By using laboratory flight mills, I tested the hypothesis that the differential distributions of knapweed gall flies, Urophora quadrifasciata and U. affinis, among knapweed sites can be predicted by the flight propensity and endurance of the flies. Results from tests did not support the prediction that the former species displays the greater propensity for flight and endurance. I discuss several reasons supporting the values obtained and their validity for use in interspecific comparisons. Finally, I point to the danger of extrapolating laboratory behaviour to the field

Roitberg, Bernard D.

Journal of the Entomological Society of British Columbia

58 J. ENTOMOL Soc . BRIT. COLUMHIA 85 (1988). AUG. 31, 1988 COMPARATIVE FLIGHT DYNAMICS OF KNAPWEED GALL FLIES UROPHORA QUADRIFASCIATA AND U. AFFlNlS(DIPTERA:TEPHRITIDAE) BERNARD D. ROITBERG CENTRE FOR PEST MANAGEMENT DEPARTMENT OF BIOLOGICAL S C IENCES SIMON FRASER UNIVERSITY BURNABY, BC CANADA VSA I S6 Abstract By using laboratory flight mills, I tested the hypothesis that the differential distributions of knapweed gall flies , Urophora quadrifasciata and U. affinis, among knapweed sites can be predicted by the flight propensity and endurance of the flies. Results from tests did not support the prediction that the former species displays the greater propensity for flight and endurance. I discuss several reasons supporting the values obtained and their validity for use in interspecific comparisons. Finally, I point to the danger of extrapolat-ing laboratory behaviour to the field. INTRODUCTION The distribution of insects in space is likely to be some function of the distribution of their resources (Karieva 1985) as well as their propensity (Jones 1977; Roitberg et al. 1984) and ability (Ralph 1977; Roitberg et al. 1979) to move among resources and resource sites. An apt example of differential distribution among similar resource sites is represented by two species of palaearctic tephritid flies in British Columbia. The larvae of Urophora quadrifasciata (Meigen) and U. affinis Frauenfeld induce galls and feed within the seed heads of diffuse knapweed (Centaurea diffusa Lam.). Adults of both species were released into knapweed-infested sites in the interior of BC during the early 1970's to control the spread of this highly pestiferous host (Harris and Myers 1984). Following release, however, each species has displayed different spatial distributions , within and between sites. Urophora affinis typically achieves moderately high population levels and low rates of spread between sites. By contrast, U. qlladrifasciata is generally found at low population densities, but spreads rapidly and can be found at remote resource sites (Harris and Myers 1984; Story and Nowierski 1984). Within-site differences in the spread of these flies has been explained in terms of differential use of seed heads (Berube 1980). Here, I propose an hypothesis to explain the different inter-site spread rates of the two species: Ha: Urophora qlladrifasciata displays greater flight propensity and endurance than U. ajjinis. If it was supported, this hypothesis could explain differential fly distribution purely on the basis of emigration tendency. Such differences have been demonstrated among closely related species of milkweed bug and in different geographic populations within the same species (Dingle 1978). In this paper, I describe a test of the differential flight hypothesis that uses laboratory flight mill s. MATERIALS AND METHODS All experiments were conducted in the laboratory with wild-type, lab-maintained flies . To obtain the flies, knapweed seed heads were collected during November 1985 from roadsides near Penticton, Be. The seed heads, some of which were infested with overwinter-ing Urophora larvae , were then held at 2°C for several months . Following this, the seed