• Australian plague locusts;
  • ecological immunity;
  • haemocyte count;
  • phenotypic plasticity

1. The density-dependent prophylaxis hypothesis predicts that individuals in high-density populations will invest more resources in immune defence than individuals at lower densities.

2. However, recent work suggests that this prediction may not apply to all situations; solitarious species may paradoxically have higher scores than crowded counterparts in certain immune assays.

3. To investigate the relationship between a key immune parameter and field population densities, the total haemocyte counts (THCs) of Australian plague locusts (Chortoicetes terminifera) from three population densities in Western Australia were compared.

4. THCs were negatively correlated with field population densities, and locusts removed from a marching band and kept in isolation had increased THCs relative to group-housed controls.

5. These results demonstrate that immune investment can inversely relate to population density in field conditions.

6. We suggest that isolated locusts increase their haemocyte densities relative to crowded conspecifics in response to potentially greater exposure to parasitoids and nematodes.