Control of insecticide-resistant German cockroaches, Blattella germanica (L.) (Dictyoptera: Blattellidae) in food-outlets with hydramethylnon-based bait stations

Ficld and laboratory performance of a commercially available hydramcthylnon-based bait station (Combat ') wcrc conductcd against insccticidc-resistant German cockroach, Blattella get.tiin~zicn (Linn.) populations in fivc coffee-shops in Pcnang, Malaysia. Bioassay screening on FI generation of the field collcctcd cockroachcs using a modified W.H.O. glass jar method dctcctcd moderate to high lcvcls of propoxur, chlorpyrifos and cypcrmcthrin rcsistancc. Synergism studies suggested possible involvcmcnt o f monooxygcnasc and cstcrasc in the rcsistancc in thcsc strains. Laboratory assays of the bait stations against all fivc strains showcd similar rcsults when compared to that of the laboratory susccptiblc strain. Ficld efficacy study was donc by placing 15 bait stations in the kitchen o f cacli prcmisc whcrc hcavy infcstation of Gcr~nan cockroachcs had previously bccn found. At 4-wcck post-trcatmcnt, a rcduction o f >90% was achicvcd in all prcmiscs. Suppression of thc cockroach population rcmaincd high up to 12 wccks with >80% rcduction in all populations. lnsecticide resistance in the German cockroacli, Blattella germanica (L.) is a prevalent problem to the pest control industry world-wide (Lee et al., 1996; 1997a). In Malaysia, tliis species is tlie most important insect pests in hotels and food-preparative industries (Lee et al., 1993). Current control strategies which rely heavily on tlie use of neurotoxic insecticides in residual foi-inulations liave been found to be ineffective against multiple resistant German cockroaclies (Cocliran, 1995). Many cases of control failures by residual insecticides due to insecticide resistance liave been reported earlier (Atkinson et a l . , 1991; Zliai & Robinson, 1991; Lee et al., 1996). A recent review on tliis subject had been reported by Lee (1997). With tlie increase in the number of German cockroach resistance to insecticides lately, there is a serious need to address the problem witli the use of clieinicals witli novel inode of actions. Baiting has become a popular mode for control of German cockroach nowadays (Rust, 1986; Milio et al., 1986; Appel, 1990; Appel, 1992; Appel & Benson, 1995) and no longer play a compliinentary role to residual sprays (Ogg & Gold, 1993). In addition, the use of baits in the form of containerized station, or gel can also lessened the l ikelihood of pesticide misapplication (Robinson, 1988). Use of toxicant baits against insecticideresistant German cockroaches have been found effective in several laboratory studies. Cocliran (1990) reported good efficacy of abamectin-based baits against pyrethroidresistant German cockroaclies. Koehler & Patterson (1991) demonstrated the performance of hydramethylnon baits against insecticide resistant German cockroach, but cautioned that low level of hydramethylnon resistance may evolve in the field populations. This present study reports tlie field and laboratory performance of a coininercially available hydrametliylnon-based bait station (Combat'"'), against f ive populations of insecticide-resistant German cockroaclles in food outlets in Malaysia. MATERIALS AND METHODS Test sites All test sites used in this study were Chinese coffee shops located in George Town, Penang, Malaysia, namely HH, CS, BA, AK and WT (names of the shops are withheld as it has been mutually agreed earlier that only initials will be used). The treatment area for the test sites ranged from 12.4 19.2 m2. All shops received pest control services on a quarter yearly basis. The insecticides used in the operation were unknown. During this study, all premise owners had been advised to stop all their usual pest control services on the quarter yearly basis. Bait stations A hundred hydrametl~ylnon-based (active ingredient: 1.65% l~ydra~net l iy lnon) bait stations (Combat") were purchased directly from three local supermarkets. A total of 15 bait stations were then randomly chosen and assigned to each premise. Collection of cockroaches Prior to baiting, pre-treatment sampling was conducted in all test sites using ten glass jar traps, left for 48 hours in the food-preparation area. Each trap was baited with a piece of white bread soaked earlier in local beer. During the collection of traps, all cockroaches were counted. The cockroaches were then brought back to the laboratory, reared for 1 2 generations and the F1 F2 generations were tested for their susceptibility to the baits and other com~nonly used insecticides. Cockroaches caught during pre-treatment were not released back into the population will not affect results of subsequent trappings. This is because German cockroaches have a very short life cycle (about 40 days) and this would allow ample time for population recovery. Laboratory bait evaluation For laboratory bait efficacy evaluation, box arenas each measuring 35 x 30 x 10 cm were used. Bait station and food were each provided at adjacent corners of the box, opposite to where harborage (folded paper) was placed. Water was placed close to the harborage. A total number of 20 adult males (aged 7 14 days) were used in this study. A susceptible strain (ICI) was used for comparison. Mortality of the cockroaches was assessed daily up to 14 days. Control experiments were done by introducing cockroaches into test arenas with similar condition as above except without bait station. No mortality was recorded in the control during the test period. Each experiment was replicated three times and conducted for all strains. Data were pooled and subjected to probit