1674 



Canadian Forestry Journal, May, 1918 



osoting of egg masses, tanglefoot- 

 ing of trees, spraying of trees, 

 intensive forestry practice, and 

 all such methods are simply out 

 of the question. Can anything 

 else be done to control, or better 

 still, to prevent such damage? 

 An answer can perhaps be found 

 by considering the nature of an 

 insect outbreak. 



In the case of any particular insect, 

 the struggle for existence is so intense 

 that an outbreak is usually impos- 

 sible. New Brunswick, for instance, 

 probably supports at least ten thou- 

 sand dilTcrent kinds of insect but the 

 competition for a livelihood is so 

 keen that most of them are uncom- 

 mon or rare. An outbreak then 

 results from a set of peculiar condi- 

 tions favoring a particular species. 



A great many dift'erent factors 

 tend to keep insects in a condition 

 of numerical stability or control, or, 

 in other words, to prevent outbreaks. 

 The more important of these factors 

 I will rapidly review. 



In the first place the vagaries of 

 climate are powerful factors in con- 

 trol. Hailstorms in July will some- 

 times free acres of forest lands from 

 a destructive caterpillar. Ever so 

 light a frost in June will often kill 

 millions of tender larvae. In fact 

 'unusual' weather at any time is 

 liable to produce direful results in the 

 insect world. 



Value of Birds 



The useful work of insectivorous 

 birds can scarcely be over estimated, 

 particularly in reducing the numbers 

 of succulent larvae feeding in the 

 north woods. The wood warblers 

 are pre-eminently useful in this respect 

 as any one can testify who has camped 

 in June and July beside one of the 

 countless brooks in the north woods 

 region. From dawn until the day 

 grows hot groups of these shy little 

 birds may be seen busily searching 

 out a meal of insects from the leaves, 

 from the twigs, or from the bark 

 according to their respective prefer- 

 ence. 



Then again epidemic diseases play 

 a,n important part in reducing the 

 numbers of insects. With the insects 



unduly abundant and the weather 

 conditions favorable for incubating 

 the fungus or bacteria organisms, 

 causing these epidemics such a death 

 rate may result that statistics for 

 the great plague of London or for 

 any human epidemic . recounted in 

 history pale into significance. At 

 times, indeed, injurious insects are 

 locally exterminated in this way. 



Predatory Insects 



Other powerful aids in reducing 

 the numerical strength of certain 

 kinds of injurious insects are the 

 predatory mites and insects. In the 

 case of the oyster shell or mussel 

 scale, for instance, the most impor- 

 tant single factor in control in Eastern 

 Canada and many other places is 

 a tiny eight legged mite. This little 

 organism ekes out an existence by 

 feeding on healthy eggs of the scale. 

 In this way the scale is often reduced 

 from a condition of great numerical 

 abundance to one of extreme scarcity. 

 As to predatory insects the impor- 

 tanc'fe of their work can be likened 

 in a general way to that of insectiv- 

 orous birds. In a favorable year 

 such insects were estimated to destroy 

 three quarters of the entire forest 

 tent caterpillar crop of New Bruns- 

 wick and better tribute could scarce 

 be paid. 



No account of the factors tending 

 to reduce or prevent insect outbreaks 

 would be complete without referring 

 to the work of insect parasites. 

 Among all the insects of the earth 

 there are two groups that stand out 

 from the rest on account of a method 

 of existence involving parasitism on 

 other insects. I refer to the parasitic 

 two-winged flies (Diptera) and to 

 the parasitic four-winged flies (Hy- 

 menoptera). In Canada there are 

 several thousand species of such 

 parasites and they are among the 

 most useful members of our wild 

 life. Some of them attack eggs, 

 others small larvae, others older 

 larvae, and some again attack only 

 pupae; so that an insect like the 

 forest tent caterpillar is liable to 

 attack by parasites in any of its 

 first three stages. Any of my readers 

 who have tried to rear adult insects 



