Crop losses due to insect pests are estimated at around 10%-30%, depending upon the crop being considered (Oerke, 2006). For example, Gangrade (1967) reported 100% pod damage in pigeonpea cultivers in Madhya Prdesh due to podfly. Most control strategies against insect pests in food crops still centre around the use of synthetic insecticides, but because of the selection pressure this exerts on insect populations, the number of resistant species is increasing; it was estimated at >500 species in 1990 and rising (Georghiou, 1990).
IPM strategies such as rotation of actives will help minimise target-site resistance and insect refugia will support susceptible populations so that they can provide again susceptible genes into resistant populations. However, insecticide resistance problems remain.
Metabolic resistance is reported to be the most common mechanism and often presents the greatest challenge (Karaağaç, 2012). It is the result of enhanced enzyme activity in the target insect resulting in degradation or increased excretion of the insecticide. Phytophagous insects have developed such enzymes as protection from naturally occurring plant toxins such as terpenes, alkaloids etc, i.e. toxins that are part of their food (Gatehouse 2002, Rane et al, 2016).
The enzyme groups primarily responsible for initial action upon the insecticide are esterases and microsomal oxidases:
Esterases are non-specific in their action and can detoxify a broad range of insecticide classes such as organophosphates, carbamates, pyrethroids, neonicotinoids and even Bt toxin (Bhatt, 2021; Philippou et al, 2010; Gunning et al, 2005). Despite many references citing that esterases only detoxify by means of cleaving ester bonds, they are derived from cell adhesion proteins and can act by simply sequestering the toxin (Suzuki et al, 1993).
Oxidases (mixed function oxidases, P450s) as a group are capable of detoxifying a wide range of insecticide classes including organophosphates, carbamates, pyrethroids and neonicotinoids (Fonseca-González, 2011; Moyes, C, 2021; Philippou et al 2010, Alptekin et al 2016). Resistant insects can reveal enhanced oxidase activity due to overexpression of P450 genes (Feyereisen, 2005) or due to modifications (mutations) to the relevant enzyme (Amichot et al, 2004).
The optimal way to mitigate the effects of metabolic resistance is through the use of a synergist that is capable of inhibiting enzyme activity. The ‘gold standard’ synergist is piperonyl butoxide (PBO).
Originally extracted from sassafras trees, this molecule was first considered a specific inhibitor of P450 oxidases (Casida, 1970). However, research on the esterases (Young et al, 2005) later showed that PBO is also a capable inhibitor of esterases, thus it effectively reduces the contributions of both major metabolic resistance enzyme groups.
Piperonyl butoxide has been used successfully against several major non-agricultural insect pests including mosquito species, cockroach etc (Thanispong et al, 2024; Hemingway and Small, 1993)
Piperonyl butoxide has also been used successfully against several major agricultural insect pests including Helicoverpa armigera, Bemisia tabaci, Aphis gossypii and Myzus persicae (Bingham et al, 2007).
Concerns that it may select for target-site resistance have shown to be unfounded, indeed it may be an opportunity to slow target-site resistance as well as overcome metabolic resistance (Zimmer et al, 2017).
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