2 BULLETIN 828, U. S. DEPARTMENT OF AGRICULTURE. 



SUMMER TRANSMISSION OF WILT. 



DIRECT INSECT TRANSMISSION EXPERIMENTS. 



The earlier experiments on insect transfer of the wilt disease have 

 been repeated many times during the past three years (1916-1918) 

 with results similar to' those already published. As an example of 

 the method used the following details may be given: 



September 17, 1916. Twelve squash bugs (Anasa tristis De G.) were fed four days upon 

 cucumber plants wilted from pure culture inoculation. Two bugs were then caged 

 (PI. I) with each of six healthy cucumber plants for two days, during which time they 

 were all observed to have fed. The plants at this time had become slightly flabby as 

 a direct result of the insect feeding. However, this condition practically always 

 results from squash-bug injury to young cucumber plants and is in no way connected 

 with bacterial wilt. Cultures and microscopical examination in such cases fail to 

 show bacteria present; the flab bin ess often begins to appear within a few hours after 

 the bugs start feeding, while wilt at the very earliest does not appear sooner than three 

 to five days after inoculation; this flabbiness at once affects the plant as a whole, 

 while in the bacterial disease the wilting is progressive from the point of inoculation ; 

 and finally, unless too far gone, plants always recover their turgor after the squash bugs 

 are removed, while vigorous young cucumber plants inoculated with virulent strains 

 of bacterial wilt have never been known to recover. In this experiment the squash 

 bugs were all removed after two days and the plants soon regained their turgor. Al- 

 though under observation until October 13, no wilt developed. This experiment was 

 repeated many times with like results. 



The squash lady bird (Epilachna borealis Fab.) was tested in 10 or more further sets of 

 experiments, but no wilt ever followed its injuries to healthy cucumber plants, and 

 negative results were obtained by inoculation with intestines of wilt-fed individuals 

 (see p. 24). 



The cotton aphis (Aphis gossypii Glov.) and the potato flea-beetle (Epitrix cucumeris 

 Harr.) were also retested by similar methods and the negative results confirmed by 

 observations in the experimental fields. Furthermore, in the field cage experiments 

 they have always had access through the wire netting to the cucurbit plants within, 

 but have never carried the disease from the numerous wilt cases in the surrounding 

 field. These observations corroborate the results of the direct experiments. 



During August, 1917, honeybees (Apis mellifera L.) were collected at random from the 

 experimental cucurbit field where wilt was prevalent, and many were taken directly 

 from blossoms of wilting vines. One to several bees were placed in 15 large cages 

 containing cucumber or cantaloupe vines (PI. II, fig. 1). No wilt followed in any 

 case. It might be noted in passing that the cucumber fruit set very much more freely 

 in these cages than where bees were excluded. 



These five different species have all given constantly negative 

 results, while the striped and 12-spotted cucumber beetles tested in 

 the same way have repeatedly given positive results. For example, 

 using the striped beetle in 10 direct experiments (1916 and 1917) 

 similar to the one detailed above for the squash bug, 7 out of the 10 

 tests gave positive results. In these cases the beetles were fed upon 

 wilted leaves and then caged with cucumber plants for a sufficient 

 length of time to determine the result. These experiments were 

 entirely separate from the successive infection experiments detailed 

 later (cf. p. 21, 22). The 12-spotted cucumber beetles have not 