analysis according to procedure described by Robertson & Preisler (1992) using a computer software, POLO-PC (LeOra Software, 1997). Baseline insecticide susceptibility and synergism tests Baseline insecticide susceptibility of the cockroacl~es to commonly used insecticides (propoxur, clilorpyrifos and cypermethrin) was determined using W.H.O. glass jar method, as described in Lee et al. (1997a). Ten adult males (aged 2 10 days) were introduced into 0.45 1 glass jars, each treated earlier with an insecticide (technical grade diluted in acetone) at 20 pg/cm2 and their knockdown activities were recorded at selected time interval. A minimurn of three replicates was done for all strains. Synergism test was conducted by applying topically 100 pg/p1 of piperonyl butoxide, or 30 pg lp l of DEF" (S ,S ,Stributylphosphorotrithioate) on the first abdominal segment of adult males, about two hours prior to W.H.O. glass jar tests. A susceptible strain (ICI) was used for comparison. Control for insecticide susceptibility tests were performed by exposing the cockroaches into glass jars treated earlier with acetone. Control for synergism test was performed by treating the cockroaches with synergists and then exposed them into acetone-treated glass jars. No mortality or knockdown was recorded for both insecticide susceptibility and synergism controls during the test period. Data were pooled and subjected to probit analysis according to similar procedure described above. Resistance ratios were calculated by dividing the LT,, values of the resistant strain witli the corresponding lethal time of the susceptible strain. Field bait evaluation Baiting was done by placing 15 bait stations in the footl-preparation area. Baits were placed in locations such as cabinets, under tlie sink, preparation table, behind tlie boiler, behind tlie refrigerator. etc. Post-treatment trapping were done uslng similar procedure to pre-treatment sampling at 1 week, 2 weeks, 4 weeks, 8 weeks and 12 weeks post-baiting. No control house was done in this study due to ethical reason and inability to obtain consent from premise owner. The cockroaches trapped were counted and percentage popi~la t ion reduction at each sampling time interval was determined by co~iiparing the number of cockroacli trapped at eacli sampling date with the pre-treatment results. All cockroaches trapped at posttreatment were released back into the population after counting. Mean percentage reduction in cockroacli count per trap between different premises was compared with Kruskal-Wallis ( K W ) one-way analysis of variance and separated witli KW multiple range test (KWMRT) (Siegel & Castellan, 1988). RESULTS AND DISCUSSION Bioassays of the five strains using glass jar method re\/ealed low to high resistance to ~ x o p o x u r (1 .7 9 . 8 X ) . low to moderate resistance to chlorpyrifos ( I . I -4.3X) and low resistance to cypermethrin ( I .2 1.7X) (Table 1). The WT strain demonstrated the highest resistance lcvel to both propoxur and chlorpyrifos. Synergism studies using PBO and DEF si~ggested the possible involvement of' both monooxygenase and elevated esterase in the resistance in all strains tested. Earlier, Lee et t r l . (1996) had reported moderate to high levels of propoxur resistance and low levels of chlorpyrifos resistance in twelve strains of field collected German cockroaches from hotels and restaurants in Kuala Lumpur and Penang, Malaysia. They also reported a similar involvement of both resistance mechanisms in the resistance in these strains. Tliis may indicate that these two meclianisms of resistance are wide-spread among various field populations of German cockroaches in the country. High levels of elevated esterase also have been reported in other Malaysian strains of the German cockroach (Lee et ul., 1997a; Lee et ( I / . , 1997b) Laboratory assays of bait stations against all strains showed comparable LT,,, values when compared to that of the [CI susceptible strains (Table 2). This indicated that all field strains tested were susceptible to hydramethylnon-based baits. Tliis results indicated that hydramethylnon-based bait stations can perform well against carbamateand organopliospliate-resistant German cockroaches. Hydrametliylnon is an inhibitolof mitochondrial respiration (Hollingsliaus, 1987). TIILIS, due to different mode of action when cornpared to carbamate and organophosphate insecticides. this may explain why good efficacy was obtained ivlien tested against these resistant insects. Initial pretreatment trap counts ranged from 35 233 cockroaches per trap per night. The bait stations demonstrated excellent performance against all field strains. At tlie first week post-treatment, the bait performance differed significantly among each other with BA having the highest reduction rate (BA > I-IH = CS = AK > WT; P < 0.05, KWMRT). However at 4-week post-treatment, a reduction of >901%r was observed in all premises (Figure 1) and did not differ significantly from each other (P > 0.05). Earlier studies by Milio et (11. (1986) also reported >8S1X, reduction in German cockroach population at 4 weeks post-treatment of using similar bait stations. Appel (1990) however, found a lower reduction rate of about 50%) i n his studies in Tallassee and Opelika. 41ab;lma. USA which concurs witli the studies by Ogg S: Gold Tablc I . lnscct~cidc rcsistancc and synergism of ficld populations of insccticidc-resistant Gcrman cockroaches from food-outlcts to commonly uscd insccticidcs using modificd W.H.O. glass jar mcthod. Insccticidc Alonc + PBO + DEF Strain n LT,, (95 % FL) ~ l o p c RR,,' n LT,, (95 % FL) Slopc RR,,' n LT,, (95 % FL) Slopc RR,,' (min) (min) (min)