Toxicological and residual effect of Deltamethrin and Chlorpyrifos against the German cockroach , Blattella germanica ( Linnaeus ) ( Insecta : Blattodea : Blattellidae )

The toxicological and residual properties of two insecticidal agents, Deltamethrin and Chlorpyrifos were evaluated against the German cockroach, Blatella germanica in laboratory bioassay. The insecticides were diluted in both aqueous and oil-based solvents and tested against the roaches in pre-determined concentrations and untreated control. Experimental cages were either completely sealed after exposure or perforated to simulate fumigation and disinfestation regimes, respectively. Mortality data generated from acute toxicity studies revealed that oil-based Deltamethrin (5%v/v) was more effective (100%) than aqueous solution (53.3%) within similar durations in both chambers. Chlorpyrifos (5%v/v) revealed an acute mortality of 100% for both oil-based and aqueous solutions in both chambers. Residual effect of both Deltamethrin and Chlorpyrifos was dose/time-dependent, with oil-based solution more effective than the aqueous solution. Computed lethal time revealed that LT50 showed significant difference (P<0.05) between aqueous solution of both insecticidal agents for fumigation treatment. A similar trend was observed for the oil-based solutions of both insecticides in the disinfestations treatment. The implication of this finding in terms of choice of insecticides for acute toxicity and residual efficacy, impact of diluting agents, and sustainable approach to roach control in Nigeria was discussed.


INTRODUCTION
The German cockroach (Blatella germanica, Linnaeus) is one of the most common species worldwide and prominent household cockroaches in the world, and can be found throughout many human settlements (Jacobs, 2007).It is particularly associated with restaurants, food processing facilities, hotels and nursing homes.It is however less prominent in temperate environments, probably due to its volume/surface area ratio which is a major hindrance to cold tolerance (Rust et al., 1995).
B. germanica is the most commonly encountered of the household pests species in Nigeria.It is also the most persistent and difficult to control (Fasulo, 2002).This is due to its larger egg ratio per capsule than other species that infest households and structures.Secondly, it has the shortest developmental period from hatching to sexual maturity; thus, populations of German cock-roaches usually build up faster than other species.Thirdly, German cockroach nymphs have relatively enhanced chance of survival than other species because the female carries the egg capsule during the entire embryonic development of the nymph.These result in the nymphs avoiding many potential environmental hazards as compared to detached and/or isolated eggs.Thus, more nymphs are likely to hatch, with resultant higher reproductive potential.Fourthly, German cockroach nymphs are smaller than most other cockroaches; thus, they are able to conceal themselves in many places which are inaccessible to those of other species.In fact, in a commercial kitchen, there may be potentially thousands of cracks and crevices young cockroaches can hide in and remain protected (Fasulo, 2002).B. germanica has aggregation pheromones associated with their droppings, which have the effect of increasing the level of aggregation or clumping of individuals in the population (Engelmann, 1970;Roth, 1970).These biological factors, combined with their highly adaptive feeding habits and other behaviors, give the German cockroach advantages toward increased chances for survival and consistently maintaining high populations.
The control of B. germanica using conventional insecticidal agents remains a challenge in many parts of the world, due to increased spate of resistance development.B. germanica resistance to insecticides was first detected in chlordane use in Corpus Christi, Texas, USA in 1952 (Heal et al., 1953).Thereafter, an increasing number of cases have been documented in the USA (Rust andReierson, 1991, Zhai andRobinson, 1991), Canada (Batth, 1977), Europe (Chapman et al., 1993), and Japan (Umeda et al., 1988).Currently, resistance to all the major groups of insecticides (organochlorines, organophosphates, carbamates and pyrethroids) by B. germanica has been reported (Cochran, 1995).Increased tolerance and potential resistance to other novel insecticides, such as sulfluramid (Schal, 1992) and abamectin (Cochran, 1994), along with behavioral changes in response to glucose attractant (glucose-aversion) in cockroach bait (Silverman and Ross, 1994) have been reported recently.
Few studies have been carried out on the control of B. germanica using different types and/or formulations of insecticides in Nigeria.This study therefore seeks to investigate the comparative insecticidal efficacies of Deltamethrin and Chlorpyrifos against B. germanica in laboratory bioassay, identify the impact of different diluents on the effectiveness of the two insecticides.Moreover, it seeks to evaluate the residual activities of the two insecticides, both as fumigation and disinfestation agents, against B. germanica

