Reports were scanned in black and white at a resolution of 600 dots per inch and were converted to text using Adobe Paper Capture Plug-in.The seasonal fluctuations of Ctenosciara hawaiiensis (Hardy) populations in two different ecosystems at the same elevation on Mauna Loa on the island of Hawaii are compared and correlated with fluctuations in rainfall, humidity, and temperature. Rainfall appears to be the most important extrinsic factor affecting seasonal changes in this species. There is a marked difference between the two populations which in part could be explained by differences in the ecosystems. Ctenosciara hawaiiensis is closely associated with Acacia koa. The larvae live under the bark of dead branches

Steffan, Wallace A.

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Technical Report No.., 7 ECOLOGICAL STUDIES OF CTENOSCIARA HAWAIIENSIS (HARDY) (DIPTERA: SCIARIDAE) Wallace A. Steffan Department of Entomology Bernice P. Bishop Museum Honolulu, Hawaii ISLAND ECOSYSTEMS IRP U. S. International Biological Program August 1972 T A B U  OF CONTENTS Page Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  i Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4 Literaturecited . . . . . . . . . . . . . . . . . . . . . . . . . .  6 Abstract The seasonal f luc tua t ions  of Ctenosciara hawaiienais (Hardy) popula- t ions  i n  two d i f f e r e n t  ecosystems a t  the 8amc e leva t ion  on Mama Jma on the i s l and  of Hawaii are compared and cor re la t ed  with f luc tua t ions  i n  r a i n f a l l ,  humidity, and temperature. Ra in fa l l  appears to  be the most important e x t r i n s i c  f a c t o r  a f f e c t i n g  seasonal changes i n  t h i s  species.  There is  a marked dif ference  between the two populations which i n  p a r t  could be explained by d i f ferences  i n  the ecosystems. is close ly  associa ted  with Acacia koat dead brznches. Ctenosciara hawaiiensis  The l a rvae  l i v e  undcr the bark of Ecological  S tudics  of Ctenosciara hawaiiensis  (Hardy) (Diptera: Sciar idae)  In t roduct ion  The Sciar idac  o r  dark-winged fungus gnats are a prominent element i n  many of Hawaii's ecosystems. commonly found around decaying p lan t  and animal mater ia ls .  The la rvae  a r e  characterized by the shiny black head and the  f e a t u r c l c s s ,  twelve-segmented, white, t rans lucent  body. Larvae general ly feed on decaying p lan t  and animal mater ia ls ,  animal excrement, molds and fungi; some feed on l i v i n g  p lan t  tissues. The adul ts  a r e  small ,  usually dark f l i e s  I am working on a revis ion  o€ thcsc f l i e s  and recognize about 40 species  & - i n  11 genera. are, Since these f l i e s  are e a s i l y  t ransported by man and h i s  a c t i v i t i e s ,  ,determination of endemic versus introduced species i s  an arduous task,  espe- c i a l l y  s ince  Sc ia r idae  i n  general  arc poorly known and d i f f i c u l t  t o  ident i fy .  None of the genera a re  endemic; however, many of the species  The l i f e  h i s t o r i e s  of 14 Hawaiian species  have been studied under labora-  tory condit ions where they were reared on Bacto-agar/corn meal agar s l a n t  cu l tu res  i n  constaiit  temperature cabinets  a t  20°C. Gcncration time from ovipos i t ion  t o  a d u l t  emergence ranged from 13 to 35 days; t h a t  of Ctenosciara hawaiiensis (Hardy) was from 23-34 days. Ctenosciara k w a i i c n s i s  was se lcc ted  as one of the species  t o  be in ten-  s ive ly  inves t iga ted  s ince  i t  i s  r e l a t i v e l y  easy t o  i d e n t i f y ,  i s  common t o  a l l  the si tes being inves t iga ted  and appears t o  be c lose ly  associated with one of the dominent elements of the p lan t  community, Acacia koa. The la rvae  - 2 -  are commonly found under thc bark of f a l l e n  koa branches and t r ees .  