44 Guayule. 



THE GUAYULE BORER. 



In the fall of 1907 it was noticed that guayule in the stack (plate 

 4, fig. A), awaiting treatment for the extraction of the rubber, was being 

 attacked by an insect, the only signs of which were the finely-powdered 

 debris escaping from minute, circular openings in the bark. It was at 

 once evident that a borer of some kind was at work. Material was sent 

 to Dr. L. O. Howard, who kindly referred the matter to Dr. A. D. Hopkins, 

 in charge of forest insect investigations, Bureau of Entomology, U. S. 

 Department of Agriculture, to whom I am indebted for the accompanying 

 notes and drawings (fig. 6, p. 43). Dr. Hopkins writes as follows: 



The beetle is Pityophthorus nigricans Bland. It has also been reported to the 

 Bureau of Entomology by H. Pittier, who found it injuring the same plant at Tor- 

 reon, Coahuila, Mexico. The insect is of special interest because of its habit of attack- 

 ing a plant of such commercial value, and on account of its being the largest repre- 

 sentative of the division of the genus to which it belongs. Those of one division 

 infest coniferous trees only, while those of the other, to which this species belongs, 

 infest only the broad-leaved plants and trees. The guayule barkbeetle evidently 

 attacks the plant after it is dead, or soon after it has been cut, and, as has been 

 shown by the specimens in the forest-insect collection of the Bureau of Entomology, 

 may continue to breed in the same bark and wood for several years. It is evident 

 that the prompt utilization of the plant for the manufacture of rubber within a few 

 days after it is cut would prevent all losses from this source. 



Inasmuch as the buyers of shrub sometimes accumulate large quan- 

 tities and place it in stacks until needed, and as this may represent large 

 investments, the amount of damage may represent no inconsiderable 

 loss. In order to determine what this loss might amount to, a piece of 

 stem of average thickness which had been attacked by the borer was 

 weighed as a whole. It was then decorticated and the insect debris was 

 carefully removed. Some of the debris had of course been lost, and thus 

 an error is introduced into the calculation of fully 5 per cent of the total 

 weight of the bark. The tunneling done by the insect was not complete, 

 however, and for this reason the figures may be regarded as the average 

 result of the damage which may occur in the space of a month or two. 



Table 19. 



mm. ! grams 



Length of sample piece of stem ....25. Weight of the whole 3 . 80 1 



Diameter of the wood 9.8 ' Weight of wood cleaned of debris . 1.703 



Thickness of the bark 2.3 Weight of bark cleaned of debris . 1 . 903 



Total diameter of the stem 14.4 



Weight of material lost (with 



probable correction) 0.2 



Of which half is bark, viz o . 1 



It can be mathematically shown that the amount of destruction in 

 the smaller twigs in which the insects work may amount to very con- 

 siderably more, indeed to the extent of 40 per cent of the volume of the 

 bark (cortex). Inasmuch as the bark contains practically all the rubber, 

 it is seen that the loss may be great enough to warrant serious considera- 

 tion. It must be observed, however, that the comminution of the cor- 

 tical tissues by the beetle does not diminish the amount of rubber in 

 the stem except by the amount that happens to escape through the en- 

 trances, so that the real question is, whether the comminution of the cor- 



