Susceptibility of Tetranychus okinawanus Ehara (Prostigmata: Tetranychidae) infesting ornamental plants to novel acaricides

Abstract

Spider mites of the family Tetranychidae are well-known agricultural and horticultural pests that feed on a wide variety of plants, including vegetables, fruit trees, and ornamentals. Many traditional insecticides and acaricides have been used to manage mite pests for several decades. The widespread usage of acaricides around the world has facilitated development of resistance in different mite species, making mite management challenging. As a result, many novel acaricides with distinct chemical structures and modes of action have been developed and marketed for mite control. However, after a few years of use, mite populations developed resistance to these newly introduced compounds too. The spider mite species, Tetranychus okinawanus, recently recorded on an ornamental plant Adenium in Thrissur district, for the first time in India, has now emerged as the predominant species of mite infesting ornamental plants in Kerala. Recently, several growers reported inefficacy of the commonly used novel acaricides against mite pests in many ornamental crops, suggesting that the mite populations might have developed resistance to acaricides. Hence a study was carried out to investigate the status of acaricide resistance in Tetranychus okinawanus infesting Adenium in horticultural nurseries and also to elucidate the biochemical mechanism involved in development of resistance. Purposive surveys were conducted in six commercial horticultural nurseries in Thrissur district viz., National Rose Garden, Mangadan Botanical Garden, Ayyappa Nursery, Saranamayyappa Nursery, Pooja Gardens and Nursery, and Manalur Adenium Garden and samples of spider mite infesting Adenium were collected. Mites were maintained as separate isoline cultures assigning unique accession numbers as NrAd1, MgAd2, AyAd3, SyAd4, PjAd5 and MnAd6. Morphological characterisation of the slide mounted mite specimens from the isoline cultures was carried out to confirm the species identity as T. okinawanus. Susceptibility of the six field populations to three acaricides viz., spiromesifen, fenazaquin and dicofol was evaluated in the laboratory, in comparison with the reference susceptible population maintained without exposure to any acaricides in the laboratory, following toxicological bioassay. Susceptibility studies with fenazaquin revealed that the accession NrAd1 recorded the highest LC50 value (27.85 ppm) and has developed moderate level of resistance (14.38-fold) to fenazaquin. This was followed by PjAd5 (9.70-fold), AyAd3 (4.06-fold), MnAd6 (3.78-fold), and SyAd4 (3.23-fold). The lowest resistance ratio was recorded by the accession MgAd2 (1.66). The toxicity studies of spiromesifen also recorded low to moderate levels of resistance in different populations of T. okinawanus. The accession NrAd1 recorded highest resistance ratio of 27.31 followed by PjAd5 (7.18), MnAd6 (3.94), AyAd3 (1.78), MgAd2 (1.42) and SyAd4 (1.06). However, the mite populations showed only low level of resistance to dicofol, recording resistance ratios in the range of 3.65 to 1.22. Biochemical basis of acaricide resistance in different populations of T. okinawanus was investigated by estimating the activity of detoxifying enzymes such as carboxyl-esterase, cytochrome P450 and glutathione S- transferase. Carboxyl-esterase enzyme showed an enhanced activity of 1.03 to 3.52-fold, while cytochrome P450 monooxygenases recorded 1.01 to 2.08-fold higher activity in the field collected populations, compared to the susceptible population. The level of these detoxifying enzymes was found to be higher in the accession NrAd1, which also recorded the highest resistance ratio in the study. However, the activity of glutathione S-transferase (GST) did not differ significantly among the field populations and also with susceptible population, indicating that GST is not a contributing factor in the development of resistance in T. okinawanus against spiromesifen, fenazaquin and dicofol. The study recorded development of resistance in the spider mite, T. okinawanus on Adenium to spiromesifen and fenazaquin, in the horticultural nurseries in Thrissur district, Kerala for the first time in the world. The significant role of the detoxifying enzymes, carboxyl-esterases and cytochrome P450 monooxygenases in imparting resistance in T. okinawanus to the two novel acaricides, fenazaquin and spiromesifen was also confirmed in the study. The study demands formulation of a suitable resistance management strategy in horticultural nurseries in the state for suppressing or delaying resistance development in mite populations.