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STATE BOARD OF' AGRICULTURE. 



under most conditions this method is impracticable. Another remedial measure is to 

 apply a tonic in the form of one of the concentrated fertilizers. Nitrate of soda, 

 muriate of potash, kainit, or some other like fertilizer to help tide the plants over the 

 strain of supporting a number of plant lice. 



Fig. 13. — Parasite of Grain Aphis. Aphidius granariaphis. Greatly enlarged. Original. 



Few better examples of the ups and downs of insect life could be obtained. The 

 explanation in a general way, is very simple. During a favorable year, the lice 

 multiply until they become very numerous, resulting in an outbreak. This continues for 

 a time until the parasites, a few of which have managed to exist in isolated places, 

 breed up sufficiently to overcome the lice. The parasites do not die out entirely although 

 they often become so few that it takes them some time to multiply sufficiently to 

 become effective, when food in the form of lice is provided. The parasites, after killing 

 off most of the lice, die for want of food, only a few are able to find suitable lice to eat. 

 For the parasites, it is a time of scarcity, they worry along in small numbers until 

 suitable weather conditions as well as their own reduced numbers, allow the lice again 

 to become plentiful. This constant warfare between the lice and their parasites results 

 in constantly recurring waves of invasion. The distance between the waves usually being 

 several years. This principle holds true with most insects, though in few cases is the 

 destruction of the host so complete as in the case of the grain louse. 