heads were held at ca. 20°C for several weeks until the flies emerged. Emergent flies were placed in IS X IS X IS cm plexiglas-sc reen cages and were provided with water and food (sugar + enzymatic yeast hydrolysate (Prokopy and Boller 1971» ad lib, and knapweed stems which served as resting and mating si tes for the flies. Upon reaching II ± I days-of-age, mature female flies were removed from the mainte-nance cages and then placed in petri dishes with water and abundant food for 2 h prior to testing. Then the flies were individually placed in glass vials and held at -SoC for ca. 30 s or J. ENTOMOL Soc. BRIT. COLU MIJIA 8S (1988). AUG. 31. 1988 59 until they became immobile. The chill ed fli es were then attached at the dorsal thorax , to fli ght mill s as described in Roitberg el al. ( 1984). Tarsa l contact was provided by a glass microscope slide. To induce fli ght , tarsal contact was removed within a few seconds aft er it appeared that the flie s had recovered from chilling. If no flight occurred , tarsal contact was reinstated and then aga in removed. If the fl y still did not initiate fli ght , that trial was terminated. Similar procedures were employed to induce further fli ght from flie s that initiated but terminated flight s. I ehose II-day-old females for testing because, at that age, fli es had mated and had been reproducti ve ly mature for several days. Since flies had been deprived of oviposition sites I reasoned th at they would be more likely to di splay fli ght response (e.g. Roitberg el al . 1984) thus providing a larger data base for comparing fliers of both species. This was an appropriate dec ision since my compari son of fli ght parameters were not absolute measures of field performance. Fli ght propensit y was indexed as the distance fl own by individuals (number of I m revolut ions) during the ini tial fli ght onl y. Flight endurance , by contrast, was indexed as the total distance flown until individuals refused to fly furt her fo llowing two tarsa l-contact removals. Following completion of each fli ght tr ial. the fly was frozen and then placed in a desiccator. Desiccated fli es were we ighed and their wi ng areas were measured through employment of an Apple™ Computer Graphics Tablet. RES ULTS Frequency d istri but ions of initial fl ight di stances are shown in Fig. I. Contrary to predictions, V . quadrifascia{(l did not fl y significantl y further during initial fli ght s than did V. ajjil1is (j( = 84.3 m ± 45.3 SE, n = 28 vs. j( = 161 .8 m ± 59.1 SE, n = 5 1: p > 0.5 Mann Whitney U). Most in iti al fli ght s covered less than 100 m for both spec ies (V. quadnfasciata , 25/30; V. affinis, 38/5 1). As with init ial fli ght. and aga in contrary to the prediction, V . quadnfasciala covered less di stance du ring lotal fli ght than did V. ajjinis (x = 140. 1 m ± 62. 1 SE vs. x = 38 1.5 m ± 108.0 SE: p > 0.05 Mann Whitney U). While the majority of total-distance fli ght s again covered less than 100 m. (V. qlladnfasciata. 24/30; V. affinis, 3 1/51 ) the distribution had an ex tended ta il fo r V. ajjinis, with a small proport ion of flies covering more than 500 m (V. quadnfascita, 2/30: V. affin is 10/ 50) (Fig. 2). No signifi cant correlati ons were found between the size characteristics of the fli es and their initial or total fli ght di stances. Two parameters, dry we ight and wing loading (= weight/ wing area) \ ' .1'. initi al and total d istance had littl e exp lanatory power as shown by the va lues be low. all of whi ch are NS: Dry we ight vs . intial di stance vs. total d istance Wing loading vs. initi al di stance vs. total distance V. qlladnfasciata r = 0.06 r = 0. 11 r = 0. 