holes into larger roots and tubers, damage which may not be apparent until harvest.

Damage to trees by adults is more frequent in northern states. In such areas the transition from cold, wet weather to warm, dry weather is abrupt, causing mass emergence of adults and defoliation of nearby trees. In contrast, in the south the gradual increase in temperatures causes a protracted emergence by adults and the defoliation of trees is less apparent. Adults prefer young foliage, feeding from the edge toward the center of the leaves. They feed at night and hide during the day, so the cause of damage is often overlooked.

Management

  1. Estimates of grub density are often made by examining 0.3 m cubes of sod and soil. Owing to the highly aggregated distribution of grubs and eggs (Guppy and Harcourt, 1970), however, extensive sampling is required to obtain a high degree of precision, often 50-100 samples (Guppy and Harcourt, 1973). Light traps are used to monitor populations of June beetles because they are nocturnal; such traps collect mostly males. Flights normally occur between sunset and midnight, and then again at sunrise, and are initiated by a combination of temperature and photoperiod stimuli (Guppy, 1982). Examples of the results of light-traps studies can be found in Kard and Hain (1990), Forschler and Gardner (1991), and Dahl and Mahr (1991).
  2. White grubs and June beetles are controlled with insecticides by application to soil or tree foliage, respectively. Soil applications predominate except perhaps for protection of foliage in fruit crops. Persistent insecticides are used in soil environments, applied preplanting (Rolston and Barlow, 1980) or at planting (Rivers et al., 1977). Insecticide resistance has developed in some regions (Lim et al., 1980b).

Cultural Practices. Because of the attraction of June beetles to grasses, particularly short or mowed grass, there is a considerable risk when crops are planted into land that supported grass during the previous year, or when crops are grown adjacent to grass pastures or sod. If clover is planted immediately fol lowing grass, the risk to subsequent crops is reduced, partly because grubs rarely injure clover, but also because June beetles prefer not to oviposit in clover fields. It is also important that crops be free of grass and other weeds during the flight of June beetles, or considerable oviposition may occur. However, if grubs are already in the soil, the presence of grass weeds can be advantageous because grubs will feed preferentially on the grass (Rivers et al., 1977).

Plowing and disking are sometimes recommended for destruction of grubs because the soft bodies of larvae are easily damaged by tillage. Also, tillage exposes grubs to birds, which can often be seen following tractors and consuming large numbers of exposed insects.

Host-Plant Resistance. There is not much useful information on the varietal susceptibility of vegetable crops to white grubs. An exception is sweet potato, where not only has resistance been demonstrated (Rolston et al., 1981), but it has been shown that inter-planting a resistant cultivar with a susceptible cultivar confers a reduction in damage to the susceptible variety (Schalk et al., 1991, 1992).

Biological Control. There have been few attempts to implement biological suppression of white grubs, other than the use of domestic animals such as poultry and swine to consume larvae. The spore-forming bacteria Bacillus popillae and Bacillus lentimorbus can infect and kill white grubs, but this expensive treatment is usually reserved for use on turf, where the bacteria can recycle through generation after generation of grubs. Entomopathogenic nematodes (Nematoda: Stei-nernematidae and Heterorhabditidae) have been evaluated for white grub suppression. Grubs are susceptible to infection and high levels of suppression have been attained by both injection and surface application, but the results are not consistent (Kard et al., 1988).

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