Mosquitoes can be controlled by adopting the following general methods: personal protection by covering all exposed parts of the body; elimination of breeding sites; taking preventive medicine or by use of chemicals to destroy the larvae or adults of the mosquitoes (Kotpal, 2010).

Insecticides are commonly used in controlling the mosquito vectors. The insecticides can be applied to nets- Insecticide Treated Nets (ITNs). Most of the nets currently in use are Long Lasting Insecticidal Nets (LLINs) or by spraying the insecticides on the walls or roof of houses and shelters of domestic animals in a given area in order to kill the adult mosquitoes that land and rest on such surfaces. The latter method is called Indoor Residual Spraying (IRS) (Kelly-hope et al., 2008).

There are four classes of insecticides used in IRS: Organochlorines- these are insecticides that contain carbon, hydrogen and chlorine. They are now primarily of historic interest, since few survive in today’s arsenal. E.g., DDT, taxophene, strobane. Organophosphates- contain phosphorous and work by inhibiting certain enzymes- cholinesterase (chE). E.g., temephos, bidrin, disulfoton, malathion. Carbamates- are derivatives of carbamic acid, their mode of action is also by inhibiting chE. E.g., Bendiocarb, adoxycarb, methiocarb, etc. Pyrethroids- they apparently work by keeping open the sodium channels in neuronal membranes. E.g., tetrametrin, bifenthrin, alphacypermeethrin (George et al., 2004).

Dichlorodiphenyltrichloroethane (DDT), malathion, fenitrothion, pirimiphos-methyl, Bendiocarb, bifenthrin, propoxur, Alphacypermethrin, cyfluthrin, Deltamethrin, etenfrox and lambda-cyhalothrin are the insecticide compounds and formulations recommended by WHO for IRS against malaria vectors. (WHO, 2009).

Pyrethroid insecticide resistance in the mosquito vectors is mainly associated with reduced target site sensitivity arising from a single point mutation in the sodium channel gene, often referred to as knockdown resistance (kdr). This resistance mechanism is widespread in West Africa and was reported for the first time in Nigeria in 2002 as cited by Awolola et al., (2007). However, information on resistance status of these vectors to the approved insecticides by WHO is lacking in Gombe State.

The selection of insecticides for the tests is line with the fact that they are part of the insecticides approved by WHO currently in use in Gombe State both as agrochemicals (pesticides) and domestic insecticides.

There have been reports of resistant strains of mosquitoes to different insecticides in Nigeria, hence the need to test those occurring in Gombe state, Nigeria. Resistance status of Culicine to Alphacypermethrin, Bifenthrin and Bendiocarb in Gombe state has not been ascertained or determined.

The study was limited to the collection of culicines mosquito larvae in three different locations in Gombe metropolis: Gombe state University, Dawaki quarters and new market area. The aim of the study was to determine the susceptibility status of culicine mosquitoes to Alphacypermethrin, Bifenthrin and Bendiocarb.

2 Study Area

Gombe State, is located in the northeastern part of Nigeria with its capital as Gombe city. It is located between latitude 9°30' and 12°30'N and longitudes 8°45' and 11°45'E of the Greenwich Meridian. It has a total land area of about 20,265 km². It is bounded by Borno and Yobe to the north and east, Taraba and Adamawa to the south, and Bauchi to the west. The projected population from the 1996 National Census is estimated at 3,225,382 at annual growth rate of 3.2 percent (NPC, 2014). Gombe has two distinct climates, the dry season (November–March) and the rainy season (April–October) with an average rainfall of 850 mm. It is an agrarian state with about 60 percent of the population engaged in agriculture. Year round cultivation is possible in some parts of the state due to favourable weather and an extensive irrigation programme. A number of food and cash crops produced in the state include: Cereals: maize, millet, sorghum, rice and wheat; Legumes: Cowpeas, groundnuts, soya beans and bambara nuts. The vegetation of Gombe metropolis can be described as Sudan savannah with open grassland. Annon (2003).