17 r = 0.14 DISCUSSION V. affinis r = 0.06 r = 0.16 r = 0.02 r = 0.08 Tethered flight can be a reli able, qualitati ve means of comparing inherent vag ility wi thin and among populations and spec ies of insec ts (Davis 198 1; Nakamori and Simizu 1983: Roitberg el al. 1984: Dingle and Evans 1987). For both species of fly studied here. the distributions of fli ght distance we re leptokurtic (i.e. skewed toward short distance ), a common fea ture of many insec t spec ies (Davis 1980). Thus. the results reported here probably reflect re lat ive within-field differences in vag ility for these two Vrophora spec ies. The conclusion is that spec ies-spec ific vagi lit y in V rophora qlladrijc/Sciata and V . ajjinis does not exp lain their differe nt ia l dist ri butions among knapweed sites. Indeed, V. affillis. the species which I predicted wo uld display lower fligh t propensi ty and endurance, actually appeared to be more vag i Ie than it s congene r. The re are reasons for be ing cau tious about 60 J. ENTOM OL Soc. BR IT . COLUMB I A 85 ( 1988), Au(, . 3 1, 1988 generali zing from these results, in that eac h indi vidual was tested onl y once (Dav is 1980) and a single age c lass was tested, but g iven the res tric ted set of conditions empl oyed, the ex perimen-tal design seems appropriate and the conc lusions warranted. A possible explanation o f my results is that the ni ght mill itse lf caused some bias in the night propensity and endurance estimates. S ince U. quadrifas icara is the somewhat smaller spec ies (Fig. 3), any fri c tion within the mill system should have a greater impact on its night. I 100 80 ---1 60 ~ o l-lL. o ~ 40 20 I2Zl U. AFFINIS _ U. QUADRIFASCIATA Fig. I Frequcncy dislribulions of initial flight di stances by tethered U C/llo(/ri/ascio{(/ and U. off/lli.l· fcmales. J. E NTOMOL Soc. BRIT. COLU MBI A !IS ( 1988). A UG. 3 1. 1988 61 tested thi s hypothes is by com parin g the proportion of fli ghts greater than 100 m (= long flight ) among the different si7.e classes o f ni es in both species. T he results showed that long fli ght s were independent of fly si7.e in bot h spec ies (U. quadrifasciata X2 = 2.29, df = 3, NS; U. affinis X2 = 6.6, df = 8, NS). S ince eve n the smallest indi vid ual within the sma ll er species appeared equall y like ly to engage in long fli ght. I conc luded that the mill es timates are not biased aga inst SI7.e. --1 ~ o l-lL. o 100 80 60 ~ 40 20 t72J U. AFFINIS _ U. QUADRIFASCIATA Fig. 2 Frequency d i'lribulion' of 100a i flig hl d i, lance, by le thered U. qllad,.,j'ascia to and U. affillis fe males. 62 50 40 --I 30 ~ o l-lL. o ~ 20 10 J. ENTOMOL Soc. BRIT. COl.UMBIA 8S ( 1988), AUG. 31, 1988 !LZJ U. AFFINIS _ U. QUADRIFASCIATA SIZE RANGE (mgX10-2) Fig. 3 Frequency di stributions for we ight c lasses (dry weight (mg x 10-2 ) of female U. ql/adnjascia/a and U. ajjillis. J. E NTOMOL Soc. BRIT. COLlI M ll l A 85 ( 1988), A UG . 3 1. 1988 63 The demonstration of size class independence from long night tendency is at odds with the findin g of Roll (1977 ), who showed that , for Drosophila melanogasler, it was the larger individuals that were most prone to disperse from release sites. Similarly, Dingle el al. ( 1980) documented a positive relationship between body size and tethered night di stance both between spec ies of milkweed bugs (Oncopeli lls spp. ) and within populations of Oncopellus fascialus. My results do indicate th at, it is indeed the larger of the two Urophora species that may be more prone to engage in long tethered night. Field samples, however, indicate the opposite trend (Myers and Harri s 1984), which points to the danger of ex trapolating lab behaviour to the fi eld . A more reali sti c ex planati on regarding the counterintuitive result s reported here derives from Mye rs and Harri s' ( 1980) obse rvat ion of the di stributions of U. qlladrifasciata and U ajjinis ga lls among plants . They reported that , although U. quadnfasciata di splayed " better di spersion" among s ites , that the ir