2.1 Study species
The three Galerucella species (Coleoptera: Chrysomelidae) are closely related, with recent divergence times: G. pusilla andG. calmariensis diverged around 77,000 years ago while G. tenella diverged around 400,000 years ago (Hambäck et al., 2013).G. pusilla and G. calmariensis are monophagous onLythrum salicaria , whereas G. tenella is oligophagous with the primary host Filipendula ulmaria . The three beetle species have similar life cycles. Adults in the area appear in May and start laying eggs on leaves or stems of their host plants. It takes a few weeks for the eggs to hatch, 2-3 weeks for the larvae to pupate, and another 2-3 weeks for the adults to emerge from the pupae. Adults then overwinter until next May. The geographic distribution in Sweden differs between species: G. pusilla occurs in the south up to central Sweden (62°N, 17°E) whereas G. calmariensis and G. tenellaoccur both in the south and north along the entire Baltic seashore (Fors et al., 2014).
The three species share an endoparasitoid wasp enemy Asecodes parviclava (Hymenoptera: Eulophidae), which lays one or more eggs in the beetle larvae (Stenberg & Hambäck, 2010). When successfully parasitized, wasp eggs hatch and wasp larvae turn the beetle larvae to black mummies containing the wasp pupae. However, if beetles manage to defend themselves, their immune system encapsulates and kills the wasp eggs, enabling the host larvae to continue growing and developing (Fors et al., 2014). Previous work show that the beetle species differ in their capacity to mount an efficient defence against parasitoid attack. Whereas G. pusilla has a strong capacity to encapsulate wasp eggs, encapsulation is rarely observed in G. calmariensis , and at an intermediate frequency in G. tenella (Fors et al., 2016; Fors et al., 2014).
We collected 45 adult male individuals, 15 samples from eachGalerucella species, during mid-May 2019 from the following sites: three G. calmariensis populations: Iggön (60°52’18”N, 17°19’29”E), Våtnäs (61°32’93”N, 17°12’77”E ) and Hölick (61°37’22”N, 17°27’18”E); three G. pusilla populations: Rastsjön (60°6’36”N, 17°53’97”E ), Lörudden (62°14’14”N, 17°39’12”E) and Haversjön (59°2’31”N, 17°9’49”E); three G. tenella populations: Umeå-1 (63°46’72”N, 20°36’00”E ), Umeå-2 (63°46’36”N, 20°37’48”E ) and Umeå-3 (63°47’18”N, 20°35’89”E ). For each population, five individuals were sampled.