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Spider mites of the family Tetranychidae are devastating pests of horticultural
crops demanding suitable control measures. Synthetic acaricides, while effective, often
cause resistance and resurgence, besides leaving residues on fruits and vegetables.
Consequently, several natural products are being evaluated as safer alternatives for the
management of mite pests. Petroleum derived mineral oils, for instance, have been used
for centuries against insect and mite pests in several crops. However, phytotoxicity
concerns have limited their use to a few selected crops. Recent advances in processing of
mineral oils have mitigated these apprehensions, enabling their wider application in crop
pest management, including vegetables.
A study was undertaken at Department of Agricultural Entomology, College of
Horticulture, Vellanikkara during 2017-18 in order to evaluate the bioefficacy of
horticultural mineral oil (HMO) against the spider mite, Tetranychus truncatus. The
study also aimed to test the phytotoxic effect of HMO on okra and its safety to important
natural enemies such as the predatory mite, Neoseiulus longispinosus and the
acaropathogenic fungus, Acremonium zeylanicum.
Laboratory bioassays were conducted to evaluate the efficacy of HMO alone and
in combination with neem oil against eggs and adults of T. truncatus. The results showed
that HMO had appreciable ovicidal and adulticidal action against T. truncatus. Mineral
oil alone, at concentrations of 1.5, 2.0, 2.5 and 3.0 per cent and combinations of HMO
with neem oil viz., HMO 2.5 per cent + neem oil 2.0 per cent; HMO 3.0 per cent + neem
oil 2.0 per cent and HMO 1.0 per cent + neem oil 1.0 per cent recorded 100 per cent
mortality of eggs. In the case of adults, HMO at 3.0 per cent recorded the highest
mortality of 92.00 per cent and was significantly superior to HMO at 2.5 per cent
(84.00%) and neem oil 2.0 per cent (81.33%), which were on par with each other.
In the field experiment, plots treated with HMO at 2.5 (92.60%) and 3.0 per cent
(93.90%) as well as HMO 2.5 per cent + neem oil 2.0 per cent (94.14%) and HMO 3.0
per cent + neem oil 2.0 per cent (96.79%) recorded significant reduction in mite
population and were superior to plots treated with either spiromesifen (91.08%) or neem
oil alone at 2.0 per cent (90.42%).
The phytotoxic effect of HMO was tested on 45 day old potted okra plants by
foliar application at concentrations of 2, 3, 4, 5 and 6 per cent. None of the concentrations
showed phytotoxic symptoms on okra.
Laboratory bioassay to evaluate the safety of HMO to the predatory mite,
Neoseiulus longispinosus revealed that HMO is relatively safer to the predator than to
T. truncatus. HMO at 3.0 per cent killed 38.89 per cent of eggs while HMO 3.0 per cent
+ neem oil 2.0 per cent caused the highest mortality (27.78%) in adults.
Combinations of HMO with neem oil were found to be more inhibitory to the
acaropathogen, Acremonium zeylanicum than HMO alone. HMO 3.0 per cent + neem oil
2.0 per cent recorded significantly higher inhibition of 86.44 per cent as against a
significantly lower 46.67 per cent inhibition by HMO at 2.5 per cent.
The high efficacy of HMO against the spider mite T. truncatus as well as its
relative safety to the predominant natural enemy brought out in the study suggests that
HMO can be an effective tool for mite management in vegetable crops.