within-plant distributions were more clumped than those of U. ajjinis. Thus, testing vag ilit y on night mills poss ibl y ignores some crucial behavioral fea ture which causes U. quadnfasciala to sw itch from a wi thin pl ant " clumper " to an active di sperse r. Mye rs and Harri s ( 1980) ci te Gilbert 's ( 1977) observation that analysis of insect di stributions does not idelllify causal mechanisms, but might identify insect behaviors dese rving further stud y. A coroll ary that ari ses from thi s study is that in vestigation of fli ght behaviour (i.e. movement) does not necessaril y lead to indcntificati on of insect distribution. Inherent tendencies must be considered against the ecological setting in which they are defined. ACKNOWLEDGEMENTS Thi s stud y was supported by an NSERC, Canada operating grant and a Simon Fraser University Pres ident 's Research Grant. I thank Richard Hamelin , Phil Lee. Chri s Steeger and Juli e Brooks for providing exce ll ent assistance, Peter Belton for technical advice and Bob Lalonde and Yves Carriere for comments on an earli er version of the MS. REFERENCES Berube. D.E. 1980. Inter' pec ifi c compe tition between Uml'/lOra afJillis and U . q lladrifasciala (Dipte ra: Tep-hriti dae l for ov ip'b ition ,i tes O il d iffu,e knapweed (Cen({Jl/re{/ d if!lis(J : Compos it ae ). Ze it. fur ang . Entom o l. 90: 299-106. Dav is. M.A. 1980. Wh y are most insect s short Ilie,, ') Evol. T heory 5: 103- 111. Davi, . M .A . 1981 . The Ili gh t capacity o f d ispersi ng m ilk weed beetles. TelraoJl('s le lraop lha/1II1I5. An n. En tomol. Soc. Amer. 74: 3XS-3H6. Dingle. H .. N. Blakl ey. and E. R. Mill e r. 19XO. Varia ti on in bod y s i/.e and !light pe rformance in milkweed bugs (O ll coJle/llIs) . Evo l. 34: 37 1-385. Di ng le. H. and Eva ns. K.E. 1987. Re'pon,e, in Ili ght to selecti on on wi ng length in no n-mi g ratory mi lk weed bugs. Ol/mpellus jilSci,,!us. Ent . ex po app . -1 5: 289-296. Gilbert . N. 1977. Appe ndi x to: Move ment pa tt ern s and egg distr ibutions in cabbage butter!lies. J. Anim . EcoJ. 46: 195-2 12. Harri s. P. and J .H. Myers. 198-1 . (' I/Illurea d!jjilS(J Lam . and C. /J/(J cu/os(J Lam . S. lat.. diffuse ,md spotted knapweed. / I/ J .S. Ke ll ehe r and M.A. Holme,. Bio logica l Contro l Programmes Against Insects and Weeds in Canada. C.A. B .. Eng land . Jones. R.E. 1977. Move ment patterns and egg dimibutio n in ca bbage butt erfl ies. J . An im . Ecol. 46: 195-2 12. Kari eva . P. 19R5. Finding and lo,i ng ho, t plants by Pltrl/ol relll : pa tch , i7e and surrou nding hab it at. Eco l. 66: 1809-1 8 16. Mye rs. J .H. and P. Harri s. 1980. Dhtrib ut ion of Uml' iIora galls in !lower heads o f cli ffu ,e and spotted knapweed in Briti sh Co lumbia . J. App l. Eca l. 17: 35<)-67 . Nakamo ri . H. and Simi/u. K. 1983. Compari so n o f the tli ght ab ilit y be twee n wild and mass- reared melon Il y. D(Jcus (" ucurhiwi' Cotjui ll ctt (Diptera: Tcp hritidae ). Appl. Ent. Zool. 18: 17 1-HI. Prokopy. R .J . an d E.F. Bo ll er. 1971. Artifi c ia l egg ing system for th e Eurnrcan che rry fruit Il y. J. Econ. Ento mol. 63 : 1413-141 7. Ralph. c. P. 1977. Effect of ho" plant den, it y on popu lation , of a spec ialilecl seed -feeding bug. Ol/cope/lus jilSt ·ialus . Ecol. SX: 7'Jl) -H09. Roll. D.A. 1977. Di spe " al in dipt e ran ,: ib co,,, and cOJhe4uence.s. J . Ani m . Eco l. 46: 443 -456. 64 J. ENTOMOL Soc. BRIT . COLUMBIA R5 (1988), AUG. 3 1, 1988 Roitberg, B.D., J .H. Myers and R.D. Frazer. 1979. The influence of predalors 011 Ihe movemenl of apte rous pea aphids between phmts . 1. Anim. Eco!. 49: 111 - 122. Roitberg, B.D. , R.S. Cairl and RJ. Prokopy. 1984. Oviposition deterrin g pheromone influences di spersa l distance in tephritid fruit !lies. Ent. ex po app!. 35 : 217-220. Story, J.M. and R.M. Nowicrsk i. 1984. Increase and di spersa l of Urol,h()ra a/fill is (Diptera : Tcphritidae) on spotted kn apweed in Weste rn Montana. Eni vron. Entomo!' 