| Summary: | Insects have a range of behavioural defences that they can use against parasites. One of these
behaviours is self-medication: the use of biologically active compounds by the host to
ameliorate the effects of parasites and diseases. Another intuitive behaviour would be to
avoid infection risks if they are recognisable in the environment. However prophylactic self-
medication (performing self-medication before the infection has occurred) has not yet been
reported or proven conclusively in insects, and avoidance of parasites has limited examples
in the literature. In this experiment, I studied if the polyphagous lepidopteran, Arctia
plantaginis, practised prophylactic self-medication by changing their diet choice when
exposed to a high infection risk caused by infected corpses of conspecifics in their
environment. I expect the larvae to recognise higher infection risks, and so respond by
consuming a diet with more dandelion that has previously been shown to give higher survival
for infected larvae. Larvae were maintained individually in either a high infection risk, a low
infection risk, and no infection risk environments. The larvae’s choice of dandelion or
plantain leaves, in response to one of these three different levels of infection risk, was
observed over a nine-day period. I also studied whether the larvae of this species could
recognise and avoid potentially pathogenic bacteria in their environment. I expected the
larvae to avoid infected leaves and feed predominantly on control leaves. The larvae in this
experiment had to choose between leaves infected with Serratia marcescens or uninfected,
control leaves. It was found that the larvae did not choose uninfected leaves over infected
leaves, so they did not avoid the free-living pathogenic bacteria. However, they did alter
their diet choice, consuming significantly more dandelion than larvae in other treatment
groups, in response to the presence of conspecifics’ corpses that had been killed by the
pathogen. The presence of an uninfected conspecifics corpse did not elicit the same
behavioural response. These larvae seem to engage in prophylactic consumption after
consuming medicative substances earlier in life, and this is the first claim for non-social
insects to use this behavioural defence against parasites or pathogens. This behaviour could
be more widespread among insects, especially polyphagous species and show that
behavioural defences play a big part in insect – parasite interactions.
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