PhD

Survey conducted in three heterogeneous rice ecosystems of Kerala
viz., Kuttanadu rice fields of Alappuzha district with high load of pesticide,
double cropped rice fields of Thiruvananthapuram district with moderate
level of pesticide and Pokkali rice fields of Ernakulam district with no
pesticide during two consecutive seasons at the vegetative, tillering and
reproductive stages of the crop revealed a distinct change in the
composition and status of natural enemies and pests in rice ecosystem.
While pests dominated in Kuttanadu and double cropped rice fields
of Thiruvananthapuram, natural enerm es dominated In Pokkali
ecosystem. Among the natural enemies, population of predators was
signi ficantly higher 111 all the three ecosystems. Population of
pnr as lt o i d s was very low. Among the predators, insect predators
dominated the predator fauna followed by the spiders in the three
ecosystems. The important insect predators observed were the mirid
bug Cyrtorhinus livi dipennis Reuter and the coccinellid beetle
Micraspis di scol or (Fabricius) and the major spider predators recorded
were Tetragnatho maxillosa Thorell and Lycosa pseudoannulata
(Boesenberg et Strand).
Among the pests seen in th~ three ecosystems, the sucking pests
were dominant followed by the defoliators. Stage specific sucking
pests like the ricebug Lept ocorisa acut a (Thunb) and earhead thrips
Hapl ot hrips ganglb aueri Schmutz too were seen infesting the crop
during the respective crop stages. Hoppers were the predominant
sucking pests in all the three rice ecosystems. Population of other
sucking pests was very low. Among the hoppers Nephot ettix sp. and
Sogatella furcifera (Horvath) were predominant. Comparatively,
population of the brown planthopper, Nilaparvata lugens (Stal) was

low. Population of other pests considered as major ones in the State
like yellow stem borer Scirpophaga incertulas (Walker), gall fly
Orseolia oryzae (Wood-Mason), case worm Parapoynx stagnalis Zeller
etc. was negligible.
The population of neutrals was low in all the three rice
ecosystems. On the other hand, high population of these 'other insects'
was seen in the adjoining vegetation.
Greater diversity of arthropod spec ies was observed in the
insecticide-free Pokkali rice ecosystem and insecticide-rationally-used
double cropped rice ecosystem of Thiruvananthapuram. Less diversity
of species was seen in Kuttanadu rice ecosystem where insecticides
were applied indiscriminately.
Studies on the suitability of different prey for mass culturing
C. l ivi dip enni s, M. discolor, T. maxillosa and L. pseudoannulata under
controlled conditions indicated that the best prey for C. lividipennis
was the first instar nymphs of N. I ugens ,for M. discol or its eggs, for
T. maxillosa, Nephotettix sp. and for L. pseudoannul at a, N. lugens.
Efficiency of the predators 111 suppressing hopper pests when
assessed in terms of searching capacity, feeding potential, prey
preference and hyperpredation revealed that the insect predators
C. lividipennis and M. discolor showed better ability to search for plant
hoppers, especially N. lugens and S. furcifera. Comparatively, the
searching capacity of the spiders for hoppers was low. The feeding
potential and prey preference of C. livi dipenni s was significantly high
for first instar nymphs of N. l ugens . Similarly, M. discolor preferred
more of its own eggs than the other prey tested. The spider,
L. ps eu do annul at a was observed to be a significantly better predator of

hoppers than T. maxill os a. None of the predators tested showed
cannibalism. Hyperpredatory activity was observed in Crocothemis sp.,
Agri ocne mis sp., Polytoxus fuscovitt at us (Stal), T. maxillosa, Oxyopes
sp. and L. pseudoannulata. The mirid bug C. lividipennis was a highly
preferred prey for hyperpredation.
The commonly used insecticides for nee pest control
viz., carbaryl, phosphamidon, monocrotophos, quinalphos and methyl
parathion caused 80 to 100 per cent mortality of predators at the dose
normally recommended for pest control.
The results of the study emphasized the need for periodical
assessment of the arthropod community Jl1 nee ecosystems as agro-
ecological conditions tend to shift the status of phytophages and
entomophages from time to time. Based on the results, growing of
varieties tolerant to major pests and regulation of incident pests by
conserving naturally occurring predators through optimal habitat
modification and use of predator-friendly insecticides when absolutely
needed will go a long way in sustainable production of rice in Kerala.