CONTROL MEASURES 



149 



keep the plantings freed from insect pests. Hogs, if allowed 

 tor oot, will clear a field of white grubs. Hop growers protect 

 the skunk because it so diligently searches out the hop borers 

 for food. A big ground beetle has been successfully imported 

 from Europe to help keep down the numbers of the gipsy 

 moth in New England. The importation of a ladybird beetle 

 into California once saved the citrus orchards from the 

 ravages of the cottony cushion scale. All these are predatory 

 enemies. There are parasitic ones as well; and these very 

 small ones are even 

 more important in re- ~^$«&&i 

 ducing the numbers 

 of injurious species. 

 How to best utilize 

 the aid of these para- 

 sites is one of the 

 live subjects of ento- 

 mological investiga- 

 tion at the present 

 time. 



10. Trap crops and 

 miscellaneous. — Trap 

 crops are sometimes 

 effective. For ex- 

 ample, early kale is 

 sometimes grown in 

 the South to attract 

 all the early harle- 



Fig. 57. — The work of Hymenopterus parasites. At <* 



t 



is shown a dead caterpillar, killed by parasites, some of 

 whictrhave spun their cocoons beside it, others, on the grass 

 blade above. At b is shown an easy way to get the adult 

 parasites : put the cocoons in a vial and stop its mouth with 

 a plug of cotton, and wait for them to appear. 



