Lethal and sublethal effects of flubendiamide and spirotetramat against the leaf worm, Spodoptera litura(Fabricius) under laboratory conditions

Introduction

The leaf worm (Spodoptera litura Fabricius) is devastating polyphagous insect pest widely distributed in South Asia with wide host range of more than hundred host plants (Ahmad, Ghaffar & Raffiq, 2013; Sang et al., 2016). During the survey of three different sites in the cotton belt of Southern Punjab, 27 host plant species of S. litura were reported belonging to 25 genera and 14 families including cultivated crops, ornamental, fruits, vegetables and weeds (Ahmad, Ghaffar & Raffiq, 2013). Female lays round or spherical eggs, covered with hairy scales. Larvae also vary in colors and length of full grown larvae is almost 40–45 mm having longitudinal bands with two dark spots present on its dorsal side. Adult moth is grayish brown in color (Simmons et al., 2018) and egg hatches within 3–5 days and life cycle complete in 5 weeks (Ahmad et al., 2021). Due to its gregarious feeding behavior, if not managed timely, serious damage on crops may occur and cause reduced crop yield (Dhir, Mohapatra & Senapati, 1992; Ahmad, Saleem & Sayyed, 2009). It causes considerable losses during the reproductive stages of the crop (Singh & Sachan, 1992). Depending upon feeding on different host plants, it has gained different names like tobacco cutworm, tobacco caterpillar, Indian leaf worm and cluster caterpillar (Ahmad, Arif & Ahmad, 2007a). In Pakistan, its infestation starts at the end of March and sustain up to November (Sayyed, Ahmad & Saleem, 2008), with an estimated lose of 20 to 60% in the total annual yield (Saleem et al., 2016). In the cotton growing areas, it is abundantly found in September and October (Islam, Ahmad & Joarder, 1984).

Among lepidopteran insect pests, S. litura was the first pest that developed resistance (Srivastava & Joshi, 1965) against organophosphates (Vijayaraghavan & Chitra, 2002) and pyrethroids (Babu & Santharam, 2002; Sudhakar & Dhingra, 2002). Extensive use of chemicals resulted in the failure of control, pest resurgence and many health hazards (Ahmad, Arif & Ahmad, 2007; Khan & Mehmood, 1999). Insecticide resistance to almost all the available insecticides has been previously recorded based on laboratory and field studies (Kranthi et al., 2002). Long field exposure to different insecticides resulted in the development of S. Litura resistance (Ahmad, Saleem & Sayyed, 2009). Use of organophosphates, pyrethroids and carbamates for more than two decades created the best environment for resistance development against these conventional insecticides and resulted in the failure of effective control (Ramakrishnan, Saxena & Dhingra, 1983; Wu, Gu & Wang, 1995; Ahmad, Saleem & Sayyed, 2009). However, the mixture of insecticides (chlorpyrifos, profenofos and fipronil) was found to be an effective alternative against S. litura management (Ahmad, Saleem & Sayyed, 2009). Field control became more difficult and expensive to later larval instars owing to their high tolerance for insecticides (Kim et al., 1998).

Chemical control still persisted as the common method because of its ease of application and quick pest control (Peter & David, 1988; Kumar & Parmar, 1996). Although, insecticides give rapid control yet there are certain disadvantages like disruption of natural balance and health hazards. Furthermore, inappropriate application of insecticides at high rates also leads to the development of resistance and environmental pollution. On the other hand, the sublethal effects of different insecticides influence the biological parameters including the larval and pupal duration, mating, pupal weight, fecundity and fertility of eggs. However, adult longevity and pupal weight were not affected by insecticides’ application but negatively affected the copulation period (Jasoja, 2002). Fluvalinate and cyhalothrin affected the biological parameters of lepidopteran pests with increased sensitivity of adult male moth in comparison with female moth and changed in the longevity of larval and pupal stages (Abro, Memon & Syed, 1997).

