Tiny pests eating up Mumbai’s mangroves

A study, called ‘Development of integrated pest management strategies against major defoliating pests of mangroves in Mumbai Metropolitan Region’ , said four mangrove species were affected the worst by the pests.

Mangrove forests, which should flourish during the monsoon months of August and September, have been wearing a dry, skeletal look across the Mumbai Metropolitan Region (MMR) for more than a decade.

These pests, during the larval stage (when juveniles become adults), feed mainly on the leaves and sometimes the bark.

Ideally, this should not happen: these trees are evergreen, which means they never shed their leaves; there is usually plenty of water during these months as Mumbai receives adequate rain; and, mangrove plants grow only near water bodies, in inter-tidal areas or wetlands.

So, what is causing this mass shedding?

Puzzled by the phenomenon, and worried if it may affect the growth and expansion of mangrove forests, the mangrove cell of the Maharashtra forest department reached out to Bengaluru’s Institute of Wood Science and Technology (IWST). “We were clueless about why the trees looked completely dry, and why its skeletal structure was visible during this time of the year,” said N Vasudevan, additional principal chief conservator of forest, state mangrove cell. IWST’s preliminary results, based on field visits to mangrove forests in Airoli and Ghansoli, along the Thane-Vashi creeks, found 14 pests — seven species of caterpillars (Lepidoptera), two beetle species (Coleoptera), one leaf miner (Lepidoptera), two species of snails and three grasshoppers (Orthoptera) — were responsible for the destruction.

The study, called ‘Development of integrated pest management strategies against major defoliating pests of mangroves in Mumbai Metropolitan Region’ , said four mangrove species were affected the worst by the pests.

These are the grey mangrove (Avicennia marina), Indian mangrove (Avicennia officinalis), Apple mangrove (Sonneratia apetala) and White mangrove (Sonneratia alba). While the plants are able to regenerate within a few months, their reproduction cycle gets severely impaired, as the defoliation coincides with flowering, the scientists from IWST said.

Just how much damage are the pests causing, and would it cause long-term harm? “These pests, during the larval stage (when juveniles become adults), feed mainly on the leaves and sometimes the bark,” said N Mohan Karnat, additional principal chief conservator of forests, group co-ordinator (research), Institute of Wood Science and Technology, Bengaluru. “They eat away the leaves completely, consuming the chlorophyll — the photosynthesis pigment that is essential for the plants to make food. This finally leads to skeletinisation of the trees,” said Karnat, adding that the loss of chlorophyll was dangerous for such trees, as later effects show a considerable drop in their girth and wood biomass.”

The results were collated after a fungus was isolated from one of the infected larvae and sent for identification to Agharkar Research Institute, Pune. The Maharashtra government has spent ₹7.7 lakh over the past year on this study, which was done through ten field visits across MMR mangroves in July, with more visits slated.

“This is the first time in India that research related to natural threats to mangrove trees is being done. We have asked the state mangrove cell for other locations, including mangrove forests in Mumbai, where similar problems are being seen. With the ongoing threat of climate change, such issues are only going to increase,” Karnat said. Vasudevan said the state mangrove cell had to undertake the study to ensure there was no lasting damage to the mangroves. “The mid-term results showed the trees were reviving after September, but these 14 pests stand as threats. However, the final results will reveal whether any conservation interventions are required, or if we should just leave it to nature,” said Vasudevan. He added the cell’s fear was that the pests would return in different seasons. “We want to understand what biological pests can be introduced to fight off these 14 pests that threaten the trees.” Similar studies in other countries show mangrove destruction by pests is not a rare phenomenon — in Hong Kong, one caterpillar was responsible for defoliation over three summer months for seven years from 1995. At Sepetiba Bay in Brazil’s Rio de Janeiro, 17 pests were identified for destroying a two--hectare mangrove swamp in 2001. In 2008, a similar outbreak was observed at mangrove forests in Ecuador, but the trees managed to regenerate.