Heather beetle (Lochmaea suturalis), has underperformed as a biocontrol agent when compared with the damage it does to native heather in Europe. Mean heather beetle body size, measured by elytron area, was 10% smaller in NZ populations compared with beetles from northern UK where the NZ beetles originated. Previous research in Europe showed that small beetles suffer higher winter mortality. Field-collected heather beetles in NZ show a positive relationship between body size and the proportion of pre-overwintering food reserves (lipids) they contained. Beetles that died in an overwintering experiment had lower proportional lipid reserves, and a smaller mean body size, than surviving beetles. Smaller body size in NZ is probably mostly due to a severe founder effect: line-rearing of beetles in NZ to eliminate a microsporidian disease, and poor establishment success, resulted in NZ beetles being derived from one or two field-collected females from one UK site. Several measures of genetic variability in NZ beetles compared with beetles from the UK indicated severe genetic bottlenecking. In particular, reductions in heterozygosity in NZ versus UK beetles were a close match to theoretical heterozygosity after a severe bottleneck. Heather beetle populations from southern UK were genetically distinct from those sampled from northern UK, and previous collecting showed higher microsporidian infestations in beetles from southern UK compared with northern beetles. Mean elytron area was 2.2% smaller in the southern UK population compared with the northern population. Genetic rescue of NZ heather beetles could use beetles from the northern UK that have slightly larger body size and lower levels of microsporidian infection.