Chemicals used for the control of lepidopteran species have some demographical effects on insect population. Chlorantraniliprole has shown reduction in survival of the offspring, fecundity and egg hatching whereas the period of oviposition has increased in Plutella xylostella with delayed development (Han et al., 2012). Chlorfluazuron when applied on S. litura at sublethal rates affected the instar development, pupal moulting and emergence of adult; however, their hazards were higher at lethal dose rates with differences in the body weight of larvae and pupae, along with the reduction in fertility of female and hatchability. While male fertility was reduced by 65–81% and hatchability by 44–66% with enhanced male sensitivity (Parveen, 2000). Flubendiamide did not exhibit the cross resistance and phytotoxicity at their recommended field doses for P. xylostella, S. litura and Pieris species yet foliar application of flubendiamide has previously been proposed for control of lepidopteran insect pests on vegetables (Khan, Ahmed & Nisar, 2011). Keeping in view the important role of new chemistry insecticides like flubendiamide and spirotetramat, lethal and sublethal effects on important biological parameters of S. litura, were observed on early larval stage of S. litura against different life history parameters like net fecundity rate, generation time, survival rate.

Materials and Methods

Results

Discussion

Lepidopteran pests grow rapidly due to short life span and high reproductive potential. These pests cause economical damage to many crops and household things. Different pesticides are used to kill these pests which are easy, short and cheap way to kill the pest of damaging entities. Long term use of insecticides provoked resistance in some pests and eventually failure in managing the specific pest in the. This is common for S. litura that use of same insecticides for long duration may result in the control failure of this pest.

Shorter life span and high reproductive potential of insects make them very efficient to increase their number in short time. Most commonly used control methods include host plant resistance, biological control and chemicals as pesticides to kill these insect pests. The latter method of pest control is common in Asian countries facing more pest problems due to good climatic conditions and variety of food resources for multiplication of such insect pests. Their wide and long term use has resulted in different problems including insecticide resistance and resurgence of insect pest. Lethal and sublethal effects of newly introduced insecticides provide more detailed and effective utilization of these chemicals for long term management and weak links to target these insect pests including this important leaf feeder of many economically important crops persisting for a long duration on different crops (Sayyed, Ahmad & Saleem, 2008).

Present study revealed a decreased larval duration and growth rate for spirotetramat and flubendiamide insecticides with the increased concentrations. Although some eggs survived at sublethal levels of spirotetramat and flubendiamide, with very low survival rate for the next generation. Such drastic decrease in number of such insects helps to manage them under less use of insecticide and at the desired recommended rate of application. Such changes have previously been observed to minimize the use of pesticides against insect pests and make our food and environment less hazardous (Stark et al., 1997). Increase in larval mortalities not only reduces the losses at that particular crop stage when that insecticide applied but also decreases them for the coming generations (Thakur et al., 2013).

Emamectin has proved to be more effective than indoxacarb, lufenuron and spinosad whereas abamectin was the least effective in previous lethal studies for Helicoverpa armigera (Ahmad, Arif & Ahmad, 1995; Ahmad, Saleem & Ahmad, 2005). It has also been observed toxic to S. litura in the surrounding country field strains (Karuppaiah & Chitra, 2013). Emamectin was previously found toxic to beneficial insect like Chrysoperla carnea in local strains of Pakistan; however, flubendiamide was moderately toxic and considered safer (Hussain et al., 2012). There is need to know more about the lethal and sublethal responses which make the present study to compare these new insecticides with novel mode of action. Such studies will be helpful for future application of these insecticides against this important leaf feeder and other economic insect pests.

Demographic toxicity is becoming a new field of toxicology (Stark & Wennergren, 1995; Forbes & Calow, 1995) because it covers all effects including all lethal and sublethal effects that an exposed insect might have on its population. The studies usually performed on complete life cycle need to be obtained under pesticide stress (Banks & Stark, 2000; Stark & Banks, 2003). The demography and other parameters of life for estimation of toxicity should be adopted more widely.

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