Phyllophaga (white grubs)

White grubs are polyphagous pests having a wide host range. There are a number of scarab beetles which cause damage in their adult and larval stages. The larvae of greatest importance are those which belong to the subfamily Melolonthinae. The larvae of the Scarabaeidea are divided into three categories by De Fluiter, but only on the basis of their feeding habits. The categories are:- grubs feeding only on dead organic matter (Cetoninae)- grubs normally feeding on dead organic matter but also attacking living roots (some Rutelinae and Dynastinae)- grubs which by preference feed on living roots of crop plants (Melolonthinae).In young plants and seedlings, attack by white grubs causes wilting which is characterized by an initial purpling of the leaves, followed by death of small plants and reduced vigour or lodging of larger ones. Sometimes affected plants produce dead hearts. In tuber and other underground crops larvae feed by making circular holes into them thus rendering them unfit for marketing. Beetles are defoliating pests and damage a large number of fruit crops and forest trees as a result of feeding on apical buds and tender leaves.

Cultural Control

Cultural techniques are useful in reducing the number of larvae as well as adult populations. In endemic areas summer ploughing exposes the larvae of different stages which may then eaten by birds. The use of nitrogenous fertilizers, especially ammonia and urea, at high doses kill the first-instar larvae. Planting of resistant crop varieties also help to reduce white grub populations, especially in sugarcane (David et al.; 1967).


Mechanical Control

Light traps can be used for collecting adult beetles during the night. The beetles can also be collected by shaking or jerking the host plants. The fallen beetles are collected and destroyed by putting them in kerosinized water or by burning them. The host trees are chopped or pruned to attract and concentrate the beetles on a limited number of host plants. The branches of these host plants can be transplanted in the endemic area to attract the adult beetles and the beetles picked up by hand from the branches and destroyed. At the time of emergence, spraying of host trees with carbaryl, fenitrothion, or quinalphos markedly reduces the beetle population (Yadava et al., 1978; Nath and Singh, 1988).

Biological Control

A number of biological control agents which attack white grubs in different parts of the world, have been reported by several workers (Vora and Ramakrishnan, 1978; Nath and Singh, 1988; Tian and Hu, 1992). The population of these agents should be encouraged in the endemic localities either by conserving the existing population or by introducing, establishing and colonizing the new kinds of parasites, predators and pathogens. Some pathogens such as Bacillus popilliae and Beauveria tenella [B. bassiana], and the nematodes Metarhizium anisopliae and Steinernema glaseri, have been very effective under field conditions for managing white grub populations (Vyas and Yadav, 1993; Tian and Hu, 1992; Chen et al., 1995).

In India, a number of biocontrol agents have been reported attacking scarabaeid beetles in different parts of the country. Nath and Singh (1988) summarized the list and stated that control of adults and grubs through their natural enemies has not yet been attempted but their importance still exists and should be incorporated in management operations. Vyas and Yadav (1993) reported effectiveness of Steinernema glaseri against P. consanguinea (Holotrichia) under field and laboratory conditions. The trials showed the nematodes to infect 40 and 100% of grubs after 10 and 20 days of treatment, respectively at an initial dose of 100,000 infective juveniles/m². Tian and Hu (1992) from Shandong, China tested the combination of 2 kg/mu of a Beauveria tenella [B. bassiana] preparation (2.0-2.5 billion cells/g) with 0.1 kg/mu of 40% isofenphos methyl, and showed that this provided the most effective control of P. morosa (Holotrichia) and P. diomphalia (Lachnosterna) in groundnuts, reducing the insect population by 83.06% and the percentage fruit infestation by 87.47% (1mu=0.067 ha).

Chen et al. (1995) reported pathogenicity of Metarhizium anisopliae against P. parallela (Lachnosterna). Application of conidia to soil at 2.5-20 million spores/g in the laboratory resulted in 100% infection of larvae.

Vora and Ramkrishnan (1978) carried out laboratory studies in New Delhi, India, to evaluate the effectiveness of milky disease bacteria, Bacillus popilliae and B. lentimorbus for the control of the larvae of P. consanguinea (Holotrichia). IC 50s calculated each week from weeks 2-6 after soil inoculation were 190.5, 162.2, 144.5, 144.5 and 138.0 million spores/100g of dry soil, respectively. Maximum disease development occurred at an incubation temperature of 20°C. The disease reduced the protein content of the haemolymph by 63.8%. Field application of bacterial spore powder in the Kapadvanj area of Gujarat, an endemic area of P.consanguinea resulted in 35-60.7% larval infection in 1976 and 20-75% in 1977.

Host-Plant Resistance

In India, a number of commercial varieties of sugarcane show partial resistance or tolerance to P. serrata (Melolontha) (David et al., 1967).

White grubs cause enormous loss to the farmers. Losses of up to 80% in crop yield have been reported in Rajasthan, India (Sharma. 1989). Moutia and Mamet (1946) considered that obvious damage becomes apparent in sugarcane fields in Mauritius when grub populations reached 50,000-60,000 per acre. In Himachal Pradesh, India, losses to potato of up to 85% have been reported (Misra and Chandla, 1989). The extent of damage caused by white grubs solely depends upon the species involved, the numbers present and host crop.