April, '15] 



McCONNELL: UNIQUE TYPE OF INSECT INJURY 



265 



destroyed before they are half grown. Later, nodules develop on 

 lateral roots and nearly all of these may be found and destroyed when 

 quite small. One larva is capable of destroying a considerable num- 

 ber of nodules. When the attack is delayed until the nodules have 

 reached their full size, the destruction may be as complete but the 

 damage is probably not as great. In any case a serious outbreak of 

 the beetle means that the nodules have little opportunity for per- 

 forming their , function of collecting atmospheric nitrogen. This 

 means that the plant will not be able through the nodules to store 

 up nitrogen later to enrich the soil. While it is true that cowpeas 

 will make a good growth in rich bottom soil irrespective of damage to 

 ,the nodules, it is in the poorer hill soils that the loss of the nodules 

 causes a marked decrease in the size of the plants,- and it is in these 

 soils deficient in nitrogen that the nitrogen-fixing ability of the nodules 

 is most valuable. 



A correct determination of the loss in this manner from the attacks 

 of the larvse involves a knowledge of the nodule-forming bacteria 

 themselves and a chemical analysis of the plants. Consequently, 

 cooperative experiments with the Bureau of Plant Industry have 

 been arranged, and these are still in progress. For the present, an 

 idea of the possibilities may be gained from what is known of the value 

 of legumes in soil renovation. Mr. Lewis T. Leonard, of the Bureau 

 of Plant Industry, who is in charge of the cooperative work on this 

 insect, has kindly supplied a statement, as follows: "Professor Bot- 

 tomley of England has estimated from German and United States 

 reports that on an average the legume nodule adds 166 pounds of 

 nitrogen per acre to the soil (166 pounds of nitrogen is equivalent to 

 about 1,844 pounds of sodium nitrate)." He also has computed from 

 reports in his office "that in the case of the cowpea the average value 

 of the nodules to the plant is 140 per cent of the crop without the 

 nodules, that is, by inoculation the crop is more than doubled." The 

 importance of the loss of the nodules on a poor soil will readily be seen. 



To this damage must also be added that caused by the larvse to 

 the roots. In addition it must be borne in mind that the adults are 

 capable of destroying a stand of young cowpeas. Ordinarily, after 

 cowpeas have made a good growth, the damage by the adults is not 

 serious. 



Suggestions for Control 



The possibilities for the successful control of this insect on a crop 

 of cowpeas are limited mainly to preventive measures. Since, as 

 Weed first pointed out,^ the first generation is in the main produced 

 upon garden beans, it would appear that a campaign of thorough 

 spraying with arsenicals against the adults as soon as they appear 



