lme - lmer mixed models

Dear Kim,

first of all, please use lmer() from library "lme4", instead of lme. I think that is currently better maintained and developed than the "nlme" library.
The model you suggest is a random regression per individual (obs_nr)?
The syntax would become

library(lme4)
cocon_mixed<-lmer(MiWi~host_spec*ins_spec+(date|obs_nr),data=cocon_nash)

It should work provided there are several observations of MiWi for each level of obs_nr, which seems to be the case given your explanation of the data.

Cheers, Tom