SB Book .lisiSL]^ UNITED STATES DEPARTMENT OF AGRICULTURE — /^V-V — t BULLETIN No. 1035 Contribution from the Bureau of Entomology L. O. HOWARD, Chief Washington, D. C. PROFESSIONAL PAPER February 17, 1922 V THE RED SPIDER ON THE AVOCADO. By G' F; Moznette, Assistant Entomoloyist, Tropical and Subtropical Fruit Insect Inrcstioations. CONTENTS. Page. Introduction 1 Economic importance 1 Nature of injury to foliage 2 Food plants and distribution 2 Description and haljits 4 Biological data 7 Page. Predatory enemies ;) Spraying experiments 11 Spray rod versus spray gun 1:; Cultural methods in the grovo 14 Recommendations 15 INTRODUCTION. The red spider Tetranychus y other si McG. is one of the fore- most enemies of the avocado and attacks a number of other plants and fruit trees in Florida. It was recognized by avocado growers as an important enemy of this fruit as early as 1909, and since that time the damage caused by it has become more pronounced each year. This bulletin records the work with this spider during the years 1918 and 1919 and the results of cooperative spraying experi- ments in connection with the station established by the Bureau of Entomology in 1917 at Miami, Fla., for the investigation of various insect enemies of the avocado and other subtropical fruits character- istic of that region. ECONOMIC IMPORTANCE. In groves where the red spider is abundant the trees frequently become defoliated prematurely during the winter season. This generally results in an abnormal development of bloom the following spring and the weakened trees are unable to set and hold a full crop 75523°— 22 2 BULLETIN 1035, U. S. DEPARTMENT OF AGRICULTURE. of fruit. To sustain the bloom and aid in the setting of fruit the older foliage should remain on the trees until a sufficient amount of new growth apparently arising from the inflorescence (fig. 1) has l)eon i:)roduccd in the spring to take its place. NATURE OF INJURY TO FOLIAGE. The red spider punctures the leaves and sucks the contents, forming Avhite spots at the point of attack. As these feeding punctures and Fic. 1. — .\vocado blossom cluster with older leaves which sustain the bloom, and developing new growth. resultant white spots become more in evidence a gradual burning and reddening of the foliage results, as if scorched by fire (PI. I, A, B). The foliage so attacked soon falls, irivin 2 70 3 Nov. 19 Nov. 23 4 75 9 Mar. 23 Mar. 25 2 72 4 Dec. 13 Dee. 18 5 70 10 Apr. 7 Apr. 9 2 75 5 Dec. 2S Jan. 1 4 68 11 .June 6 June N 2 76 6 Jan. 15 Jan. 18 3 63 12 July 14 July 16 2 79 THE SECOND NYMPHAL STAGE (THE DEUTONYMPH). The second nymphal stage (fig. 3, e) is similar to the first nymphal stage except that it is much larger and more elongate. In its full- grown condition, however, it resembles more the adult, though the color is not as deep a red. It averages 0.38 mm. in length. The habits of the second nymphal stage are likewise similar to those of the larva stage. The average length for the second nymphal stage is 2.84 days. THK rtKD SPIDER ON THE AVOC'ADO. Tabi.k 4. — Length of the second nymphal stadium. . No. Date emerged. Date molted. liura- tion. Mean temper- ature. No. Date emerged. Date molted. Dura- tion. Mean temper- ature. 1 2 3 t 6 Oct. 22 Nov. 3 Nov. 23 Dec. is Jan. 1 Jan. IS Oct. 25 Nov. 6 Nov. 25 I>ec. 21 J an . 4 Jan. 24 Days. 3 3 2 3 3 3 T. 76 77 75 70 fi8 (i3 S 9 10 11 12 Feb. 6 Feb. 2t> Mar. 25 Apr. 9 June 8 July Ifi Feb. 10 Fob. 29 Mar. 28 Apr. 12 June 10 July 18 4 3 3 3 2 65 70 72 75 76 79 BIOLOGICAL DATA. Webhhig. — Unlike the majority of red spiders this species does not spin an extensive weh and carries on its depredations on the foliage practically unprotected. The only indications of any webbing made by this species are the mere fibrils attached to the apex of the eggs Avhen deposited (fig. 3, a). Average length of life period. — The length of the life period of the adult mites varies greatly with the season and temperature and possi- bly other conditions. Experiments on the life history of this species shoM^ed that