The sewage snail
Physa acuta
is a serious threat to certain economic plants and to the purification plant
of sewage works by rendering the biofilters ineffective. Various attempts
are being made to control it. The efficacy of the predacious water bugs
Sphaerodema rusticum
was judged experimentally, in the laboratory in the potential control of
P. acuta
. It is revealed that, when supplied separately, the first, second and third
instar and the adult
S. rusticum
did not attack
P. acuta
belonging to 3.1-8 mm, 5.1-8 mm, 7.1-8 mm and £ 3 mm size classes respectively.
In the remaining trials predation rate varied from zero to eight (average
2.3) individuals per predator per day. In experiments with
P. acuta
belonging to all the size classes supplied together, none, except the first
instar
S. rusticum
, attacked the prey individuals belonging to the lowest (£ 3 mm) size
class. The first and second instar
S. rusticum
, in both trials did not attack
P. acuta
larger than 4 mm and 5 mm in shell length respectively. The water bugs belonging
to the third, fourth, fifth instar and adult stages though preyed upon
P. acuta
with 3.1-8 mm shell length. The average rate of predation by a single
S. rusticum
varied from 0.14-3.08 individuals per day depending upon the size of
P. acuta
and the stage of
S. rusticum
. A single
S. rusticum
, irrespective of instar and adult stages, destroyed on average 4.16
P. acuta
daily irrespective of sizes. It is estimated that one
S. rusticum
could destroy 1,360
P. acuta
in its life time. The results clearly indicate that the water bug
S. rusticum
may be used to control the snails
P. acuta
.
