Keith F. A. Walters

The role of climatic mapping in predicting the potential geographical distribution of non-indigenous pests under current and future climates

Abstract Climatic mapping, which predicts the potential distribution of organisms in new areas and under future climates based on their responses to climate in their home range, has recently been criticised for ignoring dispersal and interactions between species, such as competition, predation and parasitism. In order to determine whether these criticisms are justified, the different procedures employed in climatic mapping were reviewed, with examples taken from studies of the Mediterranean fruit fly ( Ceratitis capitata ), Karnal bunt of wheat ( Tilletia indica ) and the Colorado potato beetle ( Leptinotarsa decemlineata ). All these studies stressed the key role played by non-climatic factors in determining distribution but it was shown that these factors, e.g., the availability of food and synchrony with the host plant, together with the difficulties of downscaling and upscaling data, were different to those highlighted in the criticisms. The extent to which laboratory studies on Drosophila populations, on which the criticisms are based, can be extrapolated to general predictions of species distributions was also explored. The Drosophila experiments were found to illustrate the importance of climate but could not accurately determine potential species distributions because only adult and not breeding population densities were estimated. The experimental design overestimated species interactions and ignored other factors, such as the availability of food. It was concluded that while there are limitations, climatic mapping procedures continue to play a vital role in determining what G.E. Hutchinson defined as the “fundamental niche” in studies of potential distribution. This applies especially for pest species, where natural dispersal is generally less important than transport by man, and species interactions are limited by the impoverished species diversity in agroecosystems. Due to the lack of data, climatic mapping is often the only approach which can be adopted. Nevertheless, to ensure that non-climatic factors are not neglected in such studies, a standard framework should be employed. Such frameworks have already been developed for pest risk analyses and are suitable for general use in studies of potential distribution because, in order to justify the phytosanitary regulation of international trade, they must also consider the potential for pests to invade new areas and the impacts of such invasions.

Effect of temperature and cultivar on pea aphid, Acyrthosiphon pisum (Hemiptera: Aphididae) life history

Life history parameters of the pea aphid, Acyrthosiphon pisum were studied at five constant temperatures on two cultivars of peas, Scout and Sancho. The development and mortality of juveniles and the life-span, age-specific fecundity and survivorship of adult aphids were recorded and used to construct life tables. The juvenile development period (from birth to adulthood) was longest at 11.9 degrees C (16.8 days on cv. Scout and 16.2 days on cv. Sancho) and shortest at 26.7 degrees C (8.5 days on cv. Scout and 8.8 days on cv. Sancho). At all temperatures, except 26.7 degrees C, juveniles developed faster on cv. Sancho than on cv. Scout. On both pea varieties juvenile mortality was highest at temperatures above 19.6 degrees C and lowest at 19.6 degrees C. Highest cumulative juvenile mortality was recorded on cv. Scout at 26.7 degrees C when only 9% of aphids survived from birth to reproductively mature adults. Fecundity rates were unaffected by temperature in the range tested on cv. Sancho but increased with increasing temperatures between 11.9 and 19.6 degrees C on cv. Scout. These differences in life history parameters were reflected in the population growth (rm) of aphids on both pea cultivars which increased with increasing temperatures between 11.9 and 23.1 degrees C on cv. Sancho and 11.9 and 19.6 degrees C on cv. Scout, declining thereafter. Population growth was consistently greater at all temperatures for aphids reared on cv. Sancho than those reared on cv. Scout.

