PhD

A series of green house experiments were carried out for screening the
insecticides, fungicides and herbicides recommended for the control of
pests, diseases and weeds infesting rice in Kerala, for their resurgence
inducement in N. lugens. Among the thirteen insecticides screened,
each at three doses and applied at three critical growth stages of the
crop as well as at their possible combinations, methyl parathion,
deltamethrin, fenitrothion, fenthion, quinalphos and carbaryl were
identified as resurgence inducing insecticides with no apparent
differences among themselves in the intensity of resurgence caused,
HCH, dimethoate, monocrotophos, phosphamidon, phorate, BPMC
and carbofuran were found to be free from resurgence inducement
and some of them even exerted a significant suppressing effect on the
progeny production of N. lugens. There was no carry over of
resurgence effect over generations. The results of the experiments also
revealed that:
1.
The manifestation of resurgence inducing property of insecticides
was more at the higher doses than at the field doses or lower doses.
2.
The frequency of occurrence of resurgence among the different
treatments with resurgence inducing insecticides
showed that a single application at any of the three critical growth
stages of rice and two applications combining any two of the three
growth stages were on par while three consecutive treatments covering
all the three growth stages was more favourable for manifestation of
resurgence.
3.
The growth stages of the host plant had significant influence on
the manifestation of resurgence inducement of insecticides. It varied
with the properties of insecticides used. Some manifested resurgence at
tillering, some at panicle initiation and none at booting stage.
4.
The resurgence effect induced by the insecticides was found to last
in the treated plants for a period of 15 to 20 days after treatment and
the results indicated, that there was no cumulative effect by repeated
treatments on insect populations.
In the light of the above findings the method of screening resurgence
inducing insecticides in the green house was standardized as the
application of the insecticide at doses higher than the field doses thrice
covering the tillering, panicle initiation and booting stages preceeding
the exposure of insects for assessment of progeny production which
may be done at 15 days after the third application.
In further screening adopting the procedure standardized above
malathion, methyl demeton, FMC 35001, fenvalerate, permethrin and
cypermethrin were found inducing resurgence in N. lugens. The
granular insecticides were screened giving two treatments (tillering +
panicle initiation) and exposing insects at 30 DAT for egg laying.
Results revealed that diazinon, phorate, cartap and carbofuran caused
resurgence of N.
lugens.
Endosulfan,
formothion,
phosalone,
methamidophos, chlorophyriphos, DDVP and their combinations with
HCH or carbaryl (liquid formulations), aldicarb, quinalphos and
sevidol (granular) were free of resurgence hazard.
At the recommended doses and methods of application of fungicides
zineb, mancozb, captafol, ediphenphos, kitazin, carbendazim and
carboxin and the herbicides 2, 4-D (sodium salt and ester),
pendimethalin, fluchloralin, butachlor, propanil and thiobencarb did
not post any resurgence problem.
The inducement of resurgence by insecticide was seen significantly
influenced by the variety of host plants of the insects involved. The
levels of plant mediated resurgence inducement and resistance of the
plants to insect attack were not mutually related. In screening
insecticides for the control of a pest in an agroecosystem, the
interaction
of the popular varieties of the crop available in the area with
resurgence inducement also should be studied.
The resurgence inducing mechanism of six identified insecticides was
studied in detail. The results of a series of green house and laboratory
experiments revealed that :
1. Resurgence
inducing
insecticides
brought
about
some
morphological changes in the crop causing some improvements in the
stand but the magnitude of the changes were not adequate to influence
the attraction of the insects or build up of the pest population.
2. The application of the insecticides caused significant variations in
the nutrient content and biochemical constituents of treated plants
causing consistent changes in the total nitrogen, free sugars and free
amino acid contents.
3. The feeding of N. lugens on treated plants was significantly higher
as indicated by the feeding indices.
4. The correlation studies and path coefficient analysis of the data
relating to the magnitude of changes in the above factors caused by the
application of insecticides could be attributed as the major cause of
plant-mediated resurgence inducement.
The direct application of resurgence inducing insecticides revealed
that some of the insecticides which showed plant mediated resurgence
(methyl parathion, deltamethrin and carbaryl) had direct stimulating
effect also on the progeny production of N. lugens at sublethal doses
while some (fenthion and fenitrothion) did not show any increase in
progeny production and some (quinalphos) showed only marginal
effect. While carbaryl and methyl parathion were more stimulatory at
lower levels, deltamethrin stimulated reproduction at both the lower
and higher levels.
The field experiment revealed that the resurgence observed in the field
was the added effect of plant-mediated resurgence observed in green
house experiments and the direct effect of the pesticides caused by
their sublethal doses. In general the conclusions from the green house
experiments were in agreement with the results obtained from the
field. It was seen that the changes in predatory population in field
caused by the application of pesticides did not contribute significantly
to the inducement of resurgence in N. lugens. It was also observed that
the assessment of plant induced resurgence of insecticides in green
house experiments and the direct effects of the toxicants on the insects
will serve as an effective alternative elaborate field experiments for
evaluating the resurgence effect of insecticides.