CASTELO BRANCO, M.

GATEHOUSE, A. G.

English

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     An. Soc. Entomol. Brasil 28(4) 611Dezembro, 1999ECOLOGY, BEHAVIOR AND BIONOMICSFood Availability and Larval Density Affect Ovarian Development inPlutella xylostella (L.) (Lepidoptera: Yponomeutidae)MARINA CASTELO BRANCO1 and ALEXANDER G. GATEHOUSE21Centro Nacional de Pesquisa de  Hortaliças/Embrapa, Caixa postal 218, 70.359-970,Brasília, D.F, Brazil. e-mail: marina@cnph.embrapa.br2University of Wales, School of Biological Sciences, Brambell Building, Bangor,Gwynedd,  LL57 2UW, United Kingdow.An. Soc. Entomol. Brasil 28(4): 611-616 (1999)Disponibilidade de Alimentos e Densidade de Larvas que Afetam o DesenvolvimentoOvariano de Plutella xylostella (L.) (Lepidoptera: Yponomeutidae)RESUMO - Foi investigado o efeito da densidade populacional e  idade daplanta no desenvolvimento ovariano da traça-das-crucíferas (Plutella xylostellaL.). Em testes realizados em placas de Petri, a densidade de larvas por disco defolhas de repolho afetou o desenvolvimento ovariano. Quando a densidade foide sete ou 14 larvas/disco de repolho, a maior parte das fêmeas possuíam ováriosmaduros na emergência. Quando esta densidade foi de 28 larvas/disco de folha,a maior parte das fêmeas  possuíam ovários imaturos na emergência.  Não houveefeito da idade da planta sobre o  desenvolvimento ovariano.  Quando larvas datraça-das-crucíferas foram criadas sobre plantas de repolho de idades de dois equatro meses, com densidade de 30-35 larvas/planta,  mais de 94%  das fêmeaspossuíam ovários maduros na emergência. Foi sugerido que a densidadepopulacional exerce um importante efeito sobre o  desenvolvimento  ovarianoda traça-das-crucíferas.  Como migrações de insetos geralmente ocorrem quandoos ovários das fêmeas estão imaturos, em campo, fêmeas com potencialmigratório deverão ocorrer apenas quando a densidade for superior a  30   lar-vas/planta.PALAVRAS-CHAVE: Insecta, traça-das-crucíferas, desenvolvimento ova-riano, migração.ABSTRACT – The effect of Diamondback Moth (Plutella xylostella L.) popu-lation density and plant age on the rate of ovarian development was investi-gated.  In tests carried out in  Petri dishes, the density of larvae/leaf disk affectedthe rate of ovarian development.  A high proportion of moths from larvae rearedon leak disks from two or four-month-old spring cabbage at a density of sevenor 14 larvae/leaf disk were mature at emergence.  But moths reared at the sametype of plant at a density of 28 larvae/leaf disk were largely immature at emer-gence.  When the larvae were reared at a density of 30 larvae/plant on two orfour-month-old spring cabbage plants, a high proportion of females were ma-612ture at emergence. Crowding has an important effect on the rate of ovarian de-velopment in Diamondback Moth with possible consequences for the migratorypotential of moths, as insect migration generally occurs when the ovaries of thefemales are immature. The density of moths necessary to increase the propor-tion of  immature females in the field is probably higher than 30 larvae/plant.KEY WORDS: Insecta, Diamondback Moth, ovarian development, migration.house & Zhang 1995).The objective of this work was to evalu-ate the effect of population density and hostage plant on the rate of ovarian developmentof DBM in order to obtain indications of thefactors that could induce migration in the in-sect.Materials and MethodsPopulation. The population used in this ex-periment was collected in the Brasília, D.C.(Brazil), in January 1994. The rearing proce-dures were described elsewhere (CasteloBranco & Gatehouse 1997).Experimental design. Tests in Petri dishes:larvae of DBM were reared on 4.5 cm diam-eter leaf disks of spring cabbage cv. JanuaryKing at densities of seven, 14 and 28 larvaeper disk, using leaves from cabbages whichwere two and four-month-old. Eight, four andtwo replications per density were used in or-der to obtain the same number of larvae (56).The leaf disks were changed when approxi-mately 90% of the area had been eaten. Fe-males were killed in a freezer within 15 h ofemergence and were dissected under a bin-ocular at three times magnification. A solu-tion of 0.09% NaCl was coloured with neu-tral red until it was light pink (Sokolova 1994).The females were placed in the solution, theterminal part of the abdomen was pulled outand the ovary exposed.  When exposed in thisprocedure the chorionated eggs turned yellow.The  stage of ovarian development was as-sessed using the following scale (Han 1988):Grade 0: ovarioles short and empty;Grade 1: ovarioles longer with the ap-pearance of red spot-like oocytes inside;The  rate of ovarian development of in-sects can be influenced by environmental con-ditions. Hiller & Thorsteinson (1969) andMcDonald and Cole (1991) reared respec-tively larvae of Diamondback Moth (Plutellaxylostella) (DBM) and  Mythimna convecta(Walker) (Lepidoptera: Noctuidae) individu-ally and in groups in the laboratory.  