have a l so  been reared from r o t t i n g  Metroaideras, F reyc ine t i a  and severa l  o the r  r o t t i n g  woods. jthey The seasonal f luc tua t ions  of C. hawaiiensis  populations i n  two d i f -  f e r e n t  ecosystems a t  the  same e leva t ion  on Mauna Loa a re  compared and cor-  re la t ed  with f luc tua t ions  i n  r a i n f a l l ,  humidity, and temperature, Methods A modified Malaise t r a p  (Grcssict and Gressirt, 1962) i s  being used t o  sample the s c i a r i d  populations at  severa l  s i t e s  on the  i s l and  of Hawaii. This i s  an i n t e rcep t ion  t r a p  designed t o  t r a p  the  i n s e c t s  as they f ly .  Pre- sumably with t h i s  type of t r ap ,  i t  is e a s i e r  t o  ob ta in  ind ices  of absolute population s ince  i t  in terccpcs  the i n s e c t s  more o r  less randomly. f u l l i e t  (1963) compared a Malaise t r a p  with o the r s  and suggests  t h a t  for l a rge r  Hymenoptera and same Diptera t h l s  t r a p  i s  unbiased, but  he found i t  unsat i s-  fac tory  f o r  Colcoptera arid Hemiptera. Malaise t r aps  were set  up a t  two s i t e s  on the i s l and  of Hawaii: one. i n  the Kilauea XorestRcsewe (Transcct-Profi le  2, segment 11) and the o the r  near  the IBP weather s t a t i o n  on the Maunn fxls Traiscct (Transcct-Profi le  1, seg- ment 7). studied by o the r  researchers  involved In  Is lands  Ecosystems s t a b i l i t y  - and Evolution Subprogram of the  2. 2. I n t e r n a t i o n a l  Biologica l  Program. of the t r aps  a re  near IBP weather s t a t i o n s  and are a t  approximately the same a l t i t u d e ,  5400 f e e t .  The ecosystems, however, are very d i f f e r e n t .  Kilauea - Forest  Reserve is  a montane r a i n  f o r e s t  composed primari ly of Acacia &, Metroeidera and Cibotium species  ( t r e e  ferns) .  The Mauna Loa s i te  i s  i n  a These two si tes  were se lec ted  because they arc being in tens ively  Both - 3 -  mountain parkland ecosystem formed by Acacia hdonaea  ta l l- shrub communities, both. i n  a matrix of subalpine grassland. tree colonies,  Styphelia-  The f i r s t  six months, s a m p l c ~ ~  were taken continuously and col lec ted  weekly. Af ter  Ju ly  5, 1971, the sites were sampled on a l t e r n a t e  weeks only. The volume of m a t e r i a l  co l l ec ted  was too g r e a t  t o  continue on a weekly bas is .  R e s u l t s  "he numbers of a d u l t  Ctenosciara hawaiiensis  co l l ec ted  in the Malaise t r a p  a t  the  Kilauca Fores t  Reserve s i te  were r e l a t i v e l y  low throughout 1971 ranging from 0 t o  26 per '/-day co l l ec t ion  period (Pig. 1). The g r e a t e s t  adu l t  a c t i v i t y  occurred between January 25 and Apr i l  26 with two peak periods be- tween January 25 - March 1 and March 8 - Apri l  26. The numbers trapped sub- sequently were r e l a t i v e l y  low, increas ing t o  a high of 20 during the week of August 30 - September 6 .  September 27 - October '4 and again during the  week of November 8-15. There was a l so  a high of 13 during the  week of A t  the Mauna Loa Transcct Climatic S t a t i o n ,  the numbers of a d u l t s  col- lec ted  showed very pronotinccd f luc tua t ions  (Fig. 2) .  The f i r s t  and g r e a t e s t  increase began during the week of January 25 - February 1 climbing from 13 t o  131, Af ter  a decrease t o  219 during the week of  February 1, the catch rose  t o  281, the h ighes t  of the year. During the next  four weeks the numbers of adu l t s  col lec ted  dropped t o  4.6. Two similar cycles  accurred in mid-April and e a r l y  June but  t o  a lesser magnitude. The fol.lowing weck 258 adu l t s  were captured. During the  summer dry scason the catch dropped t o  a weekly low of 7 .  