Species-specific microenvironmental preferences and interactions between parasite species have been the focus of many ecological studies. Here, we studied the distribution of ectoparasite species within the gill apparatus of bream (Abramis brama) from Lake Lubāns (Latvia) to establish whether digenean metacercariae: (1) prefer specific patches within the gill apparatus; (2) co-occur in the same patches with monogeneans and copepods within a host individual; and (3) interact with monogeneans and copepods. We recorded all parasites on gill arches of the same host species and used null models to analyse co-occurrences of digenean metacercariae, monogeneans and copepods. Zero-inflated mixture models were used to define the preferred patches of parasites. We found that digenean metacercariae (Bucephalus polymorphus) prefer specific patches of the gill apparatus to encyst, and shared these preferences with monogeneans and copepods, but did not interact with them. We concluded that digenean metacercariae have a species-specific microenvironmental preference to encyst in the gill apparatus and their occurrence (even in high numbers) does not reduce the success of attachment of monogeneans and copepods in the same gill patches.

The protection of horticultural crops from slug feeding can be achieved using slug pellets; however, application of molluscicides is not always safe for the environment. There is a need for alternative methods to reduce the palatability of crop plants. Chemical properties of secondary compounds from lichens influence the feeding behaviour of slugs. Liquid extracts of three lichen species (Cladonia rangiferina (L.) F.H. Wigg., Cladonia stellaris (Opiz) Pouzar & Vězda, and Pseudevernia furfuracea (L.) Zopf) were applied to three different crops and tested for their antifeedant properties against an important agricultural pest, the Spanish slug (Arion vulgaris Moquin-Tandon, 1855). All three extracts had specific activity, showing a decrease in grazing intensity as well as slug mass loss after feeding on treated food. Slugs significantly gained mass after feeding under control condition; however, they did not gain mass when fed on extract-treated food. The most effective extract was from P. furfuracea. We propose to use properties of lichen extracts to develop new environmentally friendly molluscicides.