M.Sc.

The red flour beetle, Tribolium castaneum (Coleoptera: Tenebrionidae) is a cosmopolitan stored grain pest, causing postharvest losses of more than 20 per cent in developing countries and up to nine per cent in developed countries. Even though several practices are available for management of T. castaneum, chemical control remains the most efficient, easy and economic method to reduce the insect pest populations to acceptable levels. Selection pressure from insecticides, however has led to development of resistance in T. castaneum to insecticides. Tribolium castaneum ranks 17th among the 20 most insecticide resistant arthropods in the world and it has already developed resistance against phosphine, methyl bromide, organophosphates, pyrethroids and insect growth regulators which are the commonly used insecticides for its management. The present study entitled “Susceptibility of red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) to insecticides” was undertaken at the Department of Agricultural Entomology, College of Horticulture, Vellanikkara during October 2018 to July 2019. The objective of the study was to assess the susceptibility of red flour beetle populations to selected insecticides, to study the biochemical basis of resistance and to screen new molecules for the safe management of T. castaneum. Different populations of T. castaneum were collected from five different godowns of Food Corporation of India (FCI), viz., Thikkodi, Olavakkode, Mulangunnathukavu, Angamaly and Valiyathura. These five strains, along with the susceptible strain of T. castaneum (procured from Division of Entomology, Indian Agricultural Research Institute (IARI), New Delhi) which was maintained without exposure to any insecticides for more than 35 years, were used to conduct the experiments. Residual film bioassay with malathion, dichlorvos and deltamethrin, which were the recommended and commonly used insecticides in FCI godowns, revealed that susceptibility to malathion was lowest in the Angamaly strain of T. castaneum,
while susceptibility to dichlorvos and deltamethrin was lowest in Olavakkode and
Thikkodi strains, respectively. While, all the three recommended pesticides were
toxic to the susceptible strain. Resistance ratio for all the field collected strains, with
malathion, ranged from 10.95 in Thikkodi strain to 13.34 in Angamaly strain. There
was a significant decrease in susceptibility to dichlorvos also.
Biochemical basis of insecticide resistance was investigated by estimating the
amount of detoxifying enzymes such as carboxyl esterase, glutathione-Stransferase
and cytochrome P450 in different strains of T.castaneum. The activity
of all the three detoxifying enzymes were significantly higher in field collected
populations over that of the susceptible strain. Correlation studies indicated that
carboxylesterase and cytochrome P450 levels were significantly correlated with the
LC50 values.
Residual film bioassay was done to evaluate the susceptibility of different
strains of T. castaneum to new insecticide molecules viz., bifenthrin, chlorfenapyr,
spinosad and flubendiamide. Bifenthrin and chlorfenapyr was found to be most
toxic to all the T. castaneum strains, while, spinosad and flubendiamide were not
effective in controlling T. castaneum. When compared to malathion, bifenthrin was
104 to 378 times more toxic, while chlorfenapyr was 1214 to 2236 times toxic to
the field collected strains.
The most effective insecticides, selected based on the relative toxicity studies
along with FCI recommended pesticides, were sprayed on small jute bags
containing 1 kg of rice. A method was developed and validated to analyse the
residues of malathion, deltamethrin, chlorfenapyr and bifenthrin in rice samples
with limit of detection and limit of quantification of 0.02 and 0.08 ppm,
respectively. Pesticide residue analysis was carried out at different sampling
intervals. When the sprayed sample was analysed after 2 hours of pesticide spray,
0.084 ppm of malathion was detected, which was below the MRL level of 4 ppm.
However, residue levels, 1 and 3 days after spraying were below detection limit. In
case of chlorfenapyr, bifenthrin and deltamethrin the residue levels were below
detection limit throughout the study period.