Low temperature mortality and overwintering of the western flower thrips Frankliniella occidentalis (Thysanoptera: Thripidae)

AbstractThe UK glasshouses in which the western flower Frankliniella thrips occidentalis(Pergande) is prevalent offer protection from adverse winter conditions. As such,F. occidentalis may not have been exposed to selection for cold tolerance that wouldallow successful overwintering in the field. In this study, the cold tolerance of larvaland adult F. occidentalis has been assessed in the laboratory. Both age groups showpre-freeze mortality in chronic and acute cold exposures though adults are morecold hardy. Larvae and adults are both able to increase their cold tolerance inresponse to a combination of lower temperatures and decreased photoperiod. Fieldexperimentation confirmed tha F.t occidentalis is unlikely to survive for the durationof a harsh UK winter, but a level of cold tolerance that would be adequate forsurvival in mild winters or for short exposures at sub-zero temperatures wasobserved.IntroductionThe western flower thrip Frankliniellas occidentalis (Per-gande) (Thysanoptera wa: Thripidae firss t recorde i)n dBritain in 1986 and sinc hase becom a pese ot majof reconomic importanc ien glasshouses acros ths e UK, causingsignificant damag teo its man y host plant bsy feeding andoviposition, as wel al s bein ag vecto for planr t viruses.Control of F. occidentalis is difficult becaus of widespreae dresistance to major classe of insecticids e (Rob et al.,b 1987;Macdonald, 1993a,b). The global spread of F. occidentalis has been facilitate bdyinternational trad ien planting materia anl d the availabilityof glasshouses that provide suitabl fo popure condition- slation developmen and obviatt the e nee d to tolerat theeexternal environment prevalen in thte area of introduction.Because of this, F. occidentalis may not have been subjectedto selectio fon r th abilite tyo tolerate unfavourable fieldconditions suc ahs thos e encountered during temperatewinters.Frankliniella occidentalis has a typical thrips life cycle withan egg (produce d either sexuall or parthenogenetically) y ,two active larval instars, two relatively passive, non-feeding•Author fo correspondencer . stages (propup ana d pupa) and adult. Adults appea tor bethe dominant overwintering form, thoug ihs difficult there yin identifyin thg e immatur e stage osf thrips that have beencollected from overwinterin tgo assemblage th speciee s slevel (Chamber s Chamberli etn al., 1992 Ch; oet al., 1995) I.n the USA, F. occidentalis is able to overwinteroutdoors in Texas (Chamber & Sitess, 1989), Georgia,California (Chamberli et al., 1992)n , North Carolin (Choaet al., 1995 an) d even as far north as Pennsylvania (Fellandet al., 1993 b)y exploiting protected microclimate osn eitherplants, in soil or under leaf litter and debris. However, winterconditions in Southern Ontario (Canada and Denmar) k aretoo severe for successful overwintering by field collecte anddlaboratory cultures of F. occidentalis (Broadbent & Hunt, 1991;Br^>dsgaard, 1993) I.t is possible that the failure to establishpermanent field population at higsh latitude iss becaus e ofinadequate cold tolerance th mildee, wherear wintes rcondition s encountere at lowed latitude allow establish-ment of F. occidentalis year-round (Br^dsgaard, 1993). If establishmen otf a permanent field populatio in the nUK is prevented by winter conditions, occidentalis F. wil bl emaintained through winter primaril in glasshousesy . In thiscase, in winte anr d spring, management strategie can be sdirected specifically against glasshouse ac coloniet as s thatpopulation reservoirs, obviatin the neegd to consider field