After J u l y  5 t h e  co l l ec t ions  wcre made on a l t e r n a t e  weeks anly  and t h i s  un- doubtedly has a f fec ted  the  results, i n  e a r l y  September, mid-October and la te  December. Three add i t iona l  cycles were observed RAINFALL (1 N MM.) TEMPERATURE RELATIVE C. HUM ID ITY 0 % .\- .=: - . ./ 7 i 1 .’ om - - h) N w w P PVI ocnovromocno NUMBER OF SPECtMENS RAtNFALL (fN .MMJ IE M P E RATURE R E L AT I V E O C. HUMIDITY % . .. I i I I I 1 =----.a /-• I .’ . I \ -2 i 0 - 0 0 - m 0 hl h) v, m 0 VI NUMBER OF SPECIMENS D i  s cu s s ion Comparison of avail.abla climatic da ta  -- r a i n f a l l ,  mean weekly temperatures, and mean wcckly r e l a t i v e  humidity -- with seasonal popula- t ion  f luc tua t ions  of sa hawaiiensis adu l t s  i n d i c a t e s  t h a t  a t  the Mauna h a  s i te  r a i n f a l l  appears t o  ba the most important e x t r i n s i c  f a c t o r  a f f e c t -  ing  seasonal changcs in s. hawaiiensis  populations. The f i r s t  ha l f  of the  year  there  were three  cycles  of adu l t  a c t i v i t y ,  as r e f l e c t e d  i n  the number of adu l t s  captured i n  the  Malaise t rap.  The f i r s t  cycle,  January 25 - March 15, began about three weeks a f t e r  a period of high r a i n f a l l  i n  January. dropped s l i g h t l y ,  corresponding t o  a dccreasc i n  r a i n f a l l  three  weeks The following week the population e a r l i e r .  when the catch rose  to 281, the g r e a t e s t  of the year. Again t h i s  can be The populat ion rcaehcd a high during the  week of February 12-22 corre la ted  t o  the h ighes t  r a i n f a l l  of the year  which occurred three weeks e a r l i e r .  The rap id  decrease i n  adu l t  a c t i v i t y  the  following week was pre-  ceeded three weeks ear l ier  by a s imi la r  but  g r e a t e r  decrease i n  r a i n f a l l .  The next two cycles  i n  mid Apri l  and e a r l y  June were almost i d e n t i c a l  but  about half  the magnitude. Again thc f luc tua t ions  could be co r re la t ed  with f luc tua t ions  in r a i n f a l l  three wccks earl ier ,  the only discrepency being i n  thc last cycle. week of peak r a i n f a l l .  r e l a t i v e l y  l o w  level .  Howcver, the a c t i v i t y  of  tihe adu l t  g. hawaiiensis a f t e r  dropping only s l i g h t l y ,  increased sharply. The only explanation I The f i r s t  peak occurred about 4 weeks a f t e r  the Ra in fa l l  then decreased sharply  and remained a t  a can o f f e r  is t h a t  s ince  the population of - C. hawaiiensis  was s t i l l  f a i r l y  high, the  r e l a t i v e l y  low l e v e l  of r a i n f a l l  was s u f f i c i e n t  t o  provide another pcriod of favorable growth. - 5 -  It would appear t h a t  t h i s  species  is  very s e n s i t i v e  to  f luc tua t ions  i n  r a i n f a l l .  The u s u a l  three-week delay i n  response, i.e. a c t i v i t y  of adu l t s  as expressed i n  Malaise t r a p  captures,  poss ib ly  represent  the  gen- e ra t ion  time of s, hawaiiensis  i n  nature.  This i s  about one week shor te r  than i t s  generat ion time i n  thc laboratory,  bu t  s ince  we have considerable d i f f i c u l t y  coloniz ing t h i s  species ,  our labora tory  cvndit ions a re  undoubtedly below thc minimum required to s u s t a b  these populations. During the  second ha l f  of thc  year ,  the Malaise t r aps  were used on a l t e r n a t e  weeks only. c a l  s t a f f  t o  process the samples, but  unfortunate i n  t h a t  the  samples were not  taken Ercqucntly enough t o  de tec t  the rapid  changes t h a t  secm t o  occur i n  C. hawaiiensis  populations. This was necessary s ince  we d id  not  have the  techni-  There were again three  cycles of adu l t  a c t i v i t y  during the second h a l f  of  the ycar. three  weeks p r i o r  t o  each peak. cycle w a s  much less than expected. been dampened by the a l t e r n a t e  week sampling