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Insects that cause defoliation put neem trees at risk
FOR those who considered the neem tree invincible, a recurring phenomenon on the outskirts of Lucknow in Uttar Pradesh would come as a surprise. For three years, an insect infestation has been inducing the tree to shed leaves heavily. The leaf fall was first reported in November 2009 in Bhauli village in Bakshi ka Talab area of Lucknow. In 2010, similar leaf fall was reported in Sultanpur, Pratapgarh and Faizabad districts of the state. As the leaves are extensively used as green manure and for keeping pests away from stored grains, the insect caught the attention of local media and then of researchers.
A team from Centre of Excellence in Biocontrol of Insect Pests at the University of Lucknow analysed the infestation. “This was local problem and as entomologists, we were intrigued,” says Omkar, head of the centre. His team identified that the pest is a moth, Cleora cornaria. The moth had been earlier categorised as a minor pest for the neem tree. “But in view of previously reported tendency of the genus to be a major pest of many trees and increased incidence of its species C cornaria on neem, we should consider revising its status from a minor to that of a potential major pest,” the researchers write in the April 10 issue of Current Science. The genus Cleora has previously damaged mangroves in Thailand and Kenya and tea gardens and teak plantations in India.
Moth under lens
To understand how the moth is affecting the trees, the researchers decided to study the insect’s lifecycle first because little information is available on it. For this, they collected larvae of the insect from the infested trees in Bhauli and grew them in lab on a diet of neem leaves till they formed pupae. The adults that emerged from these pupae were housed in plastic beakers where they laid bright green eggs.
The researchers noted that the larvae that emerged from the eggs had a total of five stages in life, each stage lasting between two and five days. This gave them long feeding period and allowed the larvae to grow more. The team found that younger larvae clumped on the edges of neem leaves where the tissue was soft and relatively free from hard veins. They ate up the epidermal layer of the leaves in between the veins. Such neem leaves had no chlorophyll and so could not carry out photosynthesis and fell off. The pupa stage was 13 to 15 days long. Adults were creamish in colour with brown patterns on the wings. They mated after two to three days of emergence and laid eggs within 24 hours.
Why the infestation?
Though the researchers have been successful in determining how the infestation progresses they have failed to figure out the reasons for it. Geetanjali Mishra, co- author of the study, says there could be many reasons for it. “High-use of pesticides could have killed the natural predator or extensive development being carried out in the area might have reduced the host plant of these insects and forced them to look for alternatives,” she says. The insects might have developed mutations that allows them to consume bitter leaves, she adds. Ramesh C Saxena, chairperson, Neem Foundation, a non-profit, says it is also possible that the pest could be attacking the tree when the bitter content (azadirachtin and other triterpenoids) is possibly low.
He suggests the pest can be controlled by incorporating four to five kilos of neem cake. This manure helps increase the immunity of the tree. The researchers from Lucknow have done this experiment out of personal interest. But they suggest more research needs to be carried out before the disease spreads. Mohammed Yousuf, head of forest entomology at Forest Research Institute in Dehradun, says the disease could be self-limiting. To prove his point he gives an example of a similar defoliation in the neem trees in Madhya Pradesh five years ago. The problem did not recur, he adds. The agriculture department of Uttar Pradesh says it is keeping a tab on the disease and will take action as soon as symptoms appear.