Hemiptera.—Lygus pratensis Linn., Adelphocoris rapidus Say,
Coleoptera.—Coccinella transversoguttata Fabr.
Lepidoptera.—Eurymus eurytheme Bdv., Chrysophanus sp., Lycaena (2 species),. Libythea bachmani Kirtland, Pieris rapae Linn.
Hymenoptera.—Angochlora sp., Apis mellifica Linn., Colletes sp., Halictus lerouxi Lep., H. provancheri D. J., Halictus sp., Elis sp., Calliopsis andreniformis Smith, Polistes sp., Sphex sp., Eumenes fraterna Say, Sceliphron sp., Isodontia harrisi, Fern., Cerceris sp., Oxybelus sp.
Diptera.—Syritta sp., Paragus sp., Chrysomyia macellaria Desv., Syrphidæ (2 unidentified specimens).
EFFECT OF MOISTURE UPON THE PRODUCTION OF MELILOTUS ALBA SEED.
Careful inspection of a number of sweet-clover fields in Iowa and Illinois in the autumn of 1916 indicated that many plants were unable to obtain sufficient moisture for the proper development of their flowers. Examination of flowers that aborted shortly after reaching their mature size showed that the anther sacs had not burst, even though the pollen grains were mature. Apparently for the same reason many immature pods aborted. The precipitation for July, 1916, in Livingston County, Ill., where the sweet-clover seed crop suffered materially for lack of moisture, was 3.2 inches less than normal, while the temperature was 4.5° F. above normal. In August the precipitation was 0.96 of an inch below normal and the temperature 4.2° F. above normal. At Ames, Iowa, the precipitation was 3.54 inches below normal and the temperature 5.4° F. above normal in July. Both the precipitation and temperature were about normal at Ames in August, but most of the precipitation fell before the experiments were commenced.
In north-central Illinois the seed production of sweet clover was very irregular. Some fields produced an abundance of seed, while a large percentage of the pods on the plants in other fields near by, where the thickness of the stand, size of the plants, and conditions in general were approximately the same, aborted. It was evident that all stands producing a good seed crop were growing on well-drained soil and that those which were not yielding satisfactorily were on poorly drained land. It is well known that sweet clover will produce deep taproots only when the plants are growing in well-drained soil and that a much-branched surface root system will be formed on poorly drained land, and especially when there is an excess of moisture or a high water table during the first season's growth. During this droughty period in 1916 the upper layer of soil became so depleted of moisture that the plants with surface root systems were unable to obtain sufficient water to mature their seed. On the other hand, the lack of precipitation and the high temperatures did not affect the moisture content of the subsoil sufficiently to interfere with the normal seed production of deep-rooted plants. According to Lutts ([22], p. 47) this same condition was found to be true in Ohio in 1916.
As a rule, under droughty conditions the second crop of sweet clover will produce a higher yield of seed than the first crop, as the second growth of the plants is seldom more than half as much as the first, thereby requiring less moisture. However, if showery hot weather prevails when the first crop is cut, the end of each stub is very apt to become infected, usually with a species of Fusarium, which kills all the cortex as far back as the upper bud or young shoot and that part of it on the opposite side of this bud to the bud below. If the second bud from the top of a stub is not directly opposite the upper one the decay may extend nearly to the ground. ([Pl. IV.]) The destruction of half to two-thirds of the cortex from 2 to 4 inches below the upper bud materially reduces the quantity of water that can be conveyed to the branch above the base of the dead area. Plants thus infected obtain sufficient moisture for seed production only under the most favorable conditions. When the first crop is cut during warm dry weather, and especially when the first crop has not been permitted to make more than a 30 to 32 inch growth, the stubble seldom decays, and in no instance have the plants been observed to decay as far back as the upper buds.
An experiment was conducted at Ames in the latter part of August and first part of September, 1916, to determine the effect of watering plants that were aborting a large percentage of their flowers and immature pods. For this purpose several volunteer plants growing in a meadow were selected. A hole 12 inches square and 14 inches deep was dug 8 inches from the crown of one plant, and each evening during the experiment 2 gallons of water were poured into the hole. The top of the hole was kept covered, so as to check evaporation from it as much as possible. Another plant of the same size and growing about 15 yards from the watered plant served as a check. On both plants many of the flowers and immature buds were aborting at the beginning of the experiment. The soil in this field was so depleted of moisture that the leaves of the plants wilted during the hottest part of the days preceding the experiment. The foliage on the check plant wilted each day for the first five days of the experiment. On the sixth day 0.96 of an inch of rain fell and four days later 0.23 of an inch more. The dropping of the flowers was temporarily checked by these precipitations, but owing to the dry, compact condition of the soil the rain was not sufficient to check entirely the fall of flowers and immature pods. At the beginning of the experiment the racemes on both plants were divided into three classes, according to the development of the flowers, and labeled. They were collected and the seeds counted as soon as the pods at the bases of the racemes commenced to turn brown. [Table VIII] presents the results obtained.