Site of the experiment
Culture of the German cockroach, B. germanica was maintained in Kemabonta et al. 91 the laboratory at the Zoological Gardens, Department of Zoology, University of Lagos, where all bioassays were also carried out.

Insecticides used
Insecticides used, Deltamethrin (2.5 and 5% v/v) and Chlorpyrifos (2.5 and 5% v/v) were purchased from registered agrochemical retail stores around Lagos Island axis of Lagos in Lagos State Nigeria.The choice of diluents was water and diesel, respectively.

Insect culture
Adults and nymphs of the German cockroach were collected from students' cupboards in Fagunwa Hall, a female residential Hostel within the University Campus, as well from the insectary of the Department of Zoology, University of Lagos to set up a mass culture of B. germanica.The insects were kept in plastic vials which were smeared with petroleum jelly to prevent the cockroach from moving out of the vials.The vials were also covered with a muslin cloth and striped with rubber band to keep the insects in place.
While the culture lasted, the roaches were fed thrice a week with crumbs of bread and/or biscuits, with water also placed in the container.The containers were regularly cleaned and insect frass and faeces removed.The petroleum jelly at the edge of the vial was also renewed to prevent escape of the roaches upon opening the vial.Cleaning was done using wet foam rubbed sparingly round the containers and the dirt's packed out with a piece of serviette paper.

Experimental procedure
The experiment was carried out using the two aforementioned insecticides, two different concentrations and each treatment was replicated three times.Moreover, two different containers were used-perforated at the top (disinfestation procedure I o ) while the other was fumigation process (I c )-where the fumigation containers were not perforated.Experimental cages were either completely sealed after exposure or perforated to simulate fumigation and disinfestation regimes, respectively.The procedure was in accordance with that of Shahi et al. (2008) with slight modifications.

Toxicity test
All life stages of B. germanica (except the first, second nymphal stages, and gravid females) were used for the toxicity tests.Deltamethrin and Chlorpyrifos insecticide formulation were used in accordance with the instruction on the labels.Four concentrations 2.5 and 5% (v/v) of water; 2.5 and 5% (v/v) of diesel were used.Each formulation was impregnated on filter paper and air-dried.Excess liquid was drained off, and thereafter, ten (10) randomly picked B. germanica were confined into each jar.
The upper surface of the jar was lightly greased with petroleum jelly to prevent escape of the insects (redundancy).There were two controls-jars with only water or only diesel sprayed on the filter paper.Each treatment was replicated three times.Mortality of B. germanica was observed after 5, 10, 15, 20, 30 min to the 4 th h post treatment and the number of dead B. germanica was counted and recorded.The jars containing treated and untreated filter papers were kept aside on laboratory bench and used for the residual experiments.
At one week after treatment, German roaches were re-introduced into the various vials containing the treated filer papers including the control to check the residual effect of these insecticides on the roaches.The same procedure was used as in the first experiments (for four hours), after which the experiment was stopped and cock-

Quantal response
Mortality readings were taken for cockroaches when no part of the body or limb movement was observed upon pricking with a camel hair brush and waiting for ten minutes for any movement to occur.
Responses of cockroaches to the various experimental set up were noted and written down and the parameters used in taking records included the knock down time and rate of mortality per minutes/hours.Four hours was used, because that is the maximum time used for insect bioassay or toxicity test as long as it is in contact with the insect.

