MSc

The study entitled “Field toxicity of new generation insecticides to bee
pollinators” was carried out at AICRP on Honey Bees and Pollinators,
Department of Agricultural Entomology, College of Agriculture, Vellayani,
during 2016-18 with the objective to evaluate the field toxicity of new generation
insecticides to major pollinators namely, A. cerana indica and T. iridipennis and
to assess the insecticide residues in cucumber flowers.
The toxicity of new generation insecticides (at field concentration and half
of the field concentration) recommended for the pest management in vegetable
crops viz., thiamethoxam 25 WG, dinotefuran 20 SG, cyantraniliprole 20 SC,
novaluron 10 EC, diafenthiuron 50 WP, their combinations viz., thiamethoxam
(17.50 %) + chlorantraniliprole (8.8 %) 300 SC, flubendiamide (19.92 %) +
thiacloprid (19.92 %) 480 SC along with a conventional insecticide (dimethoate
30 EC) to the major pollinators were evaluated in terms of bee mortality in the
laboratory. From the laboratory study, two new generation insecticides with least
mortality were evaluated under field conditions along with an insecticidal check
(dimethoate 30 EC @ 0.5 mL L-1) and an untreated control in order to study their
effect on foraging activity of major pollinators of the crop. The persistence of the
insecticides was studied by assessing the residues in the flowers.
Laboratory evaluation of the insecticides revealed that upto three HAT, no
mortality was recorded in Indian bees, A. cerana indica and stingless bees,
T. iridipennis when treated with cyantraniliprole 20 SC (1.2 mL L-1 and
0.6 mL L-1) and novaluron 10 EC (2.0 mL L-1 and 1.0 mL L-1). Mortality of
Indian bees ranged from 0 to 15 per cent and that of stingless bees from 0 to 2.50
per cent at six HAT, when treated with cyantraniliprole and novaluron at their
respective concentrations. At 12 HAT the mortality of Indian bees ranged from
15.56 to 44.72 per cent while that of the stingless bees ranged from 19.44 to 30.56
per cent.
Thus, two insecticides, cyantraniliprole 20 SC @ 1.2 mL L-1 and
novaluron 10 EC @ 2.0 mL L-1 which recorded the lowest mortality in the
laboratory evaluation were selected for field evaluation.
Observations on the pollinator diversity in C. melo L. flowers, prior to the
insecticide treatment, revealed that majority of the pollinators belong to order
Hymenoptera (63.54 %) followed by Coleoptera (27.26 %) and Lepidoptera
(9.01 %).
Among the hymenopterans, T. iridipennis (9.33 m-2 5 min-1) and
A. cerana indica (7.67 m-2 5 min-1) were the dominant pollinators with their peak
time of activity being 0800 h to 0900 h and 1000 h to 1100 h respectively. For the
sufficient pollinator population, one hive of each bee species was installed near
the experimental plot which resulted in the percentage increase of field population
of Indian bees and stingless bees by 26.07 and 50.05 per cent.
Field evaluation of insecticide toxicity was assessed in terms of bee
foraging behaviour viz., relative abundance, foraging rate (no. of bees visited m-2
min.-1), foraging speed (no. of flowers visited in a single plot min-1) and the time
spent by the bees on male and female flowers separately (in sec.). Significant
variation in relative abundance of pollinators among the treatments was observed
upto five DAS in the case of Indian bees and seven DAS in stingless bees.
Lowest relative abundance was recorded for stingless bees from plots treated with
dimethoate 30 EC @ 0.5 mL L-1 which ranged from 12.33 to 23.87 per cent. The
foraging rate varied significantly among the treatments upto five DAS in both the
bees. The stingless bees showed significant variation in the time spent on flowers
upto seven DAS, while Indian bees had significant variation only upto three DAS.
Though the foraging speed of Indian bees showed significant variation among the
treatments upto three DAS, dimethoate 30 EC @ 0.5 mL L-1 recorded lowest
foraging rate when compared to other treatments. In the case of stingless bees,
significant variation in foraging speed among the treatments was observed upto
one DAS with least foraging speed from the treated check.
Observations on the number of returning foragers in the hive before and
after application of insecticides revealed significant reduction of foragers upto
five DAS in stingless bees and three DAS in Indian bees. Thus, the foraging
activities of stingless bees were found to be more affected by the insecticide
application than that of Indian bees.
The study could establish that the new generation insecticides,
cyantraniliprole 20 SC @ 1.2 mL L-1and novaluron 10 EC @ 2.0 mL L-1 which
recorded the lowest mortality in the laboratory are safe to the pollinators in terms
of their foraging behaviour when compared to the dimethoate 30 EC @
0.5 mL L-1. Considering the safety of new generation insecticides to the dominant
pollinators, they can be used for effective pest management in cucurbits, though
the residues of novaluron 10 EC was detected upto 10 DAS when compared to
dimethoate 30 EC (upto three DAS) on flowers of culinary melon.