SPRAYS FOR SCALE INSECTS AND WHITEFLIES ON CITRUS

Transcription

1 FLORIDA STATE HORTICULTURAL SOCIETY SPRAYS FOR SCALE INSECTS AND WHITEFLIES ON CITRUS TREES IN FLORIDA Bureau of Entomology, United States Department of Agriculture W. W. Yothers, Entomologist, Ralph L. Miller, Associate Entomologist, Orlando OIL EMULSIONS For over twenty-five years the premier insecti cides for the control of whiteflies and scale in sects on citrus trees have been emulsions made of lubricating oils. These oils have been emulsified with many different materials, such as soaps, kao lin, and gums. When these emulsions were first used in Florida, the citrus whitefly (Dialeurodes citri Ashm.) was spreading over the State, caus ing severe damage to crops and bringing in its wake the usual manifestations of scale insects. To control such infestations these oil emulsions were Some insects were covered with lime-sulphur.

2 FLORIDA STATE HORTICULTURAL SOCIETY 49 used extensively, and throughout the succeeding years they have furnished a reliable insecticide at a minimum cost. It is doubtful if any other in secticide of such efficient toxic properties has ever been developed at such low cost. There are, however, objections to the use of oil emulsions on citrus trees. They must be) used with considerable caution and only before the scale insects and whiteflies have become abundant enough to do much damage to the trees and fruit. Repeated warnings have been given that they should not be used so extensively as to injure the trees. It is well known that the use of oil-emulsion in secticides slightly retards the maturing of the fruit, and also delays coloring, which, of course, under the limitations imposed by the fruit-maturity law, is a serious handicap. When the oil emulsions were first used, itwas not so important to get fruit into the early markets as it is at present It is also well known that oils applied in May and June cause shadowing of the fruit. Usually these shadows disappear, but in some instances they persist even to the time when the fruit should be colored naturally, leaving the portions of the rind that were originally covered with the shadows slighty green, while the rest of the fruit is normal in color. There is also some danger of injury to the fruit and trees if a freeze follows the use of these oils. On the whole, however, they have been satisfac tory, and their effectiveness in this State has led to their use in all the citrus-growing regions of the world. Owing to some of the objections mentioned, many citrus growers have desired to use other ma terials for the control of scales and whiteflies. FISH OIL SOAP Many new citrus growers are not aware that fish-oil soap has been used in Florida for many TABLE II RELATION OF QUANTITY OF SULPHUR ON THE FOLIAGE Td SURVIVAL OF DIFFERENT STAGES OF THE CITRUS WHITEFLY WITH AND WITHOUT CASEIN-LIME SPREADER Treatment si en 3 I* I! in 1 #3 Grams Number Number Per Cent. Per Cent. Lime-sulphur, Lime-sulphur, 1-25 Lime-sulphur, 1-50 \ Lime-sulphur, Lime-sulphur, Lime-sulphur, Check (water) Horti.

3 50 FLORIDA STATE HORTICULTURAL SOCIETY TABLE III RELATION BETWEEN CONCENTRATION OF LIME-SULPHUR SPRAYS, SULPHUR RESIDUE ON LEAVES, AND MORTALITY OP THE PURPLE SCALE years and was extensively used for the control of whiteflies and scale insects before the introduc tion of oil-emulsion insecticides. Extensive ex periments have shown that fish-oil soap containing from 35 to 40 per cent, of soap is very effective in controlling severe infestations of the purple scale (Lepidosaphes beckii Newm.)1 and whiteflies when used at a concentration of three to four gal lons per 100 gallons of water. This makes the cost quite high, but no injurious results have been known to follow the use of fish-oil soap at that concentration. Slight infestations of scale insects and whiteflies can easily be controlled by using two to three gallons of fish-oil soap per 100 galloss of water, if the applications are timed when there are the greatest number of insects in the immature stages. FISH-OIL SOAP COMBINED WITH LUBRICATING OIL Although we have not conducted exhaustive ex periments with combinations of fish-oil soap and lubricating oil, several growers have done so. Their results indicate that 0.5 per cent, of lubri cating oil in the diluted spray mixture is effective in controlling whiteflies and the immature stages of scale insects. If oil in this proportion is added to one or one and one-half gallons of fish-oil soap per 100 gallons of water, an efficient spray for all stages of whiteflies and the purple scale should re sult. If a greater quantity of soap (from one and one-half to two gallons) is used, the oil can be re duced to about one quart per 100 gallons of water, or 0.25 per cent. In the past many growers have controlled all insects in their groves by using oil emulsions containing.05 per cent, of oil; it there fore seems certain that if such oil emulsions are combined with a liberal supply of fish-oil soap excellent results will be obtained. In all probabil ity, most of the commercial emulsions on sale in this State can be combined with the fish-oil soap to make a satisfactory insecticide. LIME-SULPHUR SOLUTIONS FOR CONTROL OF WHITEFLIES Lime-sulphur solution has been used for many years for the control of the San Jose scale on deciduous trees. For this purpose it is used at a l-to-8 or l-to-9 concentration. Such a concentra tion cannot safely be used on citrus trees, and probably not be entirely safe even if there were no bloom or fruit on the trees. Moreover, a 1- to-8 concentration would not kill the advanced stages of the whitefly. We have used a l-to-25 lime-sulphur solution on citrus trees in full fruit many times without damage to the fruit; we have also used it at such a concentration when much injury to the fruit re sulted. We have always considered it dangerous to use lime-sulphur solution of this strength unless the fruit is practically mature, and even then some injury has resulted. However, some people have used this material at 1 to 20 in winter and early spring without injury to the fruit. These concen trations can be used without injury to the trees if the fruit has been harvested and no young fruit