Statistical analysis
Statistical analysis of the results was also done after the results have been ascertained and its corresponding mean and standard deviation were recorded against time.This was done using the equations: Mean, =

Standard deviation=
The lethal time value (LT 50 ) and the regression slope for each treatment were obtained using probit analysis (SPSS 2000).Mean percentage of insect mortality value was subjected to arcsine transformation.Means were compared using LSD test (SPSS 2000).

Mortality in B. germanica exposed to diesel and water
Insects exposed to diesel started dying at 20 and 30 min after treatment in disinfestation (Io) and fumigation (Ic) chambers, respectively.The highest mortality of 43.3% was recorded in fumigation chambers at 4 h after treatment (I C ).No mortality was recorded on insects exposed to water treatment (Table 1).

Acute and residual effect of aqueous and diesel diluted Deltamethrin and Chlorpyrifos exposed to B. germanica
In aqueous Deltamethrin, mortality of B. germanica was recorded after 30 and 45 min for the and 5% (v/v) treatments, respectively.The highest mortality recorded (53.3%) was in 2.5% (v/v) (I O -Dis-infestation) (Table 2).
On the other hand, 100% mortality was recorded after 45 min in all the replicates and treatments with diesel diluted Deltamethrin.
At one week after treatment, mortality was recorded after 30 min (Table 3) with 5% (I C -fumigation) and 2.5% (v/v) (I C -disinfestation) having the highest (30%) and least mortalities (13.3%), respectively.On Diesel diluted Deltamethrin, mortality was recorded at 15 minutes and highest mortality (40%) was recorded at 4 h after introducing the insects in the fumigation and disinfestation chambers.
No mortality was recorded for B. germanica on water diluted Deltamethrin at 2 weeks after treatment until after 4 h with 5% (v/v) (I C -fumigation) having the highest mortality (10%).On the other hand, diesel diluted Deltamethrin had less than 20% mortality at 2 h after introducing the insects.5% (v/v) (I C -fumigation) had the highest mortality of 16.7%, respectively (Table 4).

Acute and residual effect of aqueous and dieseldiluted Chlorpyrifos on B. germanica
Mortality of B. germanica exposed to Chlorpyrifos was dose-dependent.Chlorpyrifos (5% (v/v) mixed with diesel applied to B. germanica in the fumigation and disinfestation chambers gave 100% mortality at 30 min after treatment while 100% mortality was recorded at 2 h (lc ) and 3 h (lo) at same concentration with aqueous Chlorpyrifos (Table 5).
No death was recorded in all replicates at one week after treatment until 30 min of exposure, while 5% (v/v) (I C -fumigation) had the highest mortality (33.3%) after 4 h (Table 6).
On the other hand, there was no mortality count for B. germanica introduced two weeks post-treatment, until after 4 and 3 h exposure to aqueous and oil-based Chlorpyrifos, respectively.Higher concentration of 5% v/v, recorded 23% mortality, after 4 h exposure in the fumigation chambers (Table 7).

