Introduction

In general, insecticides are classified as fast-, moderate- and slow-acting (Hannig et al. 2009) based on a most common criterion ‘time from exposure to mortality’ (Campbell 1926; Chang 1952; Bettini et al. 1958; Clinch and Ross 1970). Some insecticides cause the lethality within approximate 1 day (fast-acting), whereas some kill the pest several days later (slow-acting). The speed of toxic action of an insecticide is an indicator for control efficacy and has considerable practical importance (Cory et al. 1994; Sun et al. 2004). For agricultural pest control, fast-acting, especially the speed to cause cessation of feeding, is an important feature for an insecticide to consistently reduce the amount of feeding damage (Hannig et al. 2009). Baculoviruses, for example, exhibit slow speed of action in insect pest control (Moscardi 1999). Crop damage after application of a baculovirus spray can therefore be substantial, even if mortality in target insects is eventually high (Bianchi et al. 2000). Genetically modified baculoviruses show faster speed of action (Stewart et al. 1991; Inceoglu et al. 2006), and subsequently can provide improved protection of crops in comparison with wild type baculoviruses (Cory et al. 1994; Sun et al. 2004). In contrast, German cockroach (Blattella germanica) foraging individuals can translocate a slow-acting insecticide to other members of their aggregation, and conspecific individuals then contact the insecticide, or ingest insecticide-laden feces (coprophagy), other excretions, or dead and dying insects (necrophagy and cannibalism). Thus, slow-acting insecticides in palatable baits are most effective. For example, coprophagy by first instars is an important mechanism underlying the horizontal transmission of hydramethylnon, a slow-acting insecticide (Silverman et al. 1991; Kopanic and Schal 1997, 1999; Buczkowski et al. 2001). Similarly, termiticides are required to be slow-acting and non-repellent in order to achieve greater colony suppression or even elimination (Su et al. 1982, 1987, 1991, 1994; Su 1994; Su and Scheffrahn 1991; Yeoh and Lee 2007).

Butene-fipronil is a novel compound obtained via the structural modification of fipronil, and has a relatively low toxicity to fish (Niu et al. 2007a, c). Since butene-fipronil is a GABA-gated chloride channel blocking insecticide (Zhao and Salgado 2010), it has no obvious cross-resistance to other insecticides with different modes of action (Liu et al. 2009; Niu et al. 2007b, 2008). Up to now, butene-fipronil has been proven to exhibit high toxicity to some insect species in Lepidoptera, Hemiptera, Coleoptera, Orthoptera and Diptera (Niu et al. 2007a, c; Liu et al. 2009; Yuan et al. 2009; Li et al. 2009; Arain et al. 2014). However, the speed of action of butene-fipronil remains undetermined.

Colorado potato beetle Leptinotarsa decemlineata is a notorious defoliator of potato throughout most of the northern Xinjiang Uygur autonomous region in China, and it often causes extremely large potato yield loss each year (Jiang et al. 2010, 2011, 2012; Shi et al. 2012, 2013; Lu et al. 2011). The control of this pest has mainly relied on insecticides (Alyokhin 2009; Alyokhin et al. 2008). This inevitably leads to the development of resistance to various insecticides such as organophosphates, carbamates, pyrethroids, and neonicotinoids (Alyokhin 2009; Alyokhin et al. 2008; Zichová et al. 2010; Mohamadi et al. 2010; Jiang et al. 2010, 2011, 2012; Shi et al. 2012; Lu et al. 2011; Malekmohammadi et al. 2012; Rinkevich et al. 2012; Szendrei et al. 2012). Moreover, Drosophila melanogaster is easy to care for, breeds quickly, and lays many eggs, it is an excellent model system to assess toxicities of insecticides (Arain et al. 2014). In order to test the speed of action of butene-fipronil in different representative insects and under different mode of actions, we selected a coleopteran and a dipteran, and determined the contact toxicity in L. decemlineata and stomach toxicity in D. melanogaster, in comparison to seven other insecticides. Among them acephate, endosulfan, methomyl, α-cypermethrin and spinosad were fast-acting compounds, whereas abamectin and imidacloprid were slow-acting chemicals (Hannig et al. 2009; Franc and Bouhsira 2009).

