36 BULLETIN 746, U. S. DEPARTMENT OF AGRICULTURE. 



shows that the abundance of Diatraea is in inverse proportion to the amount 

 of rainfall. (The table mentioned shows that the infestation varies from 6 per 

 cent, where there were 101 inches of rain, to 66 per cent, where there were 

 only 21 inches.) . . . 



It is comparatively easy to demonstrate the effect of an abundance of rain- 

 full in lessening the numbers of Diatraea, but much more difficult satisfactorily 

 to account for this effect. The eggs of the borer are deposited on the leaves 

 of the cane, and when the young larvae hatch, a considerable interval elapses 

 while they crawl about on the cane before they enter the stalk or the midrib 

 of the leaf. It is quite probable that this is one of the most crucial periods 

 in its life history, and that considerable numbers of borer larva? were killed 

 in young cane by the more rapid growth of the central shoot of a cane plant 

 than of the outer leaves. Also, larva? were found which had been drowned in 

 a mixture of water and decaying cane juices which had collected in their tunnels 

 after rains. To avoid danger from these causes, many larva? were found living 

 outside the shoot, where they were exposed to the attacks of predators or 

 parasites. 



The rainfall over the sugar belt of Louisiana does not vary to any 

 great extent, and while there are annual differences both in infesta- 

 tion and rainfall at various points, a careful study of the subject has 

 failed to prove that these variations correspond as they do in Porto 

 Rico. 



Below is given the infestation by the moth borer at various places 

 in 1916 as compared with the annual rainfall. 



Infestation by the moth borer compared icith annual rainfall at various places 



in Louisiana. 



Canes 



Annual infested 



rainfall. in fall. 



Place. Inches. Per cent. 



Donaldsonville 61. 51 91 



White Castle • 50 to 60 99 



Thibodeaux 50 to 60 79 



Mathews 50 to 60 97 



Palmetto 50 to 60 37 * 



Franklin 58. 89 53. 5 



New Orleans-- 55.37 38 



Napoleonville 54. 96 95 



Lafayette 54. 68 99 



Melville 51. 98 22 



Abbeville 50. 39 98 



Morgan City 1 47. 69 93. 5 



The period during which the growing cane is attacked by the borer 

 in Louisiana is, roughly, from the month of April to the month of 

 October, inclusive. A graphic comparison of the rainfalls during 

 these months for the years 1912, 1913, 1914, 1915, and 1916 with the 

 average infestations by the borer in the fall of each year is shown 

 in figure 10. In some years the line representing infestation descends 

 as the line representing rainfall ascends, but there were exceptions, 

 especially in 1916, which the authors can not explain. Mr. Wolcott 

 suggests that the small variation in the rainfall at different points 



1 Probably flooded. 