method, but  even taking t h a t  i n t o  cons idera t ion ,  the  numbers of adu l t  C. hawaiiensis  were lower than expected . Thc f i r s t  and second were responses t o  increases  i n  r a i n f a l l  The i n i t i a t i o n  and magnitude of the  f i n a l  To sotue e x t e n t  the f luc tua t ions  may have The f luc tua t ions  of s. hnwaiionsis populat ions a t  the Kilauea s i t e  were about 1/10 the  magnitude of those a t  the  Mauna Loa s i te ;  however, they d id  r e f l e c t  similar rcsponses, Ra in fa l l  and r e l a t i v e  humidity were general ly greater a t  the  Kilauea s i tc ,  but  the mean weekly temperatures were lower. It therefore  appcars t h a t  some f a c t o r  o r  f a c t o r s  were suppressing the  Ihe d i f fe rences  i n  the magnitude population of C. hawaiiensis  a t  Kilauea. of the f luc tua t ions  a re  shown i n  Figure 3. 0 Id v, a LL - 0  300 2 50 200 150 100 '50 0 MAUNA L O A  "--. --.--. KI LAUEA 0 5 10 15 20 25 30 35 40  45 50 55 JAN APR JULY OCT J.AN NUMBER OF WEEKS Fig. 3 ,  Comparison of Ctcmoscicrn I ~ J - w n i i c n s i s  popula t ion  f l u c t u a -  . t ions ,  Mnuna Lon S t r i p  Road and 1Cilnuerc :'orest Itc: ~ Y V C .  - 6 -  As explained earl ier ,  the two si tes,  although a t  the same elevation and only a shor t  distance apar t ,  a& very d i f f e r en t  ecologically,  is  a r a in  fo res t  and usually wet. Sciaridae reared i n  the laboratory are very susccptiblc t o  excess moisture although t h i s  aspect has not been in-  vestigated quant i ta t ively .  The larvae are  highly susceptible t o  drowning. Excess moisture a l so  seems to  make them more susceptible to  pathogenic organisms. which might be suppressing the population a t  Kilauea. Kilauea These are  a few of the fac tors ,  both ex t r i n s i c  and i n t r i n s i c ,  Another fac tor  which may bc involved i s  ava i l ab i l i t y  of food and shel-  C. hawaiicnsis larvae a re  usually found under the bark of f a l l en  koa ter. trees and branches and seem to prefer  branches where thc bark i s  s t i l l  t i gh t l y  against  the wood. This probably provides them some protection from predators and paras i t cs  and provides a more su i tab le  microclimatic niche, Fallen koa trees with a tremendous number of dead branches are very typical  of the Mauna Loa si te ,  whereas they a re  not  as prominent a t  the s i te  on Kllauea. Li tera ture  Cited Gressitt, J. L. and M. K. Gress i t t .  1962. An improved Malaise trap.  Pacif ic  Insec t s  4: 87-90. J u l l i e t ,  J. A. 1963, A comparison of four types of t raps used fo r  capturing f ly ing insects .  Canadian J. Zool. 41: 219-223. Table 1. Numbers of Ctenosciara hawaiiensis adu l t s  col lected i n  Malaise t r aps  a t  Kilauea Forest  Reserve (Trap 2) and Mauna h a  S t r i p  Road (Trap 3) from January 18, 1971 to  January 10, 1972.x" Collection Collection Co Ile c tfon Period Trap 2 Trap 3 Period Trap 2 R a p  3 Period Trap 2 Trap 3 Jm. 18-25 9 13 Apr. 12-19 9 139 July 12-19 4 7 25-Feb. 1 14 131 19-26 13 96 Aug. 2-9 6 18 Feb. 1-8 20 258 26-Efay 3 0 51 16-23 11 20 8-15 11 219 May 3-10 4 57 30-Sept. 6 20 52 15-22 23 28 1 10-17 5 69 Sept. 13-20 4 22 22-biar. 1 10 18 1 17 -24 1 99 27-0ct. 4 13 20 Mar. 1-8 9 152 8-15 12 109 24-31 10 108 Oct. 11-18 2 111 31-June 7 5 107 25-Nov. 1 2 39 15-22 11 46 June 7-14 10 159 NOV. 8-15 13 28 22-29 24 93 14-21 a 65 22-29 6 25 29-Apr. 5 14 66 21-28 6 33 Mc. 6-13 1 33 20-27 a 41 Jan. 3-10 1 62 84 28-July 5 8 . Apr. 5-12 26 * After July  5, 1971, co l lec t ions  were made on a l t e rna t e  weeks only. 

Ecological studies of Ctenosciara hawaiiensis (Hardy) (Diptera: Sciaridae)

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Ecological studies of Ctenosciara hawaiiensis (Hardy) (Diptera: Sciaridae)

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