Material and methods

Experimental animals

Post-diapause L. decemlineata adults were collected from potato fields in the spring at Urumqi (43.82 N, 87.61E), Xinjiang Uygur autonomous region in China. Insects were routinely reared in an insectary at 28 ± 1 °C under a 14 h:10 h light–dark photoperiod and 50–60% relative humidity, with potato foliage at vegetative growth or young tuber stages in order to assure sufficient nutrition. The newly-eclosed fourth-instar larvae and adults at least 7 days after emergence were used in the experiments.

D. melanogaster Canton-S (CS), w 1118 and Oregan flies were raised on standard cornmeal/molasses/agar medium under controlled temperature (25 ± 1 °C), photoperiod (12 h light/12 h dark) and relative humidity (about 50%). The 5 day-old adults were used in the experiments.

Insecticides

Technical grade butene-fipronil, abamectin, acephate, chlorpyrifos, methomyl, α-cypermethrin, imidacloprid and spinosad were selected (Table 1). These chemicals were kept in a refrigerator between the experimental sessions.

Table 1 Detailed information of insecticides used in this study
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Bioassays

For L. decemlineata, a topical application was used to assess the contact toxicity in the fourth instar larvae and the adults. Insecticides were dissolved individually in analytical-grade acetone, and five concentrations within a mortality range of 0–100% based on preliminary assays were used. Larvae were treated individually with 0.22 μL of insecticide solution, which was applied to the dorsal abdominal segment using a 10-μL microsyringe connected to a microapplicator (Hamilton Company, Reno, NV). Similarly, adults were treated individually with 1.1 μL of insecticide solution, which was applied to the ventral area of the abdomen using a 50-μL microsyringe. Each control larva or adult received 0.22 or 1.1 μL of acetone. Three replications of 10 individuals per concentration were performed. After treatment, the test insects were placed in Petri dishes (9 cm in diameter and 1.5 cm in height) containing fresh potato foliage and kept under environmental conditions outlined for beetle rearing (Shi et al. 2012).

For D. melanogaster, a method described previously (Wang et al. 2013) was used to determine insecticide stomach toxicity to the adults. Insecticides were dissolved in and serially diluted two fold with acetone to obtain several solutions. An aliquot of 200 μL of test solution (treatment) was mixed with 10 g of hot conventional artificial diet to prepare artificial diets demonstrating five different concentrations within a mortality range of approximately 0–100% based on preliminary assays. Acetone was used as negative control. Thirty newly emerged flies were housed in a rearing vial (3 cm in diameter and 12 cm in height) containing 10 g of standard cornmeal-sucrose-yeast agar diet for a period of 4 days. And then, 90 adults in three replicates were transferred into 3 vials (3 cm in diameter and 12 cm in height) containing the same quantity of one of the toxic diets or acetone control diet prepared above. The adults were kept under environmental conditions outlined for the fly rearing.

To compare the speed of toxic action of the eight insecticides, the median lethal time (LT50) of these insecticides at a dose (concentration) equivalent to LD80 values were calculated.

Data analysis

The mortalities were assessed daily after treatment. Control mortality was typically less than 10%. Abbott’s formula was used to correct the data for control mortality (Abbott 1925). Probit analysis was used to calculate the concentrations needed to cause 50% mortality, their 95% fiducial limits and the slope of the line relating probit mortality to the log dose by POLO Plus logit probit software (LeOra Software Company, Petaluma, CA, USA). For median lethal time (LT50) values, we gave the data as means ± SD, and were analyzed by ANOVA followed by the Tukey-Kramer test, using SPSS for Windows (SPSS, Chicago, IL, USA).

