Biological Control: 
The biological method for pest control is based on the knowledge that in 
nature there exists a balance between mollusks and their enemies. This bal- 
ance is shifted in favor of the introduced species when they become pests, 
but can be shifted in the opposite direction by importing their foreign pre- 
dators or by conservation and augmentation of native established predators. 
In introducing predators extreme caution must be observed, however, be- 
cause of the possibility that they may become more serious pests than their 
prey. Any introduction of foreign species can be made only after careful 
study. 
Many mammals, birds, reptiles, amphibians, and insects have been re- 
corded as occasional predators of snails and slugs. In this country, how- 
ever, none offer an effective and practical means of control, except for 
small areas in which poultry, for example, ducks and chickens, can be em- 
ployed. Carnivorous snails attack and often effectively reduce populations 
of land mollusks. Members of the genus Haplotrema are common carnivo- 
rous snails inthis country. The European carnivorous snail introduced 
into this country is Testacella, 
Because of the alarm it has caused in recent years, Achatina fulica is per- 
haps the best known of all land mollusks in regard to its natural predators 
and biological control, Its most effective enemies are certain carnivorous 
snails, Edentulina affinis and Streptaxis kibweziensis, It is also fed upon 
by the monitor lizard, land crabs, a mongoose-like mammal (Bdeogale 
tenuis), carabid and drilid beetles, and firefly larvae. 

Fresh-water Snails 
Methods of control that have been tried with fresh-water snails include 
drainage, drying, and filling of their habitats, clearing of vegetation, flush- 
ing, and encouraging predators. These have often been successful in limited 
areas for short periods of time, but such measures in larger lakes and 
streams are either not practical or are prohibitive in cost. Ducks, small 
mammals, amphibians, fish, and some insect larvae feed on fresh-water 
snails, but it is doubtful if they have much effect on population densities, 
and when other methods of control are used many of these predators are 
eliminated, The effects of bacteria, fungi, and viruses on snails have re- 
ceived very little attention so far. 
The most widely used control methods practiced against fresh-water snails 
involve chemicals, and a great amount of money and research has gone into 
this method of control. Of the various chemicals known to kill fresh-water 
mollusks, copper sulfate is most commonly used because it is inexpensive 
and very toxic to snails in quantities too low to seriously affect most other 
fresh-water organisms except green algae. Perhaps the best examples of its 
effectiveness can be seen in North Central United States where ''swimmer's 
