14 



proctotrypids, ichneumonids, braconids, tachinids and syrphids ; 



(b) Probably parasitic protozoa, such as the diseases of the gypsy cater- 

 pillar, the silk-worm, the cabbage-worm, the army-worm and many others; 



(c) Predaccous mites, such as Hcmisarcopies; and 



(d) Parasitic bacteria and fungi, such a- Coccobacillus of locusts, Sporo- 

 trichum of Chinch-bugs, Sphaerostilbc of the San Jose Scale, EtnpUySa on flies,, 

 crickets and plant lice, Cordyceps on white gru!)s and wireworms, and others, 

 which sometimes cause epidemics. 



Another factor in the natural control of injurious insects in forests has been 

 brought out by several observers, and that is that many of these insects do not 

 become epidemic in forests of mixed growth. Tothill tells us that in New Bruns- 

 wick the Budworm has gained no foothold in areas where the Balsam Fir exists in 

 the primeval condition of mixed growth. 



A parasitic tacliiiia fl> 

 its puparum. 

 (.\fter Weed) 



iii'T 



Pteromalus puparum a chalcid 



parasite of Clirysalids of the 



Cabbage Caterpillar. 



Pimpla Conquisitor ovipositing on 



a cacocn of the tent-caterpillar. 



(After Fiske) 



Certain farm insects can be most effectively controlled by i)lowing under 

 infested stubble. This applies especially to the joint worms of \\4ieat, rye and 

 barley and to the Hessian Fly. As these crops are often used as nurse crops for 

 clover the practicability of such a procedure is out of the question, unless a change 

 can be made in the ordinary rotation of crops. It would appear then as if a new 

 system of rotation should be devised by expert agronomists to meet the siiuation. 



We have frequently observed that many of our most conspicuous pests like 

 the Tent caterpillar and the Tussock Caterpillar appear in cycles separated by a 

 period of years. Their disappearance after serious outbreaks is attributed to the 

 work of parasites. But may there not be other factors operating? The matter has 

 never been thoroughly investigated in a scientific manner. Not until all the factor.^ 

 are known and studied can we hope to prevent the periodic outbreaks that do 

 much damage. 



Again, in this region, the Cherry-tree Tortrix was more abundant than usual 

 during July of 1919 on wild and choke cherries. An analysis of the situation 

 seems to point to the relative absence of its normal parasites as the main reason 

 for its abundance. 



