A total number of 120 head of households were interviewed at lower Moshi covering Rau Kati (30 households), Mtakuja (31), Chekereni (30) and Mabogini (29) villages to assess the social demographic data (Table ). In these villages, plant species commonly used as mosquito repellents were Ocimum species (56.6%), Azadirachta indica (30.1%), Eucalyptus globules (11.6%) and Lantana camara (1.7%).
Summary of socio-demographic data from four villages at Lower Moshi, northern Tanzania, based on 120 households
Methods of application were mostly incense burning/smouldering considered to offer effective protection and application was mostly done around 7 pm to 10 pm in 60% of households interviewed of which majority were small scale farmers (73%). The plant parts mostly used were leaves (70%), barks (10%), mixed plant parts (13%) or roots (7%).
Knockdown effect based on exposure to filter papers impregnated with plant extract
The knockdown effect induced by the two Ocimum plants and citronella ranged from 35% to 50% in all species of mosquitoes tested. This effect however, varied significantly (P < 0.05 – P > 0.05) among the three mosquito species tested. Citronella had the highest knockdown effect followed by OS and OK. At a dose of 30 mg/m2, citronella achieved over 50% knockdown within 10 minutes, while extracts of test plants (OS and OK) achieved between 35% to 45% knockdown for all species tested. There was no significant difference between the knockdown effect due to citronella and OS in all species tested. The OK showed the lowest knockdown effect in all species, whereas citronella scored a significantly high knockdown effect (P = 0.001). The KD90 results for all species against plant extracts are summarized in Table .
The KD90 of different plant extracts based on three mosquito species
In general, the knockdown efficacy of OS was significantly higher on An. gambiae (P = 0.027), whereas that of OK was significant on An. arabiensis (P = 0.007). On the other hand, the knockdown effect of both OS (P = 0.223) and OK (0.045) was not significant on Cx. quinquefasciatus.
Mosquito mortality in experiments with treated netting materials (contact bioassays)
In all tests, citronella induced the highest mortality rate on both species, followed by OS on Anopheles species and OK on Culex species. Mortality rate was high in An. gambiae s.s up to cut off point (30 mg) for all the test products. Citronella induced the highest mortality (56.6%) followed by OS (47.5 %) and the lowest was OK (43.3%).
At a concentration of 30 mg, mortality caused by exposure to citronella were high in An. arabiensis (67.7%, P < 0.001) followed by An. gambiae ss (56.7%, P < 0.001) and Cx. quinquefaciatus 63.7% (P = 0.001).
In An. arabiensis, mortality induced by citronella was 67.7% (P < 0.0001); 62.0% for OS (P = 0.006) and 55% (P = 0.051) for OK. Mortality induced by both citronella and OS are comparable and significantly higher than mortality caused by OK.
In An. gambiae s.s., citronella achieved the highest mortality of 56.7% (P < 0.001) followed by OS 47.4% (P = 0.001) and OK 43.3% (P = 0.054) at a dosage of 30 mg. The citronella caused more than 50% mortality, but the effect of both OS and OK was comparable and below 50%.
In Cx. quinquefasciatus, citronella caused the highest mortality 63.7% (P = 0.001) followed by 56.3% for OK (0.012) and 46.3% for OS (P = 0.062). Mortality due to OK and citronella was comparable and significantly higher on Culex mosquitoes. With increased dosage to 50 mg, there was no significant increase in mortality in all species and in all treatments.
Biting inhibition of mosquitoes
This present finding of cage experiments which compared the number of mosquitoes landed on treated and untreated arms of volunteers. The highest biting inhibition rate against all tested mosquito species was achieved by DEET (ranging from 88.7% to 92.5 %) followed by OS (83.5% to 88.9%) and OK (71.2% to 85.3%). Among the natural products tested, variation in biting inhibition within the species was observed; OS was more inhibiting for the An. gambiae s.s and An. arabiensis whereas the OK was efficient in inhibiting Cx. quinquefasciatus. The feeding inhibition caused by OS (P < 0.001) was significant higher than OK (P = 0.045) on both species of Anopheles tested. Likewise OK induced significant feeding inhibition on Cx. quinquefaciatus, but overall DEET gave the highest biting inhibition (Figure ).
Biting success exhibited by three mosquito species treatments into twenty percent concentration of DEET, OK and OS.
All measured protective effects namely repellence, feeding inhibition and mortality rates of test extracts (OS or OK) were compared against CT, a standard repellent to obtain significance levels. The trend in repellency effect, feeding inhibition and mortality were found to increase with dosage (ranged from 75 mg to 500 mg/M2) of extracts (Figure ) impregnated on netting materials. The dosage of 500 mg/m2 was found to be the most effective and this was used to compare the effects of different exposures.
The mean response of mosquito repelled/inhibited from feeding in the tunnel experiment against different dosages (0 mg/m2 (Control), 75 mg/m2, 100 mg/m2, 200 mg/m2, 250 mg/m2, 500 mg/m2) of plant extracts.
In the tunnel experiment, the overall repellence effect induced by netting material treated with citronella against mosquitoes entering the bait chamber was 81% for An. gambiae s.s, 85% for An. arabiensis and 98 % for Cx. quinquefasciatus. There was feeding inhibition of 93% for An. gambiae s.s, 96 % for An. arabiensis and 100% for Cx. quinquefasciatus. Mortality rates were 63% for An. gambiae s.s, 69% for An. arabiensis and 65% for Cx. quinquefasciatus. Notably, all test effects, repellence (Table ), feeding inhibition (Table ) and morality rates (Table ) induced by citronella were significant for all three species of mosquitoes.
Repellency effects against An. gambiae, An. arabiensis and Cx. quinquefasciatus induced by 500 mg/m2 of each of citronella and two extracts of Ocimum suave and Ocimum kilimandscharicum.
Feeding inhibition of An. gambiae, An. arabiensis and Cx. quinquefasciatus induced by 500 mg/m2 of each of standard repellent citronella and two extracts of Ocimum suave and Ocimum kilimandscharicum.
Mortality rates scored for An. gambiae, An. arabiensis and Cx. quinquefasciatus induced by 500 mg/m2 for each of standard repellent Citronella and two extracts of Ocimum suave (OS) and Ocimum kilimandscharicum (OK).
Overall repellence induced by OS was 81% for An. gambiae ss, 89% for An. arabiensis and 91% for Cx.quinquefasciatus. High level of feeding inhibition was observed, indeed 88% of An. gambiae ss, 90% of An. arabiensis and 100% of Cx. quinquefasciatus could not feed on the bait. Mortality rates due to OS were 50% for An. gambiae s.s, 58% for An. arabiensis and 67% for Cx. quinquefasciatus. Notably, repellence effect (Table ), feeding inhibition (Table ) and mortality rates (Table ) induced by OS were significant for all three species of mosquitoes tested.
An overall repellence effect induced by OK was 73%, 75% and 82% for An. gambiae s.s, An. arabiensis and Cx. quinquefasciatus respectively. Likewise feeding inhibition was 77% for An. gambiae s.s, 75 % for An. arabiensis and 99% for Cx. quinquefasciatus. Mortality scored were 47%, 52% and 56% for An. gambiae s.s, An. arabiensis and Cx. quinquefasciatus respectively. Repellence effect (Table ) and feeding inhibition (Table ) induced by OK was significant for An. arabiensis and Cx quinquefasciatus only. However, mortality rates were significant higher for all three species of mosquitoes tested (Table ).