Unfortunately, the nozzles used on the two lower dosage areas 
during this second application apparently were not calibrated correctly. 
From a calculation of the insecticide used on the 0. area, it was deter- 
mined that the discharge rate was approximately one quarter greater than 
desired. This gave a dosage of about 0.53 pounds per acre instead of 0.h, 
and 0.26 pounds per acre instead of 0.2. By shutting off two of the eight 
nozzles, a nearly correct dosage was obtained for the remaining three 
quarters of the 0.2 area. These and other variables have been considered 
in evaluating the relative effects of the spray on the study plots. 
One other factor must also be considered in comparing the effects 
of DDT. The method used, while providing the desired dosages of DDI, 
resulted in a great difference in the amount of oil applied to each area. 
For example, the 1.6 area received 8 times as much oil per acre as did the 
0.2 area, or approximately twice as much as would be applied in most larval 
control. In view of the toxic properties of oil to certain forms of life, 
it is recognized that this in itself could cause an unduly high mortality 
in the areas of greater dosage. At the same time, it should be borne in 
mind that increased dosages of DDT in operational work frequently are accom- 
plished by applying a greater quantity of a standard mixture. While the 
procedure used complicated direct comparisons of the effects of different 
amounts of DDT, it had the important advantage of approximating operating 
conditions. 
Species Studied 
Special attention was focused on the invertebrates of the marsh. 
These essential links in the food chains of wildlife usually occur in larger 
numbers than do vertebrates and the majority of them are more readily 
susceptible to DDT poisoning. Decimation of invertebrate populations by 
DDT could conceivably result in serious detriment to some valuable species 
of marsh wildlife. The blue crab is the only local invertebrate with 
appreciable direct economic importance. 
Though the primary focus of population studies in relation to 
sprayings was on the lower animals, considerable attention was also given 
to fishes and birds, some of which have significant value for sport or 
commercial purposes. 
Nature of the Marsh 
Because of an extensive system of ditching, the study area is 
better drained than comparable unaltered marshlands. However, despite 
ditches, the meandering creek channels with which they connect, and the 
draining effect of ebb tide, there are numerous places where standing 
water is likely to be found. In this report, these water bodies are called 
ponds or pools; .the term tide pool being used also for those directly 
influenced by tides. Ponds or pools of the marsh vary in size as well as 
in degree of permanence. Generally, they are shallow, ranging in depth 
from a few inches to a foot or more. 
