aoe 
environmental pressures, tend to check the density of populations, thereby 
preserving the balanced community organization. Thus, in stable commu- 
nities there is a carefully adjusted balance between the reproductive poten- 
tial of the constituents and the environmental resistance which they must 
face. Interference that tips the balance in either direction may produce 
serious consequences, 
When a snail or slug species is introduced into a new area, it often leaves 
its predators and parasites behind. Therefore, these important checks are 
no longer present to curtail population growth. If other environmental factors 
are favorable, the species may then realize its full potential of increase, 
and if this potential is high, only a short period of unchecked reproduction 
will cause the population to irrupt. The invaders then proceed to consume 
certain plants in great quantity, destroying food and shelter of other organ- 
isms. Reduction in the abundance of these native species results, followed 
in turn by their predators and parasites. The whole organization of a 
community may in this way be completely disrupted by such an immigration. 
When the introduced species has exhausted its food supply, it may emigrate 
in many directions to other communities, thus leaving in its wake a path of 
devastation. 
CONTROL 
When undesirable snails and slugs become established in a new area, it is 
usually desirable to get rid of them. Sometimes this is almost impossible 
or the cost is prohibitive if they have become too well established. Never- 
theless, even in such cases it is often feasible to diminish their importance 
by curtailing their increase and spread, or by protecting valuable vegetation. 
In the past, certain methods of control have been found that are both satis- 
factory and practical, These methods vary with the types and habits of the 
various mollusks, but generally come under four classes, that is, chemi- 
cal control, physical control, cultural control, and biological control. 
Land Snails and Slugs 
Chemical Control: 
Chemical control of land snails and slugs involves poison or repellent 
sprays and paints, irritating powders, and poison baits. Since snails must 
have lime to consolidate their shells and actively search for it if it is not 
readily available to them, use has been made of this by painting or spraying 
objects with a 1 percent solution of calcium arsenate mixed with lime water. 
This serves both as a lure and a poison, Other arsenicals used as sprays 
have often been ineffective because snails and slugs avoid them. Copper 
based sprays do not give satisfactory results without also injuring plants. 
Sprays of Bordeaux mixture, kerosene emulsion, Octaklor, pyrethrum, 
DDT-kerosene emulsion, soap solutions, salt solutions, and lime sulfur are 