Theyfound that the large majority of moths devel-oping from larvae reared individually weremature at emergence but moths reared ingroups were immature at emergence. It wassuggested that a crowded environment af-fected the response and noted that, when en-vironmental conditions were favourable, fewfemales were immature at emergence.It has been suggested that migration inmost migratory insects occurs when  femalesare immature (Johnson 1969).  Coombs et al.(1993)  deployed light traps at heights of 40-50 m on the edge of an escarpment in a Na-tional Park in Australia. Helicoverpaarmigera (Hübner)  and H. punctigera (Lepi-doptera: Noctuidae) were caught in these trapsduring migratory flight and more than 88%of the females were immature.  Rankin et al.(1994) evaluated the stage of ovarian devel-opment in Anthonomus grandis (Boheman)(Coleoptera: Curculionidae) and observed thatinsects with undeveloped or partially devel-oped ovaries were the most likely to make longflights, i.e. migrate.Migration in insects can be obligatory (oc-curs every generation) or facultative (occursoccasionally).  Facultative migration occurswhen environmental conditions such as lackof food, changes in photoperiod or crowdingindicate that the habitat will be unfavourablefor the development of the insects’ offspring(Southwood 1962, 1971, Johnson 1969, Gate-Castelo Branco & Gatehouse     An. Soc. Entomol. Brasil 28(4) 613Dezembro, 1999Grade 2: these spots developed intored, rectangular oocytes;Grade 3: oocytes  further developedwith one or more chorionated yellow eggs atthe base of the ovarioles;Grade 4: fully developed yellow eggsare present  at the base of the ovarioles andhave moved into the lateral or commonoviducts.Tests on plants: fifty-five eggs of DBM wereplaced per plant. Two categories of plants,cabbage with two and four-month-old, withthree replications were used. The larvae pu-pated on the leaves and, at this stage, they werecollected and placed in a 9.0 cm diameter Petridish until the emergence of the adults. Whenthis occurred, the same procedure describedabove was used.Analysis: In order to analyse the influence ofthe several rearing procedures on the rate ofovarian development of DBM, the femaleswere grouped as immature (grades 0, 1 and2) or mature (grades 3 and 4).The results from females reared in Petridishes of different ages and with differentdensities  were analysed using χ2    in a 3x2contingency table (Zar 1984), while the re-sults from females reared on plants were ana-lysed using a two tailed Fisher Exact Test asexpected values were less than five (Zar1984).Results and DiscussionThe number of females that emerged fromPetri dishes treatments were between 20-28.More than 80% of the moths that emergedfrom larvae reared at densities of seven or 14larvae/leaf disk on two or four month-old cab-bage were mature at emergence (grades 3 and4) (Fig. 1A and B).   However at a density of28 larvae/leaf disk, 40 and 25% of the femaleswere mature at emergence when the larvaewere reared on two and four-month-old cab-bage respectively (Fig. 1A and B ).  Analysisof the contingency table showed that the rateof ovarian development was significantly af-fected by larval density on two month (χ2 =41.53, df=2, p< 0.05) or four month cabbage(χ2 = 27.26, df=2, p< 0.05).  Analysis of thecontingency table also showed that the rateof ovarian development was independent ofthe age of the plant when a density of 28 lar-vae/leaf disk was used (χ2 = 0.44, df=2, p>0.05).  Finally, the analysis using a two tailedFisher Exact Test showed that the rate of ovar-ian development in DBM was independent ofthe age of the plants at density of seven (p>0.05) and 14 (p> 0.05) larvae/leaf disk.According  to Southwood (1962, 1971)environmental factors that predict the onsetof unsatisfactory conditions, such as crowd-ing or reduced availability or quality of food,may cause a delay in sexual maturation.  Theeffect of crowding in delaying sexual matura-tion was observed by McDonald & Cole(1991) when rearing  Mythimna convecta(Walker) (Lepidoptera: Noctuidae) at highdensities. The delay in sexual maturationcould induce emigration from unfavourablehabitats.   Evidence obtained in this work sug-gests that the rate of reproduction develop-ment in DBM females is influenced by larvaldensity but not by plant age.  A high propor-tion of females reared on spring cabbbage athigh densities (28 larvae per disk) (Fig. 1And B) emerged with immature ovaries indi-cating extended pre-reproductive periodswhich may provide time for pre-reproductivemigration.  