Evolution of costly secondary defences for a cryptic prey is puzzling, if the prey is already well protected by camouflage. However, if the chemical defence is not sufficient to deter all predators, selection can favour low signal intensity in defended prey. Alternatively, if the costs of chemical defence are low or cost-free, chemical defences can be expected to evolve also for non-signalling prey, particularly if conspicuous signalling is costly. We tested these assumptions with pine sawfly larvae (Neodiprion sertifer and Diprion pini) that are cryptically coloured and chemically defended with resin acids sequestered from their host plant (Pinus sp.). Larvae feed in large aggregations, which we hypothesise could function as a signal of unprofitability. Our results show that even though the birds found N. sertifer larvae unprofitable in the controlled laboratory assays, they continued attacking and consuming them in the wild. When we tested the signal value of aggregation we found that a large group size did not offer protection for a defended larva: the survival was higher in groups of 10 individuals compared to groups of 50, suggesting increased detectability costs for individuals in larger groups. Finally, we tested how costly the production and maintenance of a chemical defence is for D. pini larvae by manipulating the resin acid content of the diet. We did not find any life history or immunological costs of the chemical defence for the larvae. In contrast, pupal weights were higher on the high resin diet than on the low resin diet. Also, larvae were able to produce higher amounts of defence fluids on the high diet than on the low diet. Thus, our result suggests high detectability costs and low production costs of defences could explain why some unprofitable species have not evolved conspicuous signals.

Bombus terrestris can cause pollination disturbance in native plants and compete with native bumblebees and other pollinators. The accompanying non-native parasites may also threaten native bees. We report new observations of the commercially used Bombus terrestris (Linnaeus, 1758) using trapping data and sporadic samples identified with a PCR–RFLP-method for degraded DNA. A total of 863 individuals (355 queens, 442 workers, 66 drones) of Bombus sensu stricto were collected during the years 2008–9, of which, 642 were B. lucorum, ten B. cryptarum, four B. terrestris and none were B. magnus. Three trap types were compared in two modified transects near areas that use the commercial B. terrestris for pollination in Southern Finland: the tree trap that was hung at approximately 3 metres height was the most effective. Regular monitoring is important in the risk assessment of B. terrestris, and for correct species identification, molecular methods are recommended.