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Journal of Agricultural Research 
Vol. XXXI, No. 
DIABROTICA BEETLES 
Preliminary experiments with the striped and 12-spotted cucumber 
beetles, Diabrotica vittata and D. 12-punctata, indicate that these 
insects do not feed as readily on the milkweed as do the cucumber 
aphids. Both species of beetles, however, are found on the milk¬ 
weed to a slight extent during the early summer, when cucurbits are 
not available as food. When caged with milkweed plants, both 
species attacked the plants to a certain extent, although the 12- 
spotted beetles feed more readily on the milkweed than the striped 
species. When striped beetles have been placed in cages containing 
mosaic milkweed and healthy cucumber plants, the cucumber plants 
in a few cases have become mosaic diseased, indicating that the 
beetle is capable of transmitting mosaic from the milkweed to the 
cucumber. The evidence to date is rather too meager to justify 
definite conclusions, but it seems probable that the beetles may 
occasionally carry infection from the milkweed to the cucumber. 
It is doubtful, however, whether they are of much importance as 
compared with the aphis as carriers of the disease from this host. 
No insects other than those mentioned above have been studied in 
relation to the transmission of milkweed mosaic. 
Occurrence of Mosaic Milkweeds in the Field 
As the cucumber aphis seems to be the chief agency by which 
mosaic is transmitted from the milkweed to the cucurbits, it is 
probable that the mosaic milkweeds which occur in the immediate 
neighborhood of the fields are the only ones of importance as sources 
of primary infection. This belief is further strengthened by the fact 
that most cucumber aphids found on the milkweed have originally 
come from adjacent cucumbers and that these insects are compara¬ 
tively limited in their flight. 
Field observations during 1921 showed that mosaic milkweeds are 
frequently found in close proximity to fields of cucumbers, and also 
that such plants are rarely found in more distant locations. The 
field studies have been made principally at Madison and Rockland, 
Wis., and at Marengo, Ill. The surveys in these localities were 
confined to areas of from 1 to 3 square miles in which there were a 
number of cucumber fields. All the milkweeds which could be 
found in these areas during June and July were examined for evidences 
of mosaic. Later surveys were also made in the neighborhood of 
cucumber fields which developed mosaic. 
At Madison only four groups of mosaic milkweed plants were 
found. The largest of these, already mentioned as occurring in the 
Olin plat, consisted of 27 plants, all of which were between rows of 
cucumbers. Approximately 100 milkweeds within 50 yards of the 
plat were examined, but no mosaic plants were found outside the 
cucumber plat. Four mosaic milkweed plants were found between 
the rows of cucumbers in another plat about a mile from the Olin 
field, and in this case also all milkweeds outside of the plat seemed 
free from the disease. Two other small groups of milkweeds showing 
mosaic were found in gardens in the neighborhood, both of which 
contained mosaic cucumber plants. Approximately 300 other 
milkweeds examined along roadsides and in open fields in the vicinity 
were all found to be healthy, so that up to the present the only mosaic 
milkweeds found at Madison have been in fields planted to cucurbits 
for several seasons. 