These results suggest the hypoth-esis that migration could occur when popula-tion levels, and, therefore, densities, are high.The same hypothesis was suggested by Hiller& Thorsteinson (1969).In order to simulate the density that couldinduce migration, a test was carried out inlaboratory on two and four month-old plants.Fourty and 45 females emerged from the twoand four month-old-cabbage respectively. Theresults showed that independent of plant age,more than 94% of the females were mature atemergence (grades 3 and 4) (Fig. 2) and  thatthe two tailed Fisher Exact Test showed  noeffect of plant age.  The small number of im-mature females at emergence indicated thatfew individuals were likely to have the po-614Figure 1: Ovarian development of Diamondback Moth at emergence when reared on Springcabbage  two (A) and four-month-old (B) at densities of seven, fourteen and twenty-eightlarvae/leaf disk.  Grades 0 (ovarioles short and empty) to 4( fully developed eggs are presentat the base of ovarioles and have moved into the lateral or common oviducts) refer to ovariandevelopment.                                                                                                                                                 A                                                                                                                   B0 1 2 3 4Grade020406080100Percentage 7  larvae 14  larvae 28  larvae0 1 2 3 4Grade020406080100Percentage 7  larvae 14  larvae28  larvaeCastelo Branco & Gatehouse     An. Soc. Entomol. Brasil 28(4) 615Dezembro, 1999tential to migrate.  In the field, higher densi-ties than those to increase the proportion offemales that emerge immature may be neces-sary.AcknowledgmentsTo Embrapa who provided the Scholar-ship for this work. To Nigel Brown from Uni-versity of Wales, who provided the Springcabbage. To Dr. Marina Sokolova, who helpedwith  the dissection of DBM.  To EmbrapaHortaliças Publication Committee for review-ing this manuscript.References CitedCastelo Branco, M. & A.G. Gatehouse.1997. Insecticide resistance in Plutellaxylostella (Lepidoptera: Yponomeutidae)in the Federal District, Brazil. An. Soc.Entomol. Brasil 26:75-79.Coombs, M.; A.P. del Socorro; G.P. Fitt &P.C. Gregg. 1993.  The reproductivematurity and mating status of Helicoverpaarmigera, H. punctigera and Mythimnaconvecta (Lepidoptera: Noctuidae)collected in tower-mounted light traps innorthern  New South Wales, Australia.Bull. Entomol. Res. 83: 529-534.Gatehouse, A.G. & X.X. Zhang. 1995.Migratory potential in insects: variationin an uncertain environment., p.193-242.In Drake, V.A. & A.G. Gatehouse.  InsectFigure 2: Ovarian development of Diamondback Moth at emergence when reared on twoand four-month-old Spring cabbage at a density of 30 larvae/plant. Grades 0 (ovarioles shortand empty) to 4 ( fully developed eggs are present at the base of ovarioles and have moved intothe lateral or common oviducts) refer to ovarian development.0 1 2 3 40100806040202 MONTHS4 MONTHSGradePercentage616migration: tracking resources throughspace and time.  Cambridge UniversityPress. Cambridge. 478p.Johnson, C.G. 1969.  Migration and dispersalof insects by flight. Methuen. London.763p.Han, E. 1988. Laboratory studies on theregulation of migratin in the OrientalArmyworm, Mythimma separata(Walker) (Lepidoptera: Noctuidae). TesePhD.  University College of North Wales.Bangor. Reino Unido. 212p.Hiller, R.J. & A.J. Thorsteinson.  1969.  Theinfluence of host plant or males  onovarian development or oviposition in theDiamondback  Moth  (Plutellamaculipennis). Can. J. Zool. 47:805-806.McDonald, G. & P.G. Cole. 1991.  Factorsinfluencing oocyte development inMythimna convecta (Lepidoptera:Noctuidae) and their possible impact onmigration in eastern Australia.  BullEntomol. Res.  81:175-184.Rankin, M.A., E.N. Hampton & K.R. Summy.1994. Investigations of the oogenesis-flight syndrome in Antho- nomus grandis(Coleoptera: Curculio- nidae) usingtethered flight tests.  J. Ins. Beh. 7: 795-810.Sokolova,M.I. 1994.  A rediscription of themorphology of mosquito (Diptera:Culicidae) ovarioles during vitelloge-nesis. Bull. SOVE 19:53-58.Southwood,T.R.E. 1962. Migration ofterrestrial arthropods in relation to habitat.Biol. Rev. 37:171-214.Southwood,T.R.E. 1971.  The role andmeasurement of migration in populationsystem of an insect pest. Trop. Sci.13:275-278.Zar,J.H. 1984.  Biostatistical analysis. 2ndedition. Prentice-Hall, Inc. EnglwoodCliff. New Jersey. 718p.Received 14/XII/98. Accepted 27/X/99.Castelo Branco & Gatehouse

Food availability and larval density affect ovarian development in Plutella xylostella (L.)(Lepidoptera: Yponomeutidae).

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Food availability and larval density affect ovarian development in Plutella xylostella (L.)(Lepidoptera: Yponomeutidae).

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