4 FLORIDA STATE HORTICULTURAL SOCIETY 51 or bloom is present. As a general practice, it is doubtful if lime-sulphur solution stronger than 1 to 33 should be used in the winter in groves with full crops of fruit, and perhaps at 1 to 50 through out the summer. Some growers have even re ported great damage at 1 to 75, but we have never found any injury at this concentration. We have known for many years that lime-sul phur solution is very effective in killing young stages of the whitefly. We have obtained very sat isfactory control when spraying at the time of the maximum hatch after disappearance of the March brood of whiteflies. Spraying later in the season, when the whiteflies are in advanced* stages of development, has never been of much value. The toxicity of lime-sulphur solutions to the different stages of the citrus whitefly, as deter mined in several groves in Florida, is shown, in Table 1. Recent experimental work has shown that when a certain quantity of sulphur is present on. the foliage the whitefly crawlers cannot attach them selves. Table 2 shows the quantity of sulphur present and the surviving whitefly stages. The results given in Table 2 show that limecasein spreader increases the toxicity of lime-sul phur solution to the crawlers of the whitefly about four times. In many instances the crawlers were killed when in the act of emerging. These could be seen clamped fast by the eggshells. The per centage of crawlers killed at this time is shown in the last column. In case lime-sulphur solution is applied when the whitefly crawlers are present, the lime-casein spreader should be added for best results. LIME-SULPHUR SOLUTIONS FOR CONTROL OF SCALE INSECTS Many growers in Florida have used lime-sul phur solutions so extensively that scale insects have practically disappeared. Observations over many years have shown lime-sulphur solution at 1 to 25 to be effective in killing the crawling stages of scale insects. Recently we have conducted ex periments to show the toxicity of lime-sulphur so lution to the crawling stages of several species of scale insects. Tables 3 and 4 show the relation of spraying with lime-sulphur to the kill of the pur ple scale and the Florida red scale (Chrysomphalus aonidum L.), respectively. An examination of Table 3 shows that the crawl ers of the purple scale can not establish them selves if somewhat more than two grams of sulphur per kilogram of leaves is present at the time of spraying. An examination of Table 4 shows that the red-scale crawlers cannot establish themselves if slightly more than one gram of sulphur is present per kilogram of leaves. This is only about half as much as is required to prevent the purple-scale crawlers from establishing themselves. In using lime-sulphur solution for the control of whiteflies and scale insects it should be borne in mind that this is the preeminent rust-mite spray. It also will completely eliminate the several species TABLE IV Date of spraying. tbate of examination.

5 52 FLORIDA STATE HORTICULTURAL SOCIETY of fed spiders, as well as improve the appearance of the fruit. If lime-sulphur solution is used ex tensively and frequently enough to control scales and whiteflies, the other pests will be controlled or will be of no economic importance whatever. Member: Have you ever found any damage from too frequent spraying with lime sulphur so lution any damage to the fruit? Mr. Yothers: We have never seen any damage to the fruit. Many years ago, however, Mr. Win ston defoliated a little nursery of sour seedlings. He had used the lime sulphur solution, I think, every two weeks throughout the summer, and I presume if you use it frequently enough damage "would follow, but I have never found any damage from a reasonable use of it. You mean cumulative damage? Member: Yes. Mr. Yothers: No, we have never caused any. We don't get damage from lime sulphur solution at 1 to 50, neither do we get it from wettable sul phur ; the only damage we got last year was from lime sulphur solution at 1-25, used in September. Member: Do you get spotting on the fruit from dusting with sulphur when the fruit is wet? Mr. Yothers: We never did. I know some people claim they have produced such damage. Member: Does lime sulphur solution control a light infestation of melanose? Mr. Yothers: Lime sulphur solution isn't con sidered very satisfactory for melanose, although if you use lime sulphur for melanose control I have never known any grower to regret it, since there are so many really good things that follow, he is perfectly satisfied with results. Member: Do you get all the good effects in combination of iron sulphate with lime sulphur solution that you get with lime sulphur alone? Mr. Yothers: Yes sir, absolutely. IRON SULFATE AND OTHER MATERIALS FOR INCREASING THE EFFECTIVENESS OF SULFUR INSECTICIDES ON CITRUS TREES Ralph L. Miller, Assciate Entomologist, W. W. Yothers, Entomologist, and lone Pope Bassett, Senior Scientific Aid Bureau of Entomology, U. S. Department of Agriculture, Orlando More money is spent for sulphur for use as an insecticide on citrus trees in Florida than for any other insecticide material. To control rust mites completely it is necessary to make several applica tions during the season, and this increases produc tion costs greatly. Now when economy is more necessary than ever before, it seemed in order to find some material that would help to increase the effectiveness of the sulphur applications. Sulphur is an excellent insecticide, but it is ef fective only when it remains on the leaves in quan tities great enough to kill mites. By experiment ing with sprayed and dusted small trees it was found that when trees were protected from wind and rain the sulphur remained on them almost in definitely. On the other hand, when trees treated in the same way were exposed to wind and rain the sulphur was blown or washed off in one or two months, so that the leaves and fruit were un protected from mites. This being the case, various materials were used in both dust and spray mixtures in an effort to prolong the period of time the sulphur remained on the leaves and thereby prolong the period of protection from rust mites. The use of some of these materials made it possible to omit one spray application or one or two applications of dust and still have bright fruit.