* Coffee output to drop over 5 pct in 2017/18 -industry official
* Robusta output seen falling 6.4 pct, arabica down 4.7 pct
* Lower rainfall encourages white stem borer pest
MUMBAI, Aug 22 (Reuters) - India's coffee output will likely fall to its weakest in eight years in 2017/18, with low rainfall hitting the development of crops and encouraging the spread of plant-consuming pests, said a senior official from one of the country's top industry bodies.
The South Asian country, famous as a tea producer, is also the world's No.6 coffee grower, mainly churning out the robusta beans used to make instant coffee, but also producing some of the more expensive arabica variety.
Although it lags well behind top planters such as Brazil and Vietnam, tepid coffee production in India could support global prices , this week trading near their lowest level in over a month.
"This year we got lower rainfall than usual," said M. M. Chengappa, chairman of Karnataka Planters' Association (KPA), forecasting the country's production could fall to around 300,000 tonnes in 2017/18 year starting from Oct. 1. That would be down 5.3 percent from a year ago and the lowest since 2009/10.
The southern state of Karnataka's Kodagu, Chikmagalur and Hassan districts, account for two-third of India's total coffee production. They have received a third weaker rainfall than usual since the start of monsoon season on June 1.
"Drought and sunshine increased infestation of the white stem borer pest," Chengappa told Reuters, referring to a beetle that can attack plants growing arabica beans.
The country's arabica output could drop to 90,000 tonnes in the 2017/18 marketing season, from 96,200 tonnes the year before, he said.
India's robusta output could fall to 210,000 tonnes, from 220,500 tonnes this year, Chengappa added.
Robusta planting was hit by lower rainfall during the key flowering stage in March and now poor monsoon rains could delay development of the crop, said Anil Kumar Bhandari, a large-scale planter.
India, which some say started coffee cultivation in 1670 with seven smuggled beans, exports three-quarters of its production. Lower output for two straight years could reduce the nation's exports in 2017/18, said an exporter based in Bengaluru.
Italy, Germany and Belgium are the main buyers of India's crop, paying a premium over global prices.
(Reporting by Rajendra Jadhav; Editing by Joseph Radford)