Results

Toxicity of eight insecticides to L. decemlineata and D. melanogaster

Ninety six hours after insecticide treatment to L. decemlineata by the topical application, the LD50 values for the eight selected insecticides in the fourth instars ranged from 0.002(0.002-0.003) to 0.691(0.371–1.014) μg/individual. According to the relative toxicity index (RTI, determined by comparing butene-fipronil LD50 value with the LD50 value of other insecticide), the eight insecticides could be classified into three groups with relatively least (RTI > 10.0), medium (5.0 < RTI < 10.0) and high (RTI < 5.0) toxicity to the larvae. Acephate, methomyl, and endosulfan fell into the first category; α-cypermethrin and spinosad belonged to the second group; whereas butene-fipronil, abamectin, and imidacloprid were placed into the third group (Table 2).

Table 2 Susceptibilities of Leptinotarsa decemlineata to several insecticides 96 h after treatment
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The toxicities of insecticides to the L. decemlineata adults also differ widely among the eight insecticides. The LD50 values ranged from 0.010 (0.008–0.014) to 2.046 (0.766–4.504) μg/individual. Butene-fipronil, abamectin, imidacloprid, and α-cypermethrin were most toxic, with the RTI less than 5, spinosad showed a moderate toxic effect, with the RTI of 7.5. In contrast, acephate, methomyl, and endosulfan were least toxic, with the RTI more than 10 (Table 2).

For the D. melanogaster adults, the LC50 values for the eight insecticides ranged from 0.05(0.04-0.06) to 20.88(18.31–23.21) μg/g. From the mean LC50 values in CS, w 1118 and Oregan strains, the order of toxicities from the most toxic to the least toxic was spinosad, α-cypermethrin, abamectin, butene-fipronil, imidacloprid, endosulfan, methomyl and acephate (Table 3).

Table 3 Susceptibilities of D. melanogaster adults to several insecticides 96 h after treatment
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Comparison of LT50 values

In order to determine the speed of toxic action of the eight insecticides, the LT50 values of butene-fipronil, imidacloprid, abamectin, acephate, endosulfan, methomyl, cypermethrin and spinosad at a dose (concentration) equivalent to LD80 values were compared.

For L. decemlineata, the values for butene-fipronil, imidacloprid, abamectin, acephate, endosulfan, methomyl, cypermethrin and spinosad were calculated to be 39.9, 36.5, 37.5, 20.2, 22.4, 23.8, 16.4 and 23.1 h, respectively. ANOVA analysis showed that butene-fipronil, imidacloprid, abamectin had similar LT50 values, whereas acephate, endosulfan, methomyl, spinosad and cypermethrin had comparable LT50 values (Figs. 1 and 2).

Fig. 1
figure 1

Median lethal time (LT50) value (hour) of butene-fipronil, compared with seven insecticides in L. decemlineata adults. A topical application was used to assess the contact toxicity in the adults. Adults were treated individually with 1.1 μL of insecticide solution. The mortalities were assessed at an interval of 8 h. The value indicate LT50 (±SD). Different letters indicate significant difference at P value <0.05

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Fig. 2
figure 2

Median lethal time (LT50) value (hour) of butene-fipronil, compared with seven insecticides in the D. melanogaster adults. A diet exposure bioassay was used to determine stomach toxicities of insecticides. See the legend in Fig. 1 for further explanation

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For D. melanogaster, the LT50 values for butene-fipronil, imidacloprid, abamectin, acephate, endosulfan, methomyl, cypermethrin and spinosad were respectively 29.8, 31.5, 29.4, 14.0, 20.3, 18.1, 13.5, and 20.1 h. ANOVA analysis revealed that the insecticides could be classified into three groups. Butene-fipronil, imidacloprid, abamectin showed the largest LT50 values, followed by acephate, endosulfan, methomyl, and spinosad, whereas cypermethrin exhibited the shortest LT50 value (Figs. 1 and 2).

