The effects of larval organic fertiliser exposure on the larval development, adult longevity and insecticide tolerance of zoophilic members of the Anopheles gambiae complex (Diptera: Culicidae)

Abstract

Zoophilic members of the Anopheles gambiae complex are often associated with cattle. As such, it is likely that the immature aquatic stages will be exposed to cattle faeces as a pollutant. This study aimed to examine the effect of cattle manure on members of the An. gambiae complex found in South Africa. In this study, a commercial organic fertiliser originating from cattle manure was used as a proxy for cattle faeces. Laboratory strains of An. merus, An. quadriannulatus as well as four An. arabiensis strains (SENN and MBN: insecticide susceptible, MBN-DDT: insecticide resistant, unselected, SENN-DDT: insecticide resistant: selected for resistance) were used in this study. The effect of larval fertiliser exposure on larval development rate and adult longevity was assessed in all three species. The effect of larval fertiliser exposure on subsequent adult size, insecticide tolerance and detoxification enzyme activity of the four strains of the malaria vector An. arabiensis was also assessed. Following fertiliser treatment, all strains and species showed a significantly increased rate of larval development, with insecticide susceptible strains gaining the greatest advantage. The adult longevities of An. merus, An. quadriannulatus, insecticide susceptible and resistant An. arabiensis were significantly increased following fertiliser treatment. Insecticide susceptible and resistant An. arabiensis adults were significantly larger after larval organic fertiliser exposure. Larval fertiliser exposure also increased insecticide tolerance in adult An. arabiensis, particularly in the insecticide resistant, selected strain. This 4.7 fold increase in deltamethrin tolerance translated to an increase in pyrethroid resistance intensity, which could exert operational effects. In general, larval exposure to cattle faeces significantly affects the life histories of members of the An. gambiae complex.

Fig 1. The effect of organic fertiliser exposure on the larval development of members of the Anopheles gambiae complex.

Larval exposure to organic fertiliser results in a significant increase in time to pupation in all members of the An. gambiae complex. Asterisks indicate a significant change from the control, with green asterisks indicating a change in insecticide susceptible strains and red asterisks indicating a change in insecticide resistant strains.

Fig 2. The effect of larval organic fertiliser exposure on the adult longevity of Anopheles gambiae complex members.

A: The effect of larval organic fertiliser (OF) exposure on the adult longevity of the SENN (An. arabiensis; insecticide susceptible) strain. B: The effect of larval OF exposure on the adult longevity of the MBN (An. arabiensis: insecticide susceptible) strain. C: The effect of larval OF exposure on the adult longevity of the SENN-DDT (An. arabiensis: insecticide resistant, selected) strain. D: The effect of larval OF exposure on the adult longevity of the MBN-DDT (An. arabiensis: insecticide resistant, unselected) strain. E: The effect of larval OF exposure on the adult longevity of the An. merus strain, MAFUS (insecticide susceptible). F: The effect of larval OF exposure on the adult longevity of the An. quadriannulatus strain, SANGWE (insecticide susceptible).

Fig 3. The effect of larval organic fertiliser exposure on subsequent adult size in the malaria vector Anopheles arabiensis.

Wing length was used a proxy for size. Asterisks indicate a significant change from the controls.

Fig 4. The effect of larval organic fertiliser exposure on the adult lethal time in the malaria vector Anopheles arabiensis.

A: LT50s of insecticide susceptible An. arabiensis. B: LT50s of insecticide resistant An. arabiensis. Asterisks indicate a significant change from the controls.

Fig 5. The effect of larval organic fertiliser exposure on adult detoxification and oxidative stress enzyme activity in the malaria vector Anopheles arabiensis.

A: Haeme peroxidase activity. Haeme peroxidase activity was used as a proxy for cytochrome P450 activity. B: General esterase activity. C: Glutathione S-transferase (GST) activity. D: Catalase activity. Significant differences to the controls of the same strain and sex are indicated by an asterisk.

Fig 6. Fourth instar larvae reared in organic fertiliser and untreated conditions.

Larvae were reared from hatchlings in either the OF-polluted of unpolluted conditions. Prior to pupation, larvae were examined and their midguts dissected. A: whole larvae reared under untreated control conditions. B: midgut of larvae reared under clean water conditions. C: whole larvae reared in organic fertiliser treated water. D: midgut of larvae reared in organic fertiliser treated water. E: comparison of untreated and treated larvae. F: comparison of untreated and treated midguts.