78 



FRUIT INSECTS 



through them. Like many other msect pests, canker-worms 

 have their '^ ups and downs," or periods of increase and de- 

 crease. Usually their destructive period lasts from 3 to 5 

 years or more before their enemies and climatic conditions 

 succeed in bringing about the ''down" period, which may last 

 10 or more years. The defoliated trees grow weaker each year, 



Fig. 



80. — Canker-worm moths caught on sticky band 



trunk. 



hile ascending tree 



mature but little fruit, and may finally succumb to the ravages 

 of the hordes of canker-worms. If disturbed, the caterpillars 

 often cling with their hind pro-legs and stand out straight, stiff 

 and motionless, this strange attitude giving them a protective 

 resemblance to the smaller twigs or leaf -stems ; or they may 

 drop suddenly and swing suspended in the air by a silken cord 

 until the threatened danger has apparently passed, when they 



