
+ rF - T T T T T T 
log Y = 0.05313X + 0.42679 
S= 0.006325 
P< 0.001 




1 1 1 so pacer 1 n Se 
2 3 4 5 6 z 8 9 TOP Sip 12813: 014" 15 S16" 617, 18-195" 2 
Miles 
Figure 4,--Number of tobacco plants per 100 showing 
attack by hornworms August 18-22, 1964, in 48 fields 
not treated with insecticides, plotted against distance 
from center of light-trap area, 
follow the standard recommendations and had 
increased stalk destruction partly as a result 
of our activities, Since it seemed better to have 
all the stalks cut rather than partofthem, each 
grower in the fall of 1963 was urgedto destroy 
all stalk fields before hornworm and budworm 
larvae were mature, Figure 5 shows the number 
of fields remaining, 
The graph indicates that the distribution of 
stalk fields in this area resembled the dis- 
tribution of hornworms and damage shown in 
the previous graphs, That is, if the hornworm 
populations were affected by the number of stalk 
fields, the effects of host-plant abundance and 
light traps were confounded in this experi- 
ment, It is not possible to separate the effect 
of these two factors because populations are not 
necessarily proportional to the number of 
fields, 
Nearly all tobacco fields in this area were 
treated with a sucker-control chemical, and 
the suckers produced were mostly small and 
198 
fst rt = 

log Y + 1 = 0.02213 + 0.02155 
Sy = 0.005316 
| P <0.001 



s*a* + + = * — +, (at et ie 
1 2 3 4 5 6 7 8 9 10 11 «#12 «#213 14°15 16 #17 «#18 19 
Miles 
Figure 5,--Number of tobacco fields per linear milenot 
cut or plowed in four directions from center of light- 
trap area, September 1963, 
unattractive to ovipositing hornworms (Rabb 
et al, 1964). Nematodes and disease further 
reduced sucker growth, so that in the area 
studied there were very few, if any, fields 
producing more than a few hornworms and no 
such fields were seen in fairly extensive 
surveys. On the other hand, there was a heavy 
reduction in insecticide usage in the light-trap 
area, so that before harvest the number of 
hornworms surviving treatment may have been 
greater inside the light-trap area, Inany case, 
it is clear that light traps or stalk destruction 
or both produced major reductionin hornworm 
numbers in the area, 
The degree of control might be estimated by 
comparing average values inside and outside, 
but this would give a biased estimate, since 
hornworm movements would reduce the dif- 
ference between high and low-population areas, 
The movement factor can be partially elimi- 
nated by using the regression equations, rep- 
resenting the population gradient, to calculate 
populations at the center of the circle and at 
some point on the outside, Theoretically the 
outside point should be the place where the 
population levels off, but there is no evidence 
that the population did level off at the farthest 
point sampled, We have arbitrarily chosen 12 
miles for the outside point, since this is the 
same distance outside the light-trap area as the 
center is inside, The results of these calcula- 
tions are given in table 1, Since counts were 
made in 1964 out to 20 miles, we have shown 
percent reductions out an additional 6 miles for 
this year for comparison, 
