Management of white grubs in peanut cropping systems in Asia and Australia

• University of Queensland, Australia
• Rajasthan Agricultural University, India
• International Crops Research Institute for the Semi Arid Tropics, India

Peanuts (also referred to as groundnuts) are a major crop in many areas of the tropics and semi-tropics. They rank in the top 10 of the world's most important food crops and are particularly important in India, where they are a major source of cooking oil and protein. About 14 million families farm 8.6 million ha of peanuts in India, but it is estimated that the crop may influence the livelihood of more than 100 million people in the country.
However, white grubs, which are soil-dwelling larvae of scarab beetles, feed on the roots of the peanut plant, killing seedlings and sometimes older plants as well as reducing drought tolerance and thereby affecting final yields. The grubs can also attack other important plants such as sugar cane and millet. In India, the value of the peanut crop lost to white grub attack is estimated to be about $800 million per year, while Australia suffers losses of at least $1 million per year. All of Queensland's current and potential peanut-producing areas have endemic populations of white grub species.
Peanuts are often grown in rotation with other crops, especially cereals. While these other crops are often less susceptible to damage from white grub attack they may contribute to the maintenance of grub populations that subsequently devastate the peanut crop. Many Indian farmers attempt to control the grubs by applying undesirable and toxic insecticides to enable them to grow peanuts.

This project sought to develop a better understanding of the white grub in the peanut-growing areas of India and Australia. The research investigated the ecology and behaviour of larvae and adults, the distribution of species, and the relationship between crop yield and pest density, and tested ways to control the pest.

After clarifying the distribution and identity of the white grub species damaging peanuts in India and Australia, the team studied the steps involved in the infection of white grub larvae by various pathogens. The best understood of these was the fungus Metarhizium anisopliae, but there are other candidates were examined for biological control of white grubs, including some bacteria.The best-performing pathogen strains were selected from those isolated from white grubs collected during the ecological surveys in both countries, with the intention of improving the formulation and application of a control agent. The efficacy of control techniques that target the adults was also assessed.
In addition, the team also investigated the biology of the pest, including the behaviour of the grubs in the soil, how they detect roots and what environmental factors (for example, soil types and temperatures) influence their development. In particular, factors that cause the pest population to vary were analysed. Computer modelling was used for the population dynamics, including an investigation of the relationships between pest density and the extent of crop damage.

In southern India the data obtained during the project in southern India established the basis for an environmentally friendly and economically-viable pest-management system for white grubs on groundnut in southern India. Researchers found that Holotrichia spp. were the main white grub species associated with groundnuts in Andhra Pradesh, Karnataka and Tamil Nadu. H. reynaudi predominated in the central Deccan area, while H. serrata was most abundant in areas to the south and west. A new, undescribed Holotrichia species near H. consanguinea occurred in mixed populations with H. reynaudi but its full distribution remains uncertain. White grub densities correlated closely with percentage of damaged groundnut plants on farms and grub densities were found to persist from year to year. The preferred host trees for Holotrichia adults in southern India were identified, and this will assist grower-initiated surveys of pest occurrence there.
Chlorpyrifos and imidacloprid insecticides were tested and found effective under specific conditions against H. serrata and H. reynaudi. Farmer surveys in Andhra Pradesh (AP) indicated use of insecticides for white grub control in 37.5% of farms sampled, but none used for this purpose in Karnataka and Tamil Nadu.
In northern India the experimental program has advanced the understanding of the pest and its damage, especially in relation to the use of insect pathogens and plant-derived adult attractants. These advances supplement the existing chemical control programs. The physiological effects of root-cutting and actual white grub damage were found to mirror the effects of water stress on the groundnut plant.
Groundnut is the most preferred monsoon crop in Rajasthan. This means that white grub populations in intercropped situations can be controlled by treatment of only the groundnut seed. Larvae of H. consanguinea were found to move freely in their endemic sandy soils in search of food and to prefer soil moisture content of 40-60%. Knowing this helped scientists determine optimum placement for inoculum of the insect pathogen Metarhizium anisopliae.
Laboratory and field bioassay of leaf extract from host trees of H. consanguinea showed Neem and khejari were the best sources of plant attractant (kairomone) for H. consanguinea, and green leaf volatiles from neem attracted more beetles than khejri. The field evaluation of the kairomone compound has been undertaken for one year and the results are encouraging.
Data obtained in Australia established the status of Heteronyx piceus as a peanut pest, and created the basis for a pest-management system for dryland peanut production in the South Burnett region of Queensland. Insecticide treatment targeting young larvae in peanuts is the chemical control option with the greatest likelihood of success.
Surveys of white grubs in southeast Queensland peanut fields found that the abundance of white grubs was similar to that recorded up to 20 years previously, i.e. peanut white grubs are a persistent rather than transient problem. Landscape position is a key risk factor; economically damaging H. piceus populations are much more likely to be encountered on the upper parts of the landscape than in the lower half. Double-cropping with peanuts also increased infestation risk.