3 Preliminary Investigation

A preliminary investigation was carried out within the study area to identify the breeding sites of culicines. This gave the information on areas in which the larvae of culicines mosquitoes were found.

4 Larval Collection Method

Larvae were collected from the identified breeding sites, using a ladle which was lowered into the water (breeding site) at an angle of about 45°C until one side is just below the surface of the water. While dipping, care was taken not to disturb the larvae which may cause them to swim downward. About 9-10 scoops were collected depending on the concentration of the larvae in the water in order to get enough quantity for rearing. The larvae collected were brought to the insect laboratory where they were reared to adults (WHO, 2008).

5 Rearing of the Larvae

All collected larvae were put into a hatching tray/larval bowl covered with a net and fed with a mixture of yeast and biscuits (10% yeast and 90% low fat biscuits). The adults emerge were transferred into the cage with an aspirator and fed with 10% glucose solution (WHO, 2008).

6 Susceptibility Test

Female non-blood feds mosquitoes from the cage were used exclusively as recommended by WHO (2008). The test was done inside the insect laboratory, each with a control. Both the test and the control mosquitoes were from the same source and belong to the same species.

In each of the holding tube, a clean white paper rolled into cylindrical shape was inserted. It was then fastened in position with a spring wire clip (silver for control, copper for test). Twenty (20) to 25 mosquitoes were introduced into the holding tubes using the aspirator provided. Mosquitoes were collected in lots of 10 and gently transferred to the holding tubes through the filling holes in each side. A pre-test holding period of one hour was necessary to guard against damage specimens, which were removed at the end of one hour period.

Impregnated paper was then introduced into the exposure tubes. Mosquitoes were then transferred into exposure tube by attaching it to the vacant screw top in the slide. The slide was pulled out to a point beyond the filling hole so that no part of it should block the tube opening; the mosquitoes were then blown gently down into the exposure tube. The slide was then closed. The holding tube was then detached and kept aside. The exposure tube was then left standing upright for the period of one hour; under moderate temperature and humidity. Knockdown mosquitoes were recorded at intervals of 5, 10, 20, 30, 40, 50, 60 minutes.

At the end of the required exposure period of one hour, both knockdown and live mosquitoes were transferred back to the holding tubes by reversing the procedure. The holding tube was set so that it stands on the side. A pad of moist cotton-wool containing sugar solution was placed on the screen. This was kept for 24 hours in a secluded place at room temperature. Final mortality counts were made 24 hour post exposure period. The result of dead and live one was recorded on the forms provided. Four replicate tests were done with each of the insecticides with control in each case.

The tests were carried out under standard laboratory condition for susceptibility test of insects at 25°C ± 2°C and 80% ± 10% relative humidity. 

7 Results

A total 1,500 female culicines mosquitoes (365 for each chemical and 135 for each control) were exposed to three different insecticide impregnated papers with Alphacypermethrin, bifenthrin and Bendiocarb with control in each case.

On exposure to the insecticides; mosquitoes from Gombe State University showed 96%, 63%, and 66% susceptibility to Alphacypermethrin, Bifenthrin and Bendiocarb respectively (Figure 1). The mosquito samples from Dawaki quarters indicated 92%, 62% and 69% susceptibility to Alphacypermethrin, Bifenthrin, and Bendiocarb respectively (Figure 2); whereas culicines mosquitoes from New market Area showed susceptibility of 90% to Alphacypermethrin, 44% and 69% to Bifenthrin and Bendiocarb respectively (Figure 3). No full susceptibility to any of the chemicals was observed; however, percentage mortality to Alphacypermethrin in all the three sites suggests the possibility of resistance that needs to be confirmed but results for Bifenthrin and Bendiocarb indicates resistance in all the study sites.

Applications of corrections to the mortality was not necessary as all the Control mosquitoes recorded below 5% mortality.