White Stem borers – bitter coffee this!

Picture: by NICE

Picture: by NICE

This year, the monsoon has not been very kind to the coffee growers in Kodagu district of Karnataka. Besides low rainfall, they are now dealing with another critical problem — most of their crops are infested with coffee white stem borers (CWSB).

“It is usually during post monsoons that this problem increases. We have tried several methods from hand-picking them to using pesticides. But feels like the problem has only escalated. This year, we have suffered more,” laments coffee estate owner, S Chengappa.

First reported in India in 1838, the coffee WSB has been causing substantial economic loss to farmers every year. Though these two-cm beetles complete their life cycle in a year, their emergence periods vary. The female beetle lays eggs in the cracks and crevices of the branches of the host trunk. That could be close to 100 eggs over 25-28 days. The larval developmental period is about 120 days. The caterpillars then feed on the bark of the host and later bore into the tissues, in which they remain for a few months, not before cutting a small circular disc below the bark for adult emergence. The adult beetle emerges, and the cycle begins all over again.

In India, over nine million trees are destroyed each year from CWSB infestation, costing over $40 million annually for replacement and loss in production (Hall et al., 2006).

Identified only later

Here is the tricky part — there is no efficient early detection for infestation, so, usually, the insect is not detected until it has caused extensive damage to the plant. “In such cases, the only recourse is to uproot the plant entirely. The major problem is that the borers elude most pesticides, so cannot be treated easily. Current management practices also require diligence and constant monitoring, which not all farmers employ,” explains Dr Shannon Olsson, Reader at Naturalist-Inspired Chemical Ecology (NICE) at the National Centre for Biological Sciences, Bengaluru.

Shannon’s students, Santosh Rajus and Sriraksha Bhagavan have been working on the project since September 2015. They have spent countless hours in Kodagu studying the beetles in a self-built field lab, and have observed over 800 beetles around coffee plants.

The Central Coffee Research Institute (CCRI), Chikkamagaluru has suggested several solutions including their recent research, which proved that wrapping the main stem of an infected plant using gunny bag strips and spraying with an insecticide (before the end of April and October) kills the adult beetles. But, can a broad scale approach of using pesticides drastically alter the local ecosystem of the crop? “Pesticides are very effective agents against pests, but they are rarely species-specific. This means that in addition to killing the pest, the chemicals can also kill beneficial insects, microbes, animals and/or predators of that pest. I would like to add that, personally, I am not against pesticide use. It can be a very effective technique for pest management. But it must be used wisely and strategically, with full knowledge of the effects it can have on ecology,” explains Shannon.

There are a few natural methods like maintaining the health of the soil that can help deter the WSBs. In a study published in BioOne in 2016, authors Sushil Thapa and Egbert Lantinga offer direct evidence that soil management plays a role in CWSB infestation rates. Adds Shannon, “Soil quality plays an essential role in any pest management. Plants stressed for nutrients will be less able to fend off attacks from pests. So, better soil quality means healthier, stronger crops.” The study proves that plants growing with more soil nitrogen, pH and organic matter were healthier than plants with less of those. Results suggested that soil attributes are crucial for management of coffee WSB.

Changed practices

Unpredictable weather conditions owing to climate change and increase in WSB has forced more and more farmers in Kodagu to switch to Robusta in an attempt to increase yield. Arabica provides better yields when grown in 40-65% shade. Robusta can be grown in the sun. But this sun-grown practice compared with shade-grown coffee has a larger impact on the land and climate as shade-grown coffee farms filter carbon dioxide, reducing global warming. Globally, several studies have repeatedly proved that shade-grown coffee farms host increased numbers and species of birds, offer better soil protection, control erosion, act as natural pest control and help purify air and water.

Though sun-grown coffee farms give better and higher yields, they do not offer the same sustainable and ecological benefits as shade method. Shade-grown coffee farms host increased species of birds (some farms host over 50-80 species) which in turn results in improved pollination and help sustain rainforests as they offer better soil protection/erosion control, and act as a natural pest control.

Shannon’s team is also studying the reasons why the borer prefers Arabica coffee to Robusta. “We believe that the coffee WSB is not a problem to be solved, but a balance that must be struck in the ecosystem. We are not looking for a specific antidote. Rather, our group is trying to understand the ecology and behaviour of the beetle in its natural environment to develop long-term ecological strategies to live with the insect in such a way that it no longer remains a pest,” she adds.

And, while the team continues to look for answers, they have chanced upon new and better possibilities, like how reducing pesticides might offer significant incentives. “We have observed that leaf volatiles appear very important for beetles to locate plants. One of the most surprising observations is the number of natural enemies that the coffee white stem borer has if pesticides are reduced. Robber flies, ants, spiders and other animals are quite ready to prey on eggs, larvae or adults. We are very excited about this observation and are currently looking at ways to promote this natural pest management,” they say.

The article was published in the Deccan Herald 

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