Effects of temperature on the establishment potential of the predatory mite Amblyseius californicus McGregor (Acari: Phytoseiidae) in the UK

Amblyseius californicus was introduced into the UK in the early 1990s as a biocontrol agent against glasshouse red spider mite Tetranychus urticae. This study investigated the effects of temperature on the establishment potential of A. californicus in the UK in the light of recent reports of their successful overwintering outside of glasshouse environments. The developmental thresholds were 9.9 and 8.6 degrees C respectively using simple and weighted linear regression. Using the day-degree requirement per generation calculated by weighted regression (143 day-degrees) in combination with climate data, it was estimated that up to seven generations would be possible annually outdoors in the UK. Non-diapausing adult females froze at -22 degrees C, with 100% mortality after reaching their freezing temperature. Up to 90% of mites died before freezing after short exposures to low temperatures. Significant acclimation responses occurred; 90% of acclimated individuals survived 26 days exposure at 0 degrees C and 11 days at -5 degrees C (acclimated mites were reared at 19 degrees C, 6L:18D followed by 1 week at 10 degrees C, 12L:12D). Non-diapausing adult females survived over 3 months outdoors in winter under sheltered conditions and oviposition was observed. The experimental protocol used in this study is discussed as a pre-release screen for the establishment potential of other Amblyseius species, and similar non-native biocontrol agents.

Effects of temperature on the establishment potential in the U.K. of the non‐native glasshouse biocontrol agent Macrolophus caliginosus

Abstract This study investigated the effect of temperature on the development and winter survival of the predatory mirid Macrolophus caliginosus Wagner, recently introduced into the U.K. as a biocontrol agent for glasshouse whitefly Trialeurodes vaporariorum. The developmental threshold for M. caliginosus calculated by three methods was between 7.3 and 8.4 °C, with a day‐degree requirement per generation varying between 472 and 524 day‐degrees. It was estimated that under outdoor conditions M. caliginosus could complete two generations per year in the U.K. All life stages of M. caliginosus had supercooling points around −20 °C, with some pre‐freeze mortality evident in both acute and chronic low temperature exposures. Acclimation increased survival of nymphal M. caliginosus from approximately 24–52 days when exposed to a constant 0 °C. Provision of prey extended survival of nymphs in the laboratory at a constant 5 °C from 39 to 64 days and in the field by c. 150 days. The results are discussed in the context of the occurrence and establishment of M. caliginosus in the U.K. and the need to develop a reliable risk assessment system for non‐native species used in glasshouse biocontrol.

PATHOGENICITY OF THE ENTOMOPATHOGENIC FUNGUS, LECANICILLIUM MUSCARIUM, AGAINST THE SWEETPOTATO WHITEFLY BEMISIA TABACI UNDER LABORATORY AND GLASSHOUSE CONDITIONS

The potential for using the entomopathogenic fungus Lecanicillium muscarium to control the sweetpotato whitefly, Bemisia tabaci has been established in the laboratory by other studies. Laboratory studies however frequently overestimate the level of control achieved by biological control agents in the glasshouse. Before full-scale commercial or field development is considered, glasshouse trials are required to confirm laboratory results. Under both controlled laboratory and glasshouse conditions high mortality of second instar B. tabaci was recorded after application of L. muscarium. The potential of incorporating L. muscarium into integrated pest management strategies for the control of B. tabaci is discussed.

Freezing induces a loss of freeze tolerance in an overwintering insect.

Cold–hardy insects overwinter by one of two main strategies: freeze tolerance and freeze avoidance by supercooling. As a general model, many freeze–tolerant species overwinter in extreme climates, freeze above −10°C via induction by ice–nucleating agents, and once frozen, can survive at temperatures of up to 40°C or more below the initial freezing temperature or supercooling point (SCP). It has been assumed that the SCP of freeze–tolerant insects is unaffected by the freezing process and that the freeze–tolerant state is therefore retained in winter though successive freeze–thaw cycles of the body tissues and fluids. Studies on the freeze–tolerant larva of the hoverfly Syrphus ribesii reveal this assumption to be untrue. When a sample with a mean‘first freez’ SCP of −7.6°C (range of −5°C to −9.5°C) were cooled, either to −10°C or to their individual SCP, on five occasions, the mean SCP was significantly depressed, with some larvae subsequently freezing as low as −28°C. Only larvae that froze at the same consistently high temperature above −10°C were alive after being frozen five times. The wider occurrence of this phenomenon would require a fundamental reassessment of the dynamics and distinctions of the freeze–tolerant and freeze–avoiding strategies of insect overwintering.