13: 11 5 1- 1156. COMPARATIVE LARVAL GROWTH OF THE VARIEGATED CUTWORM, PERIDROMA SAVe/A, FROM A LABORATORY COLONY AND A WILD POPULATION GR EG S. SALLOUM I AND MURRAY B. ISM AN DEPARTM ENT OF PLANT SCIENCE UNIVERSITY OF BRITISH COLUMBIA VANCOUVER, B.C. , CANADA V6T 2A2 Abstract Larval growth of variegated cutwom1S from a laboratory colony (maintained for over 12 generations) was compared with that of the F I generation of field-collected larvae on an artificial medium. After eleven days of feeding , larvae from the wild population weighed, on average, over three times as much as those from the laboratory colony. However, when larvae from each population were reared on med ia spiked with an inhibitory plant extract , the degree of growth inhibition re lative to their respective controls was equivalent. INTRODUCTION Insects from laboratory colonies are commonly used in both basic and applied research , especially in studies of pesticidal efficacy where large numbers of uniformly aged individuals are required for bioassay. One implicit assumption underlying such studies is that the response of insects from the laboratory colony is representative of that expected of insects from wild popUlations. Unfortunately, maintenance of a laboratory co lony of insect s often results in inadvertant selection of genotypes and pheno types which diverge from the colony founders of natural origin, Often thi s fact is overlooked, and the in sects chosen for the study are those which can be conveniently produced in the laboratory setting (Berenbaum 1986). In our laboratory, we have been using a laboratory colony of the variegated cutworm, Peridroma saucia (Hbn.)(Lepidoptera : Noctuidae), for bioassay of natural insecticides and antifeedants (lsman and Proksch 1985), This species was se lected because it is a polyphagous pest of occasional economic importance throughout North America (Simonet et al. 1981), and because it is relatively easy to maintain in the laboratory in continuous culture. In the present study, we compared larval growth and survival of cutworms from a two-year-old laboratory colony with those of the F t generation of field -co llected larvae. MATERIALS AND METHODS The laboratory colony, maintained for over 12 generations , was founded from pupae supplied by Dr. G. Ayer, Agriculture Canada, Winnipeg, They were taken from a laboratory colony maintained at Winnipeg for at leas t one year. The field population in our study consisted of the offspring of larvae collected from cabbage plants growing at the Department of Plant Science field laboratory on the University of British Columbia campus in Vancouver, as well as from unsprayed gardens in the Kitsilano di strict. Larvae were reared on an artificial medium (BioServ Inc ., Frenchtown, NJ , no. 9682) as described previously (l sman and Rodriguez 1983). In the first experiment, neonate larvae from each population were reared on the standard diet for I I days and then weighed. In the second ex periment , neonate larvae from each population were reared on either the standard di e t treated with 95% aqueous ethanol, or a di et spiked with an ethanol ic ex tract from fol iage of big basi n sagebrush, Ar,emsia ,ridenrara. at 50% of na tural concentration (dwt/dwt ). For each IPresent address: Safer Ltd ., 6761 Kirkpatri ck C rescent , R.R. 3 , Vic toria , B.C. V8X 3XI 

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Comparative Flight Dynamics of Knapweed Gall Flies Urophora quadrifasciata and U. affinis (Diptera: Tephritidae)

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Comparative Flight Dynamics of Knapweed Gall Flies Urophora quadrifasciata and U. affinis (Diptera: Tephritidae)

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