DISCUSSION
The study evaluated the effectiveness of two insecticides Chlorpyrifos (an organophosphate) and Deltamethrin (a Pyrethroid) on the control of the German cockroach, B. germanica.Only aqueous and oil-based Chlorpyrifos and Deltamethrin (oil based) gave 100% mortality of B. germanica within the four hours of exposure period.However, the efficacy of both diluents decreased with increase in residual time.
The experiment shows that Chlorpyrifos was more effective than Deltamethrin at the concentration used.The LT 50 of (5% (v/v)) aqueous Chlorpyrifos for both disinfestation (13 min) and fumigation (5 min) trials against B. germanica were found to be lower when compared with those exposed to aqueous Deltamethrin (358 and 195 min, respectively), of same concentrations and conditions.Moreover, mortality responses (5% (v/v)) of aqueous Chlorpyrifos for both disinfestations and fumigation trials against B. germanica were found to be higher than those of insects exposed to aqueous Deltamethrin.A similar trend was observed in the dieseldiluted insecticidal exposure at both concentrations for fumigation and disinfestation against B. germanica.
Diesel-diluted insecticidal treatments recorded significantly higher mortality responses than those of aqueous treatments for all concentrations against B. germanica.This result is in conformity with the findings of Limoee et al. (2001), Robison et al. (1999) and Enayati et al. (2007), who revealed that Cypermethrin was more effective than Deltamethrin (Deltamethrin) in the control of B. germanica.In a research carried out by Shahi et al. (2008) toxicity of cypermethrin, deltamethrin, diazinon, lambda-cyhalothrin and Negon® (permethrin+propoxur) commercial formulations were investigated against adult German cockroaches collected in different areas of Bandar Abbas City, southern Iran.Maximum mortality rates of 20, 35, 90 and 100% were obtained after one hour contact with label-recommended doses of cypermethrin, deltamethrin, lambad-cyhalothrin, diazinon and permethrin+propoxur insecticides, respectively.This result is not in conformity with other researches done on the B. germanica using cypermethrin and deltamethrin, as higher mortality was recorded using Deltamethrin than in Cypermethrin.The findings however, of Shahi et al. (2008) on the effects of both insecticides on B. germanica, as well as those of Spring (2010) and Cakir et al. (2008) on other insect pests revealed that Cypermethrin was more effective than Deltamethrin.This study also revealed that fumigation treatments were more effective than dis-infestation treatments in the control of B. germanica.The results on the residual effect (one and two weeks respectively) of Chlorpyrifos and Deltamethrin were in conformity with the findings of Enayati et al. (2007) and Spring (2010).
Results from this study revealed a higher insecticidal effect of Chlorpyrifos over Deltamethrin (for both fumigation and disinfestation) against B. germanica.Chlorpyrifos had a faster knock down effect and higher percentage mortality than Deltamethrin, and this was observed for both concentrations and diluents used for the study.The importance of this finding implies that Chlorpyrifos (organophosphate) insecticides may be adopted for the control of B. germanica within Lagos metropolis.The diesel-diluted treatments were more effective than aqueous treatment for both insecticides.The residual properties of both insecticides followed a similar trend observed in initial exposure.The mortality response was however relatively low (<40%) for both insecticides at the maximum time lapse of two weeks.This implies that both insecticidal agents will perhaps become ineffective over a 2-week duration, hence the need to reapply these chemicals when necessary.Ultimately, the sustainable control of B. germanica in the long-term will involve various approaches in an integrated manner in order to safeguard the ecosystem for other organisms.

Table 1 .
Mean mortality values for both water (positive control) and diesel (negative control) against B. germanica.

Table 2 .
Mean mortality of both aqueous and diesel-diluted Deltamethrin against B. germanica (initial).

Table 3 .
Residual effect (mortality) of both aqueous and diesel-diluted Deltamethrin against B. germanica (one week).
mortality was recorded in B. germanica when newly introduced to both aqueous and oil-based Chlorpyrifos as well as Deltamethrin in both fumigation and disinfestation chambers.Diesel exposed to B. germanica gave 43 and 36% mortality in fumigation and disinfestation cham-bers, respectively.Residual effect of Deltamethrin and Chlorpyrifos recorded a maximum of 40% after 4 h in one week after treatment in the

Table 4 .
Residual effect (mortality) of both aqueous and diesel-diluted Deltamethrin against B. germanica (two weeks).

Table 5 .
Mean mortality of both aqueous and diesel-diluted Chlorpyrifos against B. germanica (initial).
I O = Disinfestation; I C = fumigation.fumigation chambers.In the fumigation chambers, LT 50 of aqueous Deltamethrin and Chlorpyrifos (5% (v/v)) were 195 and 15 min respectively, while Deltamethrin and Chlorpyrifos mixed with diesel were 9 and 6 min respectively.While under disinfestation chambers,

Table 6 .
Mean residual effect (mortality) of both aqueous and diesel-diluted Chlorpyrifos against B. germanica (one week).

Table 7 .
Mean residual effect (mortality) of both aqueous and diesel-diluted Chlorpyrifos against B. germanica (two weeks).

Table 8 .
Percentage mortality after four hours of exposure.