Rapid cold hardening in the western flower thrips Frankliniella occidentalis.

A rapid cold hardening process is reported in first instar larvae of Frankliniella occidentalis. When larvae are transferred directly from 20 degrees C to -11.5 degrees C for 2h there is 78% mortality, whereas exposure to 0 degrees C for 4h prior to transfer to -11.5 degrees C reduces mortality to 10%. The response can also be induced by exposure to 5 degrees C for 4h or by gradual cooling at rates between 0.1 and 0.5 degrees C min(-1.) The acquired cold tolerance is transient and is rapidly lost (after 1h at 20 degrees C). Rapid cold hardening extends survival times at -11.5 degrees C and depresses lethal temperatures in short (2h) exposures. Rearing at 15 degrees C (12L:12D), (a cold acclimation regime for F. occidentalis), does not protect against the cold shock induced by direct transfer to -11.5 degrees C (which rapid cold hardening does) but does extend survival time at -5 degrees C (i.e. increased chill tolerance) whilst rapid cold hardening does not. The rapid and longer term cold hardening responses in F. occidentalis therefore appear to have different underlying mechanisms.

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

Thermal biology of Typhlodromips montdorensis: implications for its introduction as a glasshouse biological control agent in the UK

The recent unexpected local establishment of a non‐native predatory mite, Neoseiulus californicus (McGregor) (Acari: Phytoseiidae), in the UK prompted us to undertake this study, which investigated the thermal biology of an alien species Typhlodromips montdorensis (Schicha) (Acari: Phytoseiidae). Laboratory and field experiments on its cold tolerance were used to assess its establishment potential outside of glasshouse environments in the UK. Currently, T. montdorensis is being tested as a glasshouse biological control agent against thrips and spider mites, but is not yet licensed for release in the UK. Typhlodromips montdorensis has a developmental threshold of between 10.3 and 10.7 °C, and a thermal budget of between 108.7 and 105.3 degree‐days when estimated by weighted and simple linear regression, respectively. Under outdoor conditions, T. montdorensis could theoretically complete up to six generations a year. The supercooling points of female and larval T. montdorensis were −22 to −24 °C with 100% pre‐freeze mortality apparent in both acute and chronic low temperature exposures. Typhlodromips montdorensis were unable to enter diapause under a selected laboratory regime. No reproduction occurred in the field from November to March, with 100% mortality within 7–14 days of release during this period. It is concluded that T. montdorensis would be a ‘safe candidate’ for introduction as a glasshouse biological control agent in the UK.

Cold tolerance, overwintering and establishment potential of Thrips palmi

The cold tolerance and overwintering survival of the quarantine regulated pest and virus vector, Thrips palmi (Karny) (Thysanoptera: Thripidae), is examined and discussed in terms of its establishment potential in the U.K. Thrips palmi adults and first‐instar larvae have a wide distribution of supercooling points (SCPs) but show ‘pre‐freeze’ mortality as a result of both acute and chronic exposures to temperatures above the SCP range. Thrips palmi did not develop enhanced cold tolerance in response to cues previously shown to cold‐acclimate other thrips species. The acute cold tolerance of T. palmi is higher than that of the recently established and biologically similar species, Frankliniella occidentalis, which is thought to be capable only of very limited winter survival outdoors in the U.K. However, the more ecologically meaningful chronic assays reveal the opposite pattern. If introduced to the U.K., overwintering of T. palmi would thus be largely restricted to protected environments, as its cold tolerance is insufficient to permit outdoor survival for a complete winter. This assertion was demonstrated by caged populations that died out after as little as 25 days in outdoor winter conditions in Yorkshire, U.K. The reversal of relative tolerance of the two species when considering chronic and acute assays suggests that these forms of low temperature mortality have different physiological bases, and emphasizes the need to use both forms of assay in assessments of overwintering capacity.