Thus, butene-fipronil belongs to slow-acting insecticides.

Discussion

Insects are major biotic factors that cause losses in agricultural production. Continuous development of new insecticides (Hainzl and Casida 1996) with selective properties and specific action (Brévault et al. 2013) is necessary for their control. Butene-fipronil is a novel compound that has a relatively low toxicity to fish (Niu et al. 2007a). In the present paper, the toxicity of butene-fipronil to the coleopteran L. decemlineata and the dipteran D. melanogaster was determined, along with seven other insecticides. Among the eight insecticides, butene-fipronil belongs to the phenylopyrazoles and endosulfan is a polychlorocycloalkane. Both of them block insect GABA-gated chloride channels (Zhao and Salgado 2010). Abamectin is 16-membered macrocyclic lactones produced during fermentation by the soil microorganism, Streptomyces avermitilis (Gouamene-Lamine et al. 2003). The mechanisms of action for abamectin are also involved in GABA- and glutamate-gated chloride channel (Kass et al. 1980; Wolstenholme and Rogers 2005; Crump and Omura 2011; Moreno et al. 2010; Fritz et al. 1979). Neonicotinoid imidacloprid is a selective nicotinic acetylcholine receptor (nAchR) agonist (Casida and Durkin 2013). Spinosad is a macrocyclic lactone derived from the fungus Saccharopolyspora spinosa (Mertz and Yao 1990). It is an allosteric activator of insect nAChRs (Thompson et al. 2000; Kirst 2010). Acephate is an organophosphate and methomyl is a methylcarbamate. The toxicity of acephate and methomyl is attributable to inhibition of acetylcholinesterase (AChE) (Casida and Durkin 2013). Cypermethrin is a pyrethroid. It acts on axonal neurotransmission at insect voltage-gated sodium channel to block sodium transport (Casida and Durkin 2013).

Our results showed that the contact and stomach toxicities of butene-fipronil were among the highest ever estimated to L. decemlineata and D. melanogaster. The LD50 values of butene-fipronil for contact toxicity were 0.002 and 0.017 μg/individual to the L. decemlineata fourth instars and adults respectively. Comparably, the average LD50 values for fipronil in the L. decemlineata fourth instars and the adults were 0.012 and 0.023 μg/individual (Shi et al. 2012). Consistent with our results, butene-fipronil has been proven to exhibit high toxic to insect species in Lepidoptera, Hemiptera, Coleoptera and Orthopteran (Niu et al. 2007a, c; Liu et al. 2009; Yuan et al. 2009; Li et al. 2009). Thus, butene-fipronil is a powerful insecticide for the control of L. decemlineata.

In order to test the speed of toxic action of butene-fipronil, we selected five fast-acting compounds acephate, endosulfan, methomyl, α-cypermethrin and spinosad, and two slow-acting chemicals abamectin and imidacloprid (Hannig et al. 2009; Franc and Bouhsira 2009). We evaluated the speed of toxic action based on the most common criterion ‘time from exposure to mortality’ (Hannig et al. 2009). Our results revealed that butene-fipronil belongs to slow-acting insecticides.

It is well known that L. decemlineata consumes a small amount of foliage before fourth-instar. However, approximately 40 cm2 of potato leaves are consumed mainly by fourth instars. Moreover, close to 10 cm2 of foliage per day are consumed during the adult stage (Ferro et al. 1985). In the summer, the L. decemlineata larvae progressed through four distinct instars, with approximate periods of the first-, second-, third-, and fourth-instar stages of 2, 2, 2 and 4 days, respectively. According to our results, butene-fipronil should be used at the neonate or the second-instar larval stage. The slow-acting butene-fipronil then kills the larvae before the fourth-instar larval stage, to effectively protect potato foliage from L. decemlineata.