Rice is the most widely consumed staple food in the world, providing more than one fifth of the calories consumed by humans worldwide.
Susceptibility to Slug Damage
Snails eat young and emerging rice plants. They damage the crop by cutting the rice stem at its base, destroying the whole plant. If no control measure is taken, they can completely destroy one square metre of field overnight, resulting in a yield loss of more than 50%.
Species of Concern
Two species of snail, commonly known as golden apple snails, are widely regarded as a major threat to rice crops. Pomacea canaliculata and Pomacea insularium are species that are of predominantly South American origin. Introduced to Taiwan during the 1980s for snail farming, escapees became a serious rice pest. They are found in Japan, China, the Philippines and much of Indonesia.
Snails spread through irrigation canals, natural channels and during flooding. In the absence of water, the snail can bury itself in mud and hibernate, for as long as six months.
Symptoms of Attack
To confirm damage, look for cut leaves and cut stems. Wet-seeded rice and transplanted rice up to 30 days old is most susceptible; older plants are usually thick enough to resist eating by the snail.
Presence of distinctive pink eggs also indicate golden apple snail populations.
Field preparation and crop establishment generally provides the best opportunity for management of golden apple snails. The first ten days after transplanting, and the first 21 days after wet-seeding, are critical periods.
Biological control (domestic ducks and wild birds to eat mature snails, red ants to feed on snail eggs) can help to reduce populations.
Cultural control measures can also play a part. Handpicking snails and removing egg masses will have a direct effect on population numbers. Apple snails have difficulty moving in less than 2cm of water, so managing water levels during the most vulnerable growth stages can be beneficial. Another water-related measure is to monitor the field’s water entry and exit points, and place barriers across them, to prevent snail movement.
Chemical control can be deployed when other measures fail to reduce populations significantly. Pellets should be applied immediately after transplanting, or – in wet-seeded rice – during the seedling establishment phase. It shouldn’t be used in crops older than 30 days.