26 MONTANA EXPERIMENT STATION BULLETIN 366 



ready to lay eggs. There was a very light second generation. 



None of the commonly recommended insecticides proved of 

 much value in controlling the worms when they were present 

 in large numbers. 



As a rule only 2 to 5 per cent of the first generation enter 

 the prepupal stage, but a survey made this year showed that 

 from 45 to 50 per cent of the first generation entered this stage. 

 The reason for this large percentage entering the prepupal stage 

 is not known. However, many European workers have noted that 

 such a condition generally precedes a severe outbreak. On this 

 basis, providing conditions are favorable for the production of 

 a high percentage of fertility among females in the spring of 

 1939, a rather severe outbreak of this insect may be expected. 



CUCUMBER BEETLE 



(Diabrotica vittata Fab.) 



This species has long been recognized as the worst pest of 

 cucurbits throughout America east of the Rocky Mountains. A 

 few specimens were collected in 1932 from Custer County. 

 During the 1938 season the beetles were very abundant in the 

 truck-farming areas of Yellowstone County. They were also 

 sent in for identification from Daniels and Roosevelt counties. 



The adult beetles are about 1/5 inch long, with three black 

 stripes down the back separated by wider stripes of bright 

 yellow. They eat irregular holes in the leaves and flowers of 

 cucumbers, squash, melons, and pumpkins. The larvae live in 

 the soil and may damage the roots, although this type of injury 

 has not been noted in the Montana infestations. In some parts 

 of the country the adults carry a serious disease of cucurbits 

 called bacterial wilt. Up to the present this disease has not ap- 

 peared in association with the beetles in Montana. 



Cucumber plants are attacked as soon as they appear above 

 ground by beetles which have overwintered in trash. The most 

 successful control is a repellent and poisonous dust consisting 

 of one part calcium arsenate and twenty parts burned gypsum 

 or land plaster which should be thoroughly mixed and dusted 

 over the plants. The plants should be kept covered with this 

 dust from the time they appear above ground in the spring 

 until the first generation of beetles has disappeared. 



