Green Mountain, Irish Cobbler, Early Ohio and Bliss Triumph in the order listed, the last named variety being the most susceptible to attack by the leafhoppers. The rate of development of hopperburn on individual leaflets was determined by the size and succulence of the leaf, and not by any inherent resistance. Instead, the prevalence of hopperburn on any one of the above varieties of potatoes was a matter of host selection1 by the ovipositing females. These preferred a rapidly growing succulent plant such as Bliss Triumph. Relative freedom from this disease depended upon comparatively slow and continuous growth coupled with abundance of leaf surface as exemplified by the variety Rural New Yorker. This variety usually showed a comparatively small amount of hopperburn when planted at a favorable date. First, because it did not attain a size suitable for oviposition early in the season when the majority of spring brood eggs were laid; second, because the leaves and stems were comparatively tough due to slow growth of the plant, this being another factor unfavorable for oviposition; third, its long growth period and tendency to branch out enabled the plants to put forth a continuous new growth for some time which often counterbalanced loss of leaf surface due to hopperburn; and fourth, tuber production by this variety came between the main bulk of the first and second broods of the leafhopper.

PART III-CONTROL

HISTORICAL

Osborn (45) in discussing Empoasca mali as a pest affecting potatoes described in detail the method used to control this pest at that time. Kerosene emulsion was the insecticide used and two applications were made a few days apart early in July. A home-made spray boom fitted for spraying four rows at a time was devised and arranged so that the insecticide was directed forward and a little downward. The effectiveness of the treatment was increased by suspending a narrow board from the rear of the wagon which was carrying the spray pump. This was hung low enough to brush over the tops of the four rows being sprayed and disturbed the plants causing the adults to fly into the midst of the spray and also exposed many of the nymphs which were on the underside of the leaves so that they were hit and killed. Good control of the pest was reported. Ball (3, 4, 5) who was associated with Osborn while the latter was conducting the above tests with kerosene emulsion for the control of this insect, recommended a 7 or 8 percent solu

Kotila, Michigan Technical Bulletin 56, 1922, states that ovipositing females prefer plants showing mosaic symptoms.

tion of this insecticide and also the use of nicotine sulfate diluted with soapy water or added to bordeaux mixture.

Fluke (23) tested out bordeaux1 and nicotine sulfate sprays for the control of this pest in Wisconsin. He found that in the plots treated with the former solution little hopperburn was present and few leafhoppers. On the other hand the nicotine spray failed to give relief from the disease. He suggested the possibility that bordeaux mixture repelled the leafhoppers. Parrot (49) tested out 4-4-50, 5-5-50, and 4-4-50 excess lime bordeaux mixtures as well as a china clay solution for the control of this insect in New York. All of these preparations gave protection but of them the lime-bordeaux and china clay solutions gave the best results. The former caused injury to the foliage while the latter washed off easily in rainy weather.

The senior author (17) tested out the effect of several bordeaux formulæ and a milk of lime mixture upon the ovipositing females. These preparations were used both with and without the addition of nicotine sulfate at the rate of 1 to 800 parts. In all cases very few eggs were laid in the sprayed leaves by the females. However, these had access to unsprayed leaves at that time and they showed a decided preference for oviposition in these. There was also the possibility of egg laying in the sprayed leaves, the ova not hatching owing to some ovicidal action of copper in the bordeaux. However, the fact that no nymphs hatched from plants treated with milk of lime solution seems to preclude this.

The authors (20) reported on more extensive tests in which the above experiments were repeated and in addition female leafhoppers were confined with plants completely covered with the spray. In the latter case a few eggs were apparently laid in sprayed leaves but only in those imperfectly covered with the spray or in the unfolding bud which was also not completely protected. Even under these conditions very few hatchings from sprayed leaves were recorded showing that the mixture either repelled the ovipositing females or acted as an ovicide. During this same year the writers reported on preliminary tests which indicated that bordeaux mixture was toxic to the nymphs but not to the adults.

SPRAYING EXPERIMENTS

Control experiments were started in 1919 but were not developed until 1920 when more was known about the life history of the insect. The following tests are described in chronological order to show the developments that took place from year to year as the different spray compounds were tried out. Three

14-4-50.

#23cc to 5 gals.

methods were used to determine the effectiveness of any insecticide as a control for the leafhopper, namely: comparative nymph counts, percentage of hopperburn, and yields. The first two were valuable when small plots were used, while the latter was relied on only when the tests were on a field basis.

SPRAYING TESTS FOR 1919

During 1919, owing to the fact that a small compressed air sprayer and a barrel pump were the only available spray outfits, the plots were necessarily small. Also some of these were located at the edge of a large potato field where they could be quickly reinfested after the insecticides were applied. The results on the whole were very unsatisfactory so far as control of the leafhopper was concerned, even when three sprays were applied at intervals of four to seven days respectively as in plot C (table XV). At most, but temporary relief was secured and counts made after spraying showed that enough nymphs survived to cause considerable burning.

SPRAYING TESTS FOR 1920

During 1920, at Ames, a total of 12 individual plots of about 100 plants each and separated from each other by several rows of corn were sprayed with whale-oil soap-nicotine sulfate solution (1-9-50), kerosene emulsion (6 percent) and bordeaux mixture-nicotine sulfate (4-4-50-9)2. The first two mixtures were applied once, twice, or three times respectively to different plots, while one plot was treated once with bordeaux, and two others twice. The spraying was done July 2, 16 and 28. A

TABLL XV.

SPRAYING TESTS FOR POTATO LEAFHOPPER CONTROL,
AMES, IOWA, 1919

11

lbs. whale-oil soap; 9 oz. nicotine sulfate; 50 gals. water. 29 oz. nicotine sulfate in 50 gals. bordeaux mixture.