adults emerging November 25 came to their natural death during the period January 1 to 15, while those emerging Junt> 10 succumbed between July 1 and 15. This shows that during the dry winter months approximately two months are required from the time of emergence to the completion of the life period, while during the humid summer months approximately a month is required. Molting process. — Before molting the mite securely attaches itself to the leaf. In emerging from the quiescent stage the old skin splits transversely along the cephalothoracic-abdominal suture. Following the splitting of the skin the anterior end of the mite is slowlj'^ drawn from the old skin. AVitli the use of its fore legs the mite forces its way out from the shell. Pafthenogenesis. — Some immature individuals were isolated on a number of plants. From these individuals virgin females were ob- tained. These females produced eggs and in each instance the re- sultant individuals were males. Migration. — There does not seem to be an alternate host of this spe- cies. Individual red spiders may be found on the avocado at any time during the year in varying numbers, and never leave the tree for want of a new or alternate host plant on which to feed. In a grove the red spiders are spread from tree to tree by the wind, birds, etc. Generations of the species. — The generations of the avocado red spider fluctuate as to number and overlap considerably. In years of little rain during the fall the red spiders come in evidence more quickly than when rains occur earlier. Intermittent rains frequently 8 BULLETI:N 1035, U. S. department of AGRldULTT'Ri:. occurring' during the red spider season also interfere with the regu- larity of the generations. Activity of the red spider usually com- mences during the latter part of August and ceases the first part of April, giving an active season of about 240 days. The average dura- tion of the life cycle is 14.2 days, which would give 17 generations. This would be true where no interruptions due to climatic conditions occurred, and when no other factors interfered with the normal activi- ties of the mites in the field. Shedding of the foliage. — During the winter months the foliage may be termed " dormant," no new^ growth being present on the trees. Usually during the latter part of March and April the avocado com- mences to bloom and the older leaves, which have served their purpose to the trees, commence to fall. With the shed leaves many mites are lost and do not regain positions on the trees. During the latter part •^wresr/rr/oh oW/V. /=-£B. /V/?/?. /9/V?. /V/?K ^UN£ UULY /9UG. SEPT. OCT: A/OU DEC — — ^ /9/a V "'>.^ _,,--'^ /9/9 — — . , — — "" ~ , X / / /yo/?Af/9iso^ . ^v ^^ / » ^* y "V.^ y .»'"' \ y /^ ♦• \ ^ --— -^ .■^' €XT/NCr/-«rr ~—'^ Fig. 4. — Curves showing 2-yGar composite soasonal status of the avocado red spider la southern Florida. The decimations arising through the amount of preiipitation are the most importani controlling factor in tlie activities of the species. of April very little old foliage is present and a remarkable reduction of red spiders is apparent. A fcAv old leaves, however, always re- main on the trees until the newer growth has hardened and thus enable the red spiders to remain continuously on the trees and to in- fest the newer growth when it hardens. Climatic control. — Climatic conditions existing in Florida influence the development and activity of the red spicier to a marked degree. This particular species, as has already been stated, confines its depre- dations to the upper surface of the foliage. The species so working is exposed to the weather conditions. Hence during the period of the life cycle or seasonal cycle there is a series of fluctuations in numbers. In April, as the rainy season approaches, the red spider barely maintains existence. (Fig. 4.) During the months of June. July, and August no pronounced gain is made, but toward the latter part of October the avocado ceases to produce new growth, the red spiders commence to make their appearance in greater numbers, and increase during November and December. They usually reach the maximum number during January and February, and decrease again toward March. Precipitation is the one climatic factor im- jDortant in reducing the red spiders during the spring and summer. THE RED SPIDER ON THE AVOCADO. 