Natural Crop Protection in the Tropics: Snails and Slugs

Along with slugs, terrestrial snails cause considerable damage to vegetable gardens, agricultural crops and fruit orchards. Ariophanta madraspatna, a terrestrial snail found in Southern India grazes rapidly on tender shoots of garden plants. The giant African snail Achantina fulica, a voracious eater of paddy, started out from the Andaman Islands, came to north-east India and is now migrating westwards creating serious problems for farmers. This snail eats the leaves and fruits of cultivated plants in fields and gardens. It destroys paddies and many other commercial plants, preferring the families Cruciferae, Leguminosae and Cucurbitaceae along with mulberry and tea. In the Philippines, the golden snails (Pomacea canaliculata, P. cuprina and P. gigas) were introduced in 1982 as protein food for people, but soon became important pests, particularly in rice.

The economic importance of slugs and snails, which together form a large group (approx. 12,000 species of pulmonates), ranks next only to insects. They are both agricultural pests and carriers of diseases. The pests are by and large terrestrial while the carriers are mostly aquatic.

Host plants

Fabaceae, Cruciferae, Cucurbitaceae, Solanaceae, cotton, tea, coffee, rice etc.

Snail and slugs can be very destructive in bean cultivation, particularly during the first 20 days.

Control measures

There is no magic bullet in the control of snails and slugs. Instead, an integrated approach is required, using a number of tactics. These are, as generally in pest control, a systematic combination of cultural methods, biological control and curative measures.

Natural enemies

Birds, ducks, chicken

Cultural control

• Never leave plant debris in the field when snails and slugs are a problem without regularly removing these, otherwise the plant material acts as an attractant.

• Attract birds

Snail-controlling plants

Saponins, tannins, alkaloids, alkenyl phenols, glycoalkaloids, flavonoids, sesquiterpene lactons and terpenoids have been found to be poisonous to snails at acceptable doses ranging from <1–100 ppm. Of these, saponins appear to be the most potent class of chemical compounds. Plants containing these compounds belong mostly to the plant families Euphorbiaceae, Phytolaccaceae, Polygonaceae, Rutaceae and particulary Mimosaceae, Papilionaceae and Caesalpiniaceae. The latter three families contain many easily cultivated, drought-resistant trees with high-quality seed proteins .

In the following, the results of two research projects are presented: one was conducted in the Philippines and the other in China. The following standard screening method was used in China: a 1% stock solution of dried plants is made by adding 3 g of plant powder to 300 ml boiling water, boiling it for two hours in a water bath, and then adding water making it up to 300 ml. Varying concentrations of the solution are tested 24 to 120 hours after application on adult snails.

In the Philippine-study 12 plant species were identified which were toxic as water extract at 2% or below. Derris was the most promising of the plants tested. It could be shown that the toxicity was not caused by rotenone. Nevertheless, the high toxicity of rotenone to fish has to be taken into consideration .

In the Philippines also, berries of endod showed an outstanding toxicity as water extract of the seeds, causing as high as 100% mortality at 20 mg/l. The seeds for the study were obtained from Ethiopia. Because of these encouraging results, some seeds were used for planting endod trees on four selected sites. Those planted in the highlands, 1 500 m above sea level, bore fruits after 18 months whereas those planted in the mid-elevation and lowlands grew very well but produced barely any fruits. The leaves however do not show any molluscicidal effects.

SHEHATA confirmed the promising potential of endod as molluscicide. Unripe but fully grown and green berries were collected and dried for about 2 weeks. Water extract was obtained by soaking the carefully crushed berries in water for 24 hours at concentrations of 150 mg/l, 75 mg/l and 37.5 mg/l. In this experiment the first two concentrations killed 98% of the snails (Schistosomum haemotobium and S. mansoni) within 24 hours. At a concentration of 37.5 mg/l a mortality of only 67% was obtained.

Tannin - bearing plants

are considered to have a potential as molluscicides that is worthy of further exploration. Water extracts of Krameria triandra, Hamamelis virginiana and Quercus spp. showed a 100% mortality at 50 ppm. Other promising species against the snail Biomphalaria glabrata are Camillia spp., Potentilla erecta, Alchemilla spp., Acacia catechu, Dalbergia nitidula, Arctostaphylus uva-ursi and Chinchona succirubra.

The advantage of using tannins is that these can be extracted in water and their toxicity to non-target animals is generally lower than the toxicity of potential saponine molluscicides.

The printed version contains more information about the following themes:

Water-soluble plant extracts for snail control

Traps, baits & lures

Other methods