Integrated pest management (IPM), is defined as the selection, integration and implementation of pest control based on predicted economic, ecological and sociological consequences. Application of insecticidal baits in IPM can be an effective approach for controlling
Fipronil, a phenylpyrazole-type insecticide, kills insects by interacting agonistically withγ-aminobutyric acid-gated chloride channels [5]. Fipronil slowly degrades in soil and water, with a half-life ranging between 36 h and 7.3 mo. The toxicity of fipronil to laboratory mammals by oral exposure is moderate (LD50 = 97 mg/kg for rats, LD50 = 91 mg/kg for mice) [6]. Fipronil is classified as a quick action bait by Stejskal et al. [7].
Imidacloprid is a neonicotinoid in the chloronicotinyl nitroguanidine class of insecticides with a high selective toxicity to insects [8, 9]. Imidacloprid acts on several types of postsynaptic nicotinic acetylcholine receptors in the nervous system [10]. The toxicity of imidacloprid to laboratory mammals is moderate by oral exposure (LD50 = 4000 mg/kg for rats, LD50 = 131 mg/kg for mice). Imidacloprid has low vapor pressure with a half-life of 30 days in water and 27 days anaerobically in soil [9]. Imidacloprid is classified as a rapid action bait by Stejskal et al. [7] and as toxic to all developmental stages of the German cockroach by Appel and Tanley [9].
Hydramethylnon is an amidino hydrazone type pesticide that acts by disrupting energy production (inhibiting the formation of ATP) at the cellular level. Hydramethylnon has low mammalian toxicity with a half-life more than one year (from 375-391 days) in aerobic soil and is stable at high temperature [11]. This bait is classified as a slow action bait by Stejskal et al. [7]. Toxicity of hydramethylnon gel bait against the German cockroach was reported by Appel [12]. Khadri and Lee [13], and Sulaiman et al. [14].
The purpose of this study was to determine the comparative toxicity of the three toxicants on the German cockroach.
All the tested German cockroaches (field strain) were collected from dormitories (in the Yasuj University of Medical Sciences) in Yasuj city, Iran. Cockroaches were collected using jar traps, which were installed overnight. They were one liter glass jars each containing a slice of bread and beer and with inner upper surface of the jar coated with petroleum jelly to prevent escape. The study was conducted in the medical entomology laboratory of the Yasuj University of Medical Science.
Adult males and nongravid females and small nymphs (2–4th instar, 3.5–10 mm in length) and large nymphs (5–6th instar, 10.5–14 mm in length) were used in the toxicity tests. Ten German cockroaches at each developmental stage were placed in separately labeled glass rearing jars of the same size (one liter). The jars were provided with a mouse pellet as food (1g), water (a moistened cotton wick) and a 140 cm2 cardboard cylinder as harborage [9]. Mortality was observed at 12 h intervals for 5 days (until all cockroaches were dead) at the same temperature (25°C-28°C), humidity (%50 ± 5 RH) and photoperiod (12:12 h, light: dark). The upper inside surface of the jars was lightly greased with petroleum jelly to prevent the cockroach from escaping. Additionally, muslin cloth covers were secured with rubber bands over the opening to provide air. There were three replicates (jars) for each study, i.e. (stages of the German cockroaches). Before the test was started, cockroaches were left to habituate to test conditions for one day [15].
Three most popular gel baits that were introduced for the IPM programme were evaluated for their toxicity against the German cockroach. Chemicals used were hydramethylnon gel bait 2% (commercialized as Siege, BASF, Shah Alam, Malaysia), imidacloprid gel bait 2.15 % (Bayer, Leverkusen, Germany), and fipronil gel bait 0.05%, (commercialized as Goliath, Rhone-Poulenc Rhodia, Lyon, France). Before treatment, the German cockroaches were fasted for 24 h to increase their hunger level and thereby optimizing their response during the test [15]. Initially after fasting they exposed to food and then 0.5 g of each bait was placed in a plastic boat (plug) in treatment jars [9, 16, 17]. The two kinds of foods (mouse pellet and bait) were introduced allow the cockroaches to make a choice and more accurately reflect the situation in the “field”. Control treatments contained only water, mouse pellet, and cardboard. For each developmental stage, each replicate comprised 10 cockroaches. There were three replicates (jars) for each stage totalling 120 cockroaches in 12 colony jars for the three gel treatments and an equal number of cockroaches for each control, i.e. hydramethylnon, fipronil, and imidacloprid bait treatment.
