436 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 31 



The variation in the beginning of the growth period from the 

 north to the south extreme of the United States amounts to several 

 weeks. For example, between southern Ohio and Illinois and the 

 northern boundaries of these States there is, in the instance of some 

 , crops, a difference of three weeks. As an illustration, let me cite the 

 instance of the imported cabbage worm and its Apanteles parasite in 

 Ohio. In the Muskingum Valley of that State early cabbage harvest 

 is begun by the 4th of July, whereas some cabbages are only being 

 set into the field in the north part at the same time. The chief 

 pest of cabbage there is the imported cabbage worm, whose very 

 efficient parasite was intentionally introduced from Europe many 

 years ago to check the worm. In the first two generations the worm 

 gradually develops injurious numbers, but the parasites also multiply 

 rapidly. By the time that the cabbage crop is removed the parasites 

 dominate the situation, but must soon be weakened in numbers again 

 because the host grows scarce due to parasitism and many thousands 

 of parasites die without reproducing. At this time the worm is 

 probably doing its worst damage 200 miles north. Valuable cab- 

 bages grown in the vicinity of large cities like Cleveland and Toledo 

 are probably being injured, or artificial control may be practiced. 

 Inasmuch as many thousands of the parasite may be gathered in a 

 few days at Marietta, it would seem feasible to ship or carry such 

 for release in the north parts to greatly supplement the work of the 

 individuals present there and perhaps prevent severe damage to the 

 crop and possibly avoid the extensive use of insecticides. 



Perhaps the most feasible mode of favoring parasites of insect 

 pests is to modify slightly the application of certain other insect 

 control measures. When a pest is known to possess one or more 

 effective parasites, wholesale slaughter of the hosts should be avoided 

 in order to permit the parasites to increase. For example, the large 

 green injurious tomato and tobacco horn worms are freely parasitized 

 by a small wasplike species whose mature larvae issue through the 

 back of the host and spin their white cocoons there. When such 

 cocoons begin to appear, many caterpillars could easily be assembled 

 and placed in a screened cage, from which the parasites can go forth 

 but which retains any moth that might have escaped the parasite. 

 Many other insects might be kept at a minimum in this manner or a 

 modification of it without excessive costs, if their parasites were 

 better known and this method of biological control were studied with 

 reference to them. Probably no other plan for the use of para- 

 sites has been more frequently suggested in earlier times. 



In spite of careful State and interstate inspection service to pre- 

 vent the spread of insect pests, some of these inevitably penetrate 

 into new territory. Trade within and between States has been 



