﻿157 
  

  

  ON 
  THE 
  USE 
  OF 
  EXPERIMENTAL 
  PLOTS 
  WHEN 
  STUDYING 
  

  

  FOREST 
  INSECTS. 
  

  

  By 
  Ivar 
  Tragardh, 
  D.Sc, 
  F.E.S., 
  

  

  Swedish 
  Institute 
  of 
  Experimental 
  Forestry, 
  Entomological 
  Department, 
  

  

  Experimentaljdltet, 
  Sweden. 
  

  

  The 
  problems 
  of 
  how 
  to 
  obtain 
  accurate 
  records 
  of 
  the 
  infestation 
  of 
  injurious 
  

   insects 
  has 
  during 
  recent 
  years 
  received 
  considerable 
  attention 
  ; 
  and 
  when 
  we 
  

   take 
  into 
  consideration 
  that 
  it 
  is 
  quite 
  as 
  impossible 
  to 
  study 
  the 
  course 
  of 
  an 
  

   outbreak 
  during 
  its 
  different 
  stages 
  as 
  it 
  is 
  to 
  compare 
  two 
  outbreaks 
  occurring 
  in 
  

   different 
  localities 
  or 
  at 
  different 
  times, 
  without 
  possessing 
  methods 
  of 
  recording 
  

   in 
  an 
  accurate 
  way 
  the 
  degree 
  of 
  infestation 
  and 
  injury, 
  it 
  is 
  evident 
  that 
  every 
  effort 
  

   must 
  be 
  made 
  to 
  solve 
  this 
  difficulty 
  and 
  devise 
  means 
  of 
  obtaining 
  accurate 
  records 
  

   in 
  these 
  respects. 
  

  

  In 
  the 
  case 
  of 
  the 
  insects 
  injurious 
  to 
  forest 
  trees 
  this 
  need 
  of 
  accurate 
  information 
  

   is, 
  for 
  various 
  reasons, 
  perhaps 
  still 
  more 
  urgent 
  than 
  in 
  the 
  case 
  of 
  herbivorous 
  

   insects, 
  but 
  at 
  the 
  same 
  time 
  the 
  problem 
  seems 
  to 
  present 
  fewer 
  difficulties. 
  The 
  

   necessity 
  for 
  possessing 
  accurate 
  methods 
  of 
  recording 
  the 
  infestation 
  and 
  the 
  injury 
  

   done 
  is 
  obvious, 
  because 
  the 
  attacks 
  made 
  on 
  the 
  trees 
  by 
  injurious 
  insects 
  often 
  

   continue 
  for 
  several 
  years, 
  which 
  means 
  that 
  the 
  degree 
  of 
  infestation 
  and 
  the 
  amount 
  

   of 
  injury 
  done 
  must 
  be 
  continually 
  observed 
  and 
  accurately 
  recorded 
  during 
  several 
  

   years, 
  if 
  we 
  are 
  to 
  obtain 
  a 
  reliable 
  idea 
  of 
  the 
  course 
  of 
  the 
  outbreak 
  and 
  to 
  be 
  able 
  

   to 
  predict 
  the 
  probable 
  seriousness 
  of 
  the 
  attack 
  during 
  a 
  succeeding 
  year. 
  It 
  is 
  

   also, 
  in 
  the 
  case 
  of 
  forest 
  insects, 
  necessary 
  to 
  investigate 
  the 
  consequence 
  of 
  the 
  

   injury 
  to 
  the 
  health 
  of 
  the 
  trees 
  and 
  to 
  observe 
  the 
  infestation 
  of 
  secondary 
  insects 
  

   following 
  upon 
  the 
  primary 
  ones, 
  in 
  order 
  to 
  know 
  what 
  measures 
  are 
  to 
  be 
  adopted 
  

   to 
  prevent 
  the 
  spreading 
  of 
  an 
  outbreak 
  or 
  the 
  deteriorating 
  of 
  the 
  timber 
  if 
  the 
  

   trees 
  are 
  killed. 
  

  

  But 
  on 
  the 
  other 
  hand, 
  trees 
  undoubtedly 
  present 
  fewer 
  difficulties 
  to 
  such 
  

   investigations 
  than 
  do 
  herbaceous 
  plants, 
  because, 
  at 
  least 
  in 
  many 
  instances, 
  the 
  

   degree 
  of 
  infestation 
  may 
  easily 
  be 
  fairly 
  estimated 
  without 
  in 
  the 
  least 
  interfering 
  

   with 
  the 
  trees 
  or 
  causing 
  any 
  disturbance 
  in 
  the 
  prevailing 
  conditions 
  at 
  the 
  period 
  

   when 
  the 
  insects 
  have 
  temporarily 
  migrated 
  from 
  the 
  trees 
  and 
  are 
  hibernating 
  

   in 
  the 
  ground. 
  

  

  When 
  investigating 
  an 
  outbreak 
  of 
  the 
  pine- 
  tree 
  looper 
  (Bupalus 
  piniarius, 
  L.), 
  

   for 
  instance, 
  the 
  degree 
  of 
  infestation 
  is 
  comparatively 
  easy 
  to 
  ascertain 
  by 
  counting 
  

   the 
  pupae 
  in 
  the 
  ground 
  in 
  May 
  in 
  different 
  parts 
  of 
  the 
  forest 
  ; 
  all 
  that 
  is 
  required 
  

   being 
  a 
  sufficient 
  number 
  of 
  girls 
  or 
  boys 
  to 
  examine 
  the 
  ground 
  thoroughly. 
  The 
  

   amount 
  of 
  injury 
  done, 
  viz., 
  the 
  degree 
  of 
  defoliation, 
  on 
  the 
  other 
  hand, 
  is 
  more 
  

   difficult 
  to 
  estimate, 
  especially 
  when 
  the 
  same 
  trees 
  are 
  to 
  be 
  examined 
  during 
  

   several 
  years, 
  which 
  makes 
  it 
  impossible 
  to 
  fell 
  them 
  and 
  count 
  the 
  number 
  of 
  

   devoured 
  or 
  half-devoured 
  needles. 
  

  

  During 
  the 
  recent 
  outbreak 
  of 
  Bupalus 
  piniarius 
  in 
  Sweden 
  in 
  1916-1917 
  the 
  

   author 
  had 
  occasion 
  to 
  devote 
  some 
  time 
  to 
  the 
  studying 
  of 
  these 
  problems, 
  and 
  in 
  

   the 
  following 
  paragraphs 
  the 
  method 
  employed 
  will 
  be 
  briefly 
  described. 
  

  

  