HANDBOOK OX INSECT ENEMIES OF FLOWERS AXD SHRUBS 37 



have been formed. Eventually the 

 borer tunnels downward in the branches 

 and finally reaches the pith cavity of the 

 main stem, causing the stem to break. 



Treatment. — On dahlia the young 

 caterpillar is susceptible to insecticides 

 at the time it feeds on the surface of 

 developing foliage and blossom buds 

 and before it tunnels deeper 'into the 

 pith of the main stem. The sprays 

 found effective for this purpose contain 

 either derris powder or phenothiazine. 

 These sprays may be made up in small 

 lots at the rate of 2 ounces, or 12 level 

 tablespoonfuls, of either derris or 

 phenothiazine to 3 gallons of water in 

 which a small amount of spreader, such 

 as -odium lauryl sulfate, has been 

 previously added. It is best to make up 

 a thin paste of each before adding it to 

 the spray water. Time the spray 

 applications to begin on the first sign of 

 wilting and repeat five or six times at 

 intervals of o days. Repeat the next 

 day after a heavy rainfall if this occurs 

 soon after spraying. 



Prevention. — Grow choice or valu- 

 able plants under cloth shelter. Prun- 

 ing and disbudding serve to remove some 

 of the borer population but do not pro- 

 vide protection for the remaining 

 blooms. Cut off the stalks close to the 

 crown of the plant to prevent the larvae 

 from hibernating over the winter in root 

 clumps. 



Cocklebur Billbus 



The cocklebur billbug (Rhodobaentis 

 tredecimpunctatus (111.), though of rare 

 occurrence on dahlia, has become im- 

 portant in some regions during the last 

 few years. It is a common pest of 

 cocklebur, ironweed, sunflower, joe-pye- 

 weed, leaf cup, thistle, ragweed, evening 

 primrose, and the seed stalks of sugar 

 beet, and has recently been reported on 

 chrysanthemum. The adult (fig. 60) is a 

 reddish weevil, or snout beetle, with 13 

 black spots on the back and a long 

 curved snout on the head, and is about 

 % inch long. The adults overwinter in 

 trash, appear in May or June, and lay 



Figure 60. — Cocklebur billbug: Adult. 

 About 3 times natural size. 



Figure 61. — Portion of dahlia stem 

 split to show tunneling by grubs of 

 the cocklebur billbug. 



eggs on the tender stalks. The eggs 

 hatch into white, legless grubs, which 

 bore into the pith and may hollow out 

 the -talk (fig. 61) for a foot or more near 

 the base of the plant. The weakened 

 stems break over easily. The grubs 

 pupate in August, and the beetles 

 emerge from the stems late in August 

 and early in September. 



Treatment. — Once the stems have 

 become infested, there is little that can 

 be done except to stake the plants well 

 to prevent their breaking, so that the 

 plants will not be entirely lost. Slitting 

 the stems and removing the grubs is 

 suggested where the infestation is light. 

 Hand-picking of the conspicuous beetles 

 is also useful. 



Prevention. — Since the adults de- 

 posit their eggs in May or early in June, 

 and since dahlias may be planted rather 

 late in the season for the production of 

 good plants, delaying the planting of 

 tubers, so that the young shoots will 

 not be present until after the beetles 

 have disappeared, will be helpful. 



Other Pesrs of Dahlia 



Page 



Yellow woolly bear 3 



Flower thrips 74 



Red spiders 11 



Asiatic garden beetle 



Green peach aphid 