9 (Fig. 5.) Durino; the summer in Florida drenching and frequent rains usually prevent the red spiders from establishing themselves on the trees. During late fall and winter and early spring it rains seldom, and the interference with the activities of the red spider is slight. PREDATORY ENEMIES. A number of predatory enemies of the avocado red spider aid at various times in keeping down the species on the avocado to a small degree. /A/CHES J^N. F£8. /^/9/?. /9PR. Af^Y S / / V ^ V \ ^.S / 1 K 1 \ V \ ^ / A ^ / \ M ^-^^ \ 3.S / / \/ \ \ s.^^ "r \ S ^^ > ^ V \ \ > \ z.s / "a. / \ \ \ ;? y , /> v/ v \ /.5 ' / V \ V / / .5 / O Dottei /;«o I'/'i Stro-'lj hir li./ ^ \V3 1 Fig. 5. — Precipitation chart for the yrars 1918 and 1019 in southern Florida. Scolothrijis semnaculatus Pergande is one of the predatory thrips and is not abundant during the height of the red-spider season. Nevertheless it is present doing its share of destruction. It is a light- colored thrips possessing six dark spots on its body. It feeds on the red spiders in both the larva and adult stages. ClirysoiKi JateiaViH Guer. is one of the so-called lace- wing flies and is predatory in the larva stage on the red spider. The larvte, while feeding and wandering about the foliage in search of their prey, carry with them a protection consisting of foreign material, such as cast red spider skins, etc. The larA^se have a voracious appetite and 10 BULLETIN 103"), U. S. DEPARTMENT OF AnRICULTUEE, possess powerful jaws with which they attack their prey. This species is quite beneficial. Scymnus utilis Horn. — The most important enemy of the red spicier found up to this time is a very small black ladybird beetle Fig. fi. — ficiimnuK utilis: a. Larva: b, pupa; c. adult. Greatly enlarged. (fig. 6, c) about Vij-iiit'h long. With the beetles may be found their dark brown larvEe (fig. 6. a), also feeding on all stages of the red spider. Seymiiu-H klnseli Casey is another ladybird beetle found feeding in both the larva and adult stages on the red spider. It is larger than Fir -Lcptothrips mali: Adult thrips. Cieatl.v enlarged. the former, the abdomen is black, and the head reddish. It is not a very abundant species and is not as beneficial as the former beetle. Leptothrips mali Hinds is a large black thrips (fig. 7) predatory in both the larva and adult stages, and when present is very acti^-e THE RED SPIDER. ON THE AVOCADO, 11 on the foliage in searc-li of red spiders. It is not very abundant at any time. SPRAYING EXPERIMENTS. A number of insecticides have been tried out to ascertain their rela- tive merits against the avocado red spider. The experiments Avere all conducted cooperatively with growers and in groves where the mite was abundant. Fiu. 8. -Power duster in operation using sulphur dust against the red spider iu an avocado grove. SULPHUR DUST. An impalpable sulphur dust was tested on both the West Indian and Guatemalan races of avocadoes. In applying the material a power duster (fig. 8) was used. The sulphur proved to be very effective against the species, killing 99 per cent of the spiders, and remaining effective on the foliage over as long a period as did any of the liquid sprays tested. Experiments Avith this material showed that it was not necessary to apply the sulphur dust when the foliage was wet with dew. IMiero an avocado grower has a large acreage and the red spider is the only serious enemy to contend with, the dusting method is very practical and by far the quicker method. At the present time, however, the avocado grower has other insect pests with 12 BULLETIN 1035, U. S. DEPARTMENT OF AGRICULTUKE. Tvhich lie has to contend, making it necessary to use liquid insecticides in combination with a sulphur spray in