Lythrum salicaria L., purple loosestrife, an invasiveEurasian perennial, is degrading wetlands across temperate NorthAmerica. Chemical, physical, and mechanical methods failed to providelong-term control. Therefore, four host-specific insect species (twoleaf feeders, a root feeder, and a flower feeder) were introduced asbiological control agents. To increase the availability of adults ofthe root feeding weevil Hylobius transversovittatus Goezefor field releases, we developed a semiartificial diet. Suitability ofdifferent diet formulations (varying vitamin mixes, salt mixes,antimicrobials, water content, root content) and temperatures forlarval development were evaluated. We also monitored how rearing onartificial diet and the number of larvae per container affected larvaldevelopment time, larval survival, adult weight, and incidences ofdeformities. Rearing larvae on artificial diet reduced development timefrom 1–2 yr to 2–3 mo. Larval development was fastest and survivalrates highest under constant temperatures of 25°C. Hatch rate andlarval survival decreased if eggs were surface sterilized. Using FABCOantimicrobials could not prevent fungal contamination; use of methylparaben and sorbic acid was successful in suppressing fungal andbacterial growth throughout larval development time to <10%. Themoisture content of the diet did not significantly affect larvalsurvival, development, or adult weight. Decreasing the proportion ofpurple loosestrife roots in the diet reduced adult weight and theproportion of larvae completing development, and increased developmenttime; no larvae were able to complete development in root-free diet.With an increase in the number of larvae per cup, survival rates werereduced. The experiments revealed a female biased sex ratio: femalesconsistently developed faster and were heavier than males. Incidence ofadult deformities was consistently below 5%. Increased availability ofadults for field release as a result of mass production using thesemiartificial diet will accelerate the biocontrol program targetingpurple loosestrife. We are able to produce several hundred weevils perweek and have adapted the diet to rear other root-feeding weevils.