Mortality data were analyzed by probit analysis, using SPSS software version 15.0 (SPSS, Chicago, IL, USA) to determine LT50s and LT90s of treatments and subsequent slopes of probit regressions. Normality of data and homogeneity of variances was detected by using exploratory and Levine’s test (SPSS). Data was transformed to obtain a normal distribution and homogeneity of variance. To determine significant differences between means, a Mann-Whitney
All gel baits produced 100% mortality of German cockroaches within 1-5 days. There was no mortality for the controls. The LT50 values for various developmental stages of the German cockroach exposed to imidacloprid, fipronil, and hydramethylnon gel baits ranged from 12.5 to 14.5 h, 31.2 to 35 h and 57 to 64.8 h respectively
Toxicity of the three gel baits to various developmental stages of the German cockroach
Baits | Developmental Stages | n | Slope ± SE | LT50 (95%CI) hours; means of LT50s and LT90s among the three baits were significantly different at |
LT90 (95%CI) hours; means of LT50s and LT90s among the three baits were significantly different at |
---|---|---|---|---|---|
Imidacloprid | Male | 30 | 0.33 ±0.44 | 12.5 (10.3−14.4) | 19.3 (16.3−23.3) |
Female | 30 | 0.12 ±0.02 | 12.7 (9.5−15.3) | 25.2 (21.1−33.1) | |
Large | 30 | 0.14 ±0.02 | 13.7 (11−15.9) | 23.9 (20.3−31.1) | |
Small | 30 | 0.15±0.02 | 14.5 (12.1−16.7) | 24.2 (20.7−31.1) | |
Fipronil | Male | 30 | 0.1±0.01 | 35 (31.9−37.9) | 49.7 (45.3−56.7) |
Female | 30 | 0.1±0.01 | 32.3 (29.2−35.2) | 47.4 (42.9–54.8) | |
Large | 30 | 0.13 ±0.02 | 31.2 (28.4−33.8) | 43 (39.1−49.8) | |
Small | 30 | 0.11 ±0.02 | 31.4 (28.6–34.2) | 44.4 (40.3−51.4) | |
Hydramethylnon | Male | 30 | 0.17 ±0.06 | 63.9 (60.6–67.1) | 79.6 (75.2−86.4) |
Female | 30 | 0.07 ±0.01 | 64.8 (60.9–68.5) | 89 (83.2−97.4) | |
Large | 30 | 0.05 ±0.01 | 57 (52.5−61.3) | 91.4 (83.8−102) | |
Small | 30 | 0.06±0.01 | 60.8 (43.3−77.3) | 91.2 (72.6–168.1) |
Our findings are consistent those of Durier and Rivault [18] who reported cockroach age did not affect performance in tests for hydramethylnon and fipronil. However, in the present study the LT50 for large nymph stage of the German cockroach treated with hydramethylnon showed the lowest value (
Delayed action of the hydramethylnon gel bait (no mortality) was observed within 24 to 48 h. These results confirmed the different modes of action for these three baits as was substantiated and nominated as rapid action bait (such as imidacloprid), quick action bait (such as fipronil), and slow action bait (such as hydramethylnon) by Stejskal et al. [7]. This finding is also consistent with those reported by Lee [19] and Durier and Rivault [18] who reported a faster effect of fipronil than hydramethylnon gel bait against the German cockroaches. A similar result was reported by Sulaiman et al. [14] for the American cockroach.