India is the seventh largest producer of coffee, exporting 3,95,000 tonnes annually. Of the two varieties of coffee grown widely in India, namely, arabica (Coffea arabica) and robusta (Coffea canephora), the arabica plants suffer infestation by a variety of longhorn beetle called the coffee white stem borer (Xylotrechus quadripes).
The lifecycle of the beetle lasts between 142 and 390 days. The female lays about 50-100 eggs in the cracks in the bark of coffee plants. Once the larvae hatch and grow, they chew the bark and tunnel into the stem where they end up obstructing the flow of food, which may even kill the plant. The damage done by the coffee white stem borer amounts to $17-40 million lost annually.
Managing this pest is a difficult problem. The larvae burrow deep into the wood, therefore, it is difficult to kill them using insecticides. Several strategies are being tried to manage the pest. These include placing shady trees at strategic points, as the beetles prefer open, sunlit areas; using insecticides to kill adults; bark scrubbing to get rid of the eggs laid in the cracks; stem wrapping to prevent adults from laying eggs in the stems; mass trapping of adults using pheromones and even getting rid of affected plants. “Though these methods are effective, for various compounding reasons the coffee white stem borer has become one of the toughest pests to control,” says Shannon Olsson from the National Centre for Biological Sciences, Tata Institute of Fundamental Research (NCBS) Bengaluru.
Prof. Olsson and a team of researchers have been studying the coffee white stem borer. Their aim is to study the insect and the way it approaches various plants and its response to plant volatiles so that a ecologically derived management strategy can be worked out. According to her, historically, it has been noted that the beetle attacks only arabica unless the area is heavily infested. However, recent studies by the team, published in the journal Frontiers in Ecology and Evolution, have shown that this is not because the beetle is more attracted to one variety than the other.
“We found that [the beetle] is attracted to both varieties and uses the volatiles released by the host plants to locate them,” she says. Further, the studies showed that the beetles preferred to approach robusta over arabica. “Subsequent experiments in our lab have also shown that the beetle can and will lay eggs on robusta, but that robusta apparently can partially defend itself against the beetle as it begins to bore through the wood,” Prof. Olsson explains. “However, it is important to note that this defence is not absolute – not all plants we examined prevented the beetle from developing into an adult and emerging.”
Recent reports have shown evidence of infestation in robusta, although not at levels seen in arabica. Hence, a complete shift in cropping from arabica to robusta can potentially face a similar infestation problem unless different management practices are followed, she explains. “While it should not be a total replacement for arabica, the lower infestation rates of [the beetle] in robusta, indicate that it could serve a suitable trap host plant if organised strategically.”
The paper also provides the interesting statistic that coffee plantation area in India has increased by 56% in the past 25 years, converting the diverse forest regions of Western Ghats. In this, the planted area of robusta has increased by 810% from 1950 to 2019, replacing arabica farms that experience severe damage from pests.