water-impermeable covers were not used for this pur- 

 pose, for they would liave promoted the formation of 

 condensed water on the leaf. Covers or caps made from 

 textile, cardboard, or paper, including filter paper, 

 proved satisfactory. 



An insecticide or insect pathogen could be sprayed 

 or dusted on the surface of the corn leaf either before 

 or after the edges had been treated. In tests involving 

 insecticides or pathogens, 10 first instar borers were 

 placed, with the help of a wet camel's-hair brush, on 

 the floating leaf or preferably on the top of the glass 

 rod. Diffused light or darkness prevented phototropic 

 responses in the young larvae and helped to reduce the 

 length of a period of wandering during which larvae 

 occasionally got into the water. The length of this period 

 of wandering was reduced also by pollen on the leaf. If 

 pollen was not available, finely ground corn meal, 80- 

 100 mesh, served the purpose. Pollen or corn meal was 

 deposited on the leaf with a small sieve. Small pieces 

 of thin polyethylene film (freezer bag material) or tissue 

 paper placed on the leaf over the larvae appeared to 

 satisfy the thigmotactic responses of the young larvae. 



Most of the larvae started to feed in about 6 to 24 

 hours. Their behavior upon treatment was easily ob- 

 served. When fast-acting insecticides or pathogens were 

 used, the larvae fed for only a short time and then wan- 

 dered around aimlessly until eventually they drowned 

 in the water. The number of drowned larvae served as 

 an indication of the effectiveness of the treatment. In 

 untreated checks or in tests where slow-acting ingredi- 

 ents were used, larvae could be reared on floating 

 leaves until the second instar. 



The conditions prevailing in the whorl of a young 

 corn plant could, with fair success, be duplicated by 

 still another experimental device. For this device, 

 20 X 90 mm. double-strength cellulose extraction thim- 

 bles were selected. Each extraction thimble, fig. 7, was 

 reinforced with lacquer to preserve its original shape 

 when it was exposed to high humidities. The thimble 

 was closed with a softened cork stopper wrapped in 

 tinfoil. 



As a support for the corn leaf, a coil of about 10 

 turns was made from enameled copper wire. (Stainless 

 steel or aluminum wire would have been preferable.) 

 The coil was about 2-1/4 inches long, constricted at 

 the lower end, and of such diameter that it would fit 

 loosely into the thimble. One or two pieces of corn leaf 

 cut transversely into 4-inch lengths were rolled up and 

 slipped inside the coil. The coil was then inserted into 

 the thimble, first instar corn borer larvae were trans- 

 ferred to the corn leaf, and tlie mouth of the thimble was 

 closed with a cork stopper. The thimble was then sus- 

 pended by its cork stopper and a wire hook in a 25 x 

 150 mm. culture lube. In tiie bottom of this culture tube. 



a 1-inch level of water or saturated salt solution was 

 kept in order to help maintain the required degree of 

 humidity. The culture tube was covered with an alumi- 

 num cap. 



It was found that almost no condensed moisture 

 accumulated inside the extraction thimble and that the 

 corn leaf remained fresh up to 8 days. The wire coil 

 prevented the leaf from touching the side of the thimble. 

 The corn borer larvae fed on the leaf in such a way as 

 to make characteristic feeding patterns. They could be 

 reared in this manner to at least the third instar. When 

 the time arrived for checking the larvae, the coil was 

 lifted out of the thimble, and the leaf was removed from 

 the coil and unrolled. This type of handling apparently 

 did not disturb the larvae. 



Each leaf could be treated with an insecticide or 

 pathogen on one or both surfaces before it was placed 

 in the coil. Ry using either the floating leaf technique 

 or the extraction thimble technique, it was possible to 

 estimate the area of leaf surface, and thus the amount 

 of active ingredient, consumed by young larvae, which 

 tended to remain in the same place for a considerable 

 time after they had started to feed. The area of feeding 

 could be measured by the grid method or by a planim- 

 eter, if the pressed and dried leaf displaying the feed- 

 ing marks was placed in a photographic enlarger and 

 the image was enlarged to a suitable size. The leaf 

 itself could be filed in an envelope. 



ADAPTATIONS OF CONTINUOUS 

 MASS REARING TECHNIQUE 



The method of rearing European corn borer larvae 

 and the experimental techniques described in the pre- 

 ceding paragraphs were designed to satisfy primarily 

 the requirements of research in the field of biological 

 control, P>oth the corn borers reared by this method and 

 the techniques described might have wider use. 



Since it was found possible to rear 8 to 10 succes- 

 sive generations of the E.uropean corn borer in the lab- 

 oratory under controlled conditions, this insect might 

 be used as a standard experimental subject. 



The split bean technique of testing insecticides 

 might be adopted for use in a study of systemic insec- 

 ticides. If a systemic insecticide is applied to the foli- 

 age or stems of a bean plant grown in a greenhouse, or 

 to the soil in which the plant is grown, the amount of 

 translocated insecticide in the pods might be evaluated 

 with a fair degree of accuracy by subjecting the pods 

 to feeding by corn borer larvae. 



As it is easy to rear nymphs or even adults of the 

 potato leafhopper, Fmpoasca fabae (Harr.) on whole or 

 sectioned green bean pods, experiments could be de- 

 signed to test an insecticide against a chewing insect 



10 



