Control of Disease Vectors using Fungi - Biological and Environmental Control of Disease Vectors - page 75

transmission potential could be reduced to zero
(Blanford et al. , 2011). Fungal impacts on
feeding and fecundity have also been
demonstrated in An . gambiae s.s. mosquitoes
exposed to Metarhizium . Using excreted
haematin to estimate blood uptake, smaller
blood meals were taken by fungal-treated
mosquitoes and the proportion blood-fed at 72 h
post-exposure was 50% compared to 100% of
controls (Scholte et al. , 2006). Reduced feeding
propensity over eight gonotrophic cycles and
decreased egg laying was observed in fungal-
exposed mosquitoes. Reduced responses to
feeding cues and decreased foraging due to B .
bassiana and M . anisopliae infections were
associated with impaired abilities to detect
external stimuli such as CO 2 and 1-octen-3-ol
(George et al. , 2011). Similar fi ndings have been
reported from laboratory and semi-fi eld studies
of B . bassiana -infected St . aegypti (Darbro et al. ,
2012). Fungi, therefore, can also af ect vector
competence by reducing dispersal and blood
feeding, thereby reducing the probability of
becoming infected or passing on infections, and

reducing egg laying, thus decreasing the
population size of the vector.

Isolates that target a sub-set of the
population
The lifespan of an insecticide may be prolonged
if it disproportionately af ects a sub-set of the
population. It has been shown that older
mosquitoes, the proportion of the population
most likely to harbour infective disease
pathogens, have a greater risk of succumbing to
fungal infection compared to younger insects.
The relative impact of both M . anisopliae and B .
bassiana was greater in older mosquitoes, with
corrected median survival times 20% lower in
older mosquitoes than younger ones (Stevenson,
2008) (Fig. 5.5). Similarly, when An . gambiae s.s.
were infected with the microsporidian parasite
Vavraia culicis , a pathogen closely related to
fungi, the ef ect of infection increased with age
from a dif erence of 16% in clutch size during
the fi rst gonotrophic cycle compared to
uninfected females to 45% fewer eggs in the

1.2

3-7 day old

1

17-21 day
old

0.8

0.6

0.4

0.2

0

M. a. acridum high dose B. bassiana low dose B. bassiana high dose

Fig. 5.5. Mean survival of young and old Anopheles stephensi mosquitoes 14 days after exposure to
fungi. Abbott's corrected mean proportion survival (± SE ) 14 days after exposing 3-7-day-old and
17-21-day-old female Anopheles stephensi to untreated cardboard pots or cardboard pots treated with
Metarhizium anisopliae var. acridum 330189, or Beauveria bassiana , at low or high dose. (Adapted from
Stevenson, 2008, with permission from LSHTM.)