plants are dying rapidly and the adults migrate to fields that are still green or to late varieties.

Field counts of the number of young hatching each day were made during the summers of 1919-20-21 (figs. 3, 4). Six branches of approximately the same size, from as many hills, were labelled and the number of nymphs noted and killed daily. A total of 1,4471 young were counted in 1919, as compared with 1,485 in 1920 and 2,154 in 1921. The first figure represents a population of about 10,000,000 per acre. The maximum hatched from a single branch in 24 hours was 121. The length of the nymphal season is also graphically shown by the curves. For instance, in 1921 the period extended from the middle of June until the middle of July for the first brood and from the forepart of August until the third week in September for the second brood. It follows from this and observations by others that the potato leafhopper is found in some stages on potatoes from the time the plants appear above ground until the vines die in the fall.

1Second brood only.

NATURAL ENEMIES

PREDATORS

The potato leafhopper has comparatively few natural enemies of importance so that biological control cannot be depended upon to keep the insect in check. Among the predators, several species of spiders were observed capturing all stages and sometimes seriously interfering with cage experiments. Occasionally the common brown ant1 was seen carrying an individual to its nest. Anthocorids, pentatomids and assassin-bugs also contributed a share in natural control. By far the most voracious, however, are chrysopid and coccinellid larvæ. A number of experiments were conducted with different predaceous insects to determine their role in relation to E. mali. A chrysopid3 larva consumed 23 nymphs and two adults in five days, while during the same period a coccinellid larva ate ten nymphs.

PARASITES

The eggs were parasitized by Anagrus armatus Ashm. This species (plate II, fig. C) occurred in considerable numbers during July and August, 1919, and again in 1922. In some cases 40 percent of the eggs were parasitized so that this was the most important natural check observed. This para

site was reared from material collected at Ames and St. Ansgar and is probably the same as found in Wisconsin by Ball (3). A dryinid parasite of the nymph has been rarely observed by other writers but never reared.

DISEASE

During August, 1919, a fungous disease of adults and nymphs was very common thruout the state, especially at St. Ansgar. Counts made in the field disclosed a mortality of 37 percent. Of these attacked 80 percent were nymphs. The disease appeared after a period of moist, warm weather. Diseased adults were quite conspicuous as the wings were partly expanded and covered with a heavy fungous growth, which was iridescent in appearance. After death the body turned yellow and on its surface appeared a glistening mass of conidiophores and conidia.

Lasius brunneus.

2Triphleps insidiosus.

3Chrysopa plorabunda. The chrysopid larva, Aug. 15. 1919, was observed eating a nymph of the fifth instar which it had captured at 3:28 P. M. (standard time), by inserting its mandibles along the suture between the thorax and abdomen. The victim was held securely in position by the right fore leg while the left pressed firmly on its head to keep it from struggling. The legs of the nymph were next bitten off and the larva continued eating between the head and prothorax, severing the head from the body at 3:57 and eating the vicera. By 4:06 nothing was left but the legs and the hollow abdomen. The whole process required about 26 minutes. Hippodamia 13-punctata.

Entomophthora sphaerosperma Fresenius.

PART II. HOPPERBURN

HISTORICAL

In 1896 Osborn (45), then entomologist for the Iowa Agricultural Experiment Station, discussed Empoasca mali under the caption, "A New Pest of Potatoes. His observations concerning this species were significant and interesting. He said:

* * *It is a minute greenish insect so nearly the color of the leaves on which it rests that it easily escapes observation until the withering of the leaves indicates its presence. "The injuries of this pest were first called to my attention in the latter part of June by Mr. Kegley, of Ames, who had a small patch of early potatoes badly injured by them, but a few days later they were also found plentiful in the college garden, causing serious wilting of some of the early varieties and more or less plentiful over several acres of potato crop."

However, no one followed up Osborn's findings until 1918 when Ball (2, 3, 4, 5) definitely proved that Empoasca mali LeB. was the cause of potato tipburn. Previous to Ball's findings it was commonly assumed that this disease was due to hot, dry summer weather and was non-parasitic in origin. It was believed that such unfavorable climatic conditions accelerated leaf transpiration of the potato plant to such an extent that water was lost more rapidly than it could be supplied by the roots. One of the several inconsistencies of this theory was the well-established fact that while bordeaux spraying was known to control this disease, this fungicide actually accelerated leaf transpiration.

Jones (35) described a "new potato disease" which he stated was distinct from blight'. Parts of this early description conform to what is now known about hopperburn. Sturgis (50) referred to what was probably this disease in a discussion of early blight of potatoes. He believed that "extreme heat and dryness, accentuated by lack of moisture in the soil, may induce the death of tissues at a distance from the roots (the tips and edges of leaflets); that this damage may be increased and even initiated by the attacks of predatory insects (flea-beetles)".' Jones (36) gave the name "tip-burn" to this "new" disease which he separated from early blight, with which it had previously been confused. From his description and figure of the disease, it is apparent that he was dealing with hopperburn. Goff (31) noted the "resistance" of Rural New Yorker and Green Mountain potatoes to this disease and stated that in Wisconsin the older varieties were more easily affected. Parrott (48) described an outbreak of Empoasca mali in nurseries

1Early and late blight of potato were not differentiated at that time. 2Phytophagous.

Identified by Jones as tipburn.

in New York and also recorded this species as being seriously injurious to potatoes there. He was the first to observe severe infestation of leafhoppers and destructive outbreak of "tipburn' in the same field, but he did not connect the two.

Ball (2) recorded experiments in which a leaf-burn of potato leaves was produced in four days' time by introducing large numbers of leafhoppers of the species Empoasco mali into cages over potato plants. In field observations he noted that this injury was always directly proportional to the number of leafhoppers present on the vines. He said, "the relation of this injury to what has been previously diagnosed as 'tipburn' is an interesting subject for future determination." He suggested the possibility that there might be a pathological relation between the potato leafhopper and this leaf-burn, but not "the same specific relation as exists between the beet leafhopper and curlyleaf of beets." Again Ball (3, 4, 5) gave a definite description of this disease which he named "hopperburn" and stated that there were no doubt many other causes that will produce burning of the foliage. ."-and mentioned the fact that some other leaf burns were distinct from this in that

they avoided " . . the midrib and veinlets, while hopperburn appeared just on the veinlets and spread later to the area of membrane which they supplied." In this paper he also described in detail experiments and observations which proved his conclusions that tipburn was caused by the leafhopper. Ball and Fenton (6) suggested the substitution of the name hopperburn for potato tipburn because this name " .. covers practically all that has formerly been designated as tipburn on this plant."

Fenton (17) stated that all nymphal stages of the leafhopper were capable of producing symptoms of the disease, that the adult insect was not as effective in this respect as the older nymphs, that atmospheric or soil conditions had little direct influence on the severity of the disease unless E. mali was present, and that hopperburn was not systemic, but rather localized, being confined to that part of the plant exposed to leafhopper attack. Dudley and Wilson (12), in a series of experiments in which Triumph, Early Ohio, Irish Cobbler, Green Mountain and Rural New Yorker varieties of potatoes were used, found them to vary in "resistance" to this disease in the order named, the first being the most susceptible and the last the most "resistant." They also reported that this burning was much more severe on vines grown from tubers produced by hopperburn-susceptible plants than from those that were comparatively free from this malady. Thus, according to them, poor seed usually predisposed the plant for severe infestation by leafhoppers and invariably these vines suffered more from hopperburn.

Fenton and Ressler (21, 22) were able to produce a leaf necrosis that was very similar to hopperburn on potato by injecting leafhopper extract into healthy leaves. Inoculum prepared from macerated adults produced more injury than that from the nymphs in these tests. Eyer (15, 16) produced hopperburn on potato leaves by inoculating healthy plants with solutions prepared by macerating either nymphs or adults of this species in dilute alcohol or water. Burning was more severe on those leaves injected with inocula prepared from the nymphs. He was also able to produce typical burning by injecting healthy leaves with solutions prepared from hopperburned tissues. He stated that the action of the sun's rays was necessary for the browning of the leaves after the yellowing had been produced by inoculation. Inocula prepared by macerating potato aphids (Macrosiphum solanifolii Ashm.), tarnished plant bugs (Lygus pratensis Linn.), or apple leafhoppers (Empoasca unicolor Gill.) gave negative results. When potato leaves were injected with the solution prepared by macerating false chinch bugs (Nysius ericae Schill.), injury resulted which, however, did not resemble hopperburn.

DESCRIPTION OF HOPPERBURN

Hopperburn always begins at the margin or tip of the leaflet and is correlated with the principal veins. The first symptoms are usually noticed at the tip of the apical leaflet on the lower and older leaves of the plant (plate 1, fig. A). At first the disease is distinguished as a small triangular or diamond-shaped discolored area, centered in the midrib, which is seen to be somewhat crinkled and distorted for a short distance beyond the affected portion. Within this infected area, the tissue is of a yellowish green color and is usually more or less flaccid. Later the green tint entirely disappears, giving way to a deep yellow which rapidly changes to brown. At this stage the burned tissue is very fragile and brittle. While these changes are taking place, other affected areas appear along the margin usually triangular in outline and centered in a lateral veinlet which is invariably seen to be crinkled and collapsed beyond the immediate diseased area. As the injury progresses, these portions coalesce, involving the entire periphery of the leaf (plate 1, fig. B). Coincident with this, the margin begins to roll upward and inward. In extreme cases the entire leaflet is flilled (fig. 5), but often a central area around the midrib, especially near the petiole, still retains its green color (plate 1 fig. C). The leaves then present a characteristic linear appearance. When the insect infestation is heavy, the entire plant is killed in a comparatively short time. The exact progress of the disease as outlined