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.