Structure-Repellence Studies of Δ-Octalactone Analogues and Blends against Tsetse Flies, Glossina Pallidipes and G. Morsitans Morsitans
Wachira, Benson Mureithi
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Tsetse flies (Glossina spp.) are serious vectors of trypanosome parasite, threatening over 60 million people with Human African Trypanosomosis (sleeping sickness) and 45-50 million cattle with Animal African Trypanosomosis (Nagana) annually in the sub-Saharan Africa. It is estimated that 50,000-100,000 people and 3 million cattle die from trypanosomosis annually. Studies on vaccine development for parasite control have proved unsuccessful due to antigenic variations of the parasite. Control of tsetse vectors is considered a better option. However, some of the earlier methods based on the use of synthetic insecticides are ecologically harmful. A number of alternative methods based on combination of visual and odor baits have shown some success among settled farming communities, although they have been less successful among pastoralists and isolated farmers. Use of a repellent or repellent blend is now considered an effective approach to control tsetse flies at individual farmer or pastoralist level, and may even have potential in ‘push-pull’ strategy to control the flies more effectively. A recent study on body odors of waterbuck, a tsetse-refractory animal, led to the identification of a potent repellent blend. One of the key components in the odor is (RS)-δ-octalactone. In the present study, structure-activity with δ-octalactone analogues was carried out with the aim of identifying more potent repellents. This involved evaluation of the effect of chain length of the alkyl moiety and ring size of the δ-octalactone on the repellent activity against Glossina pallidipes and G. morsitans morsitans. The responses of the two Glossina spp. to (RS)-δ-octalactone and six analogues comprising (RS)-γ-hexalactone, (RS)-γ-heptalactone, (RS)-γ-octalactone, (RS)-δ-heptalactone, (RS)-δ-nonalactone and (RS)-ε-nonalactone were compared in a two-choice wind tunnel. (RS)-δ-heptalactone and (RS)-ε-nonalactone were synthesized and their structures confirmed using conventional spectroscopic techniques while the rest were acquired commercially. Data obtained were subjected to Analysis of Variance and means ranked using Student-Newman-Keuls test. (RS)-δ-octalactone gave RD50 of 0.34 g/ml and 0.22 g/ml for G. pallidipes and G. m. morsitans respectively. (RS)-δ-nonalactone was found to be the most potent analogue with RD50 of 0.26 g/ml and 0.20 g/ml for G. pallidipes and G. m. morsitans respectively. Reduction of ring size and chain length on (RS)-δ-octalactone led to a decrease in repellency while an increase in chain length from propyl to butyl increased its repellency. (RS)-ε-nonalactone showed attraction to both G. pallidipes and G. m. morsitans with AD50 of 0.38 g/ml and 0.36 g/ml respectively. An increase in ring size from six to seven member lactone had a converse effect of converting the compound from being a repellant to an attractant. Blending (RS)-δ-nonalactone with 4-methylguaiacol (a known tsetse repellent), showed synergistic effects against both G. pallidipes and G. m. morsitans with RD50 of 0.13 g/ml and 0.08 g/ml respectively. The study identifies a structural feature associated with enhanced repellency against the two Glossina spp. and lays down useful groundwork for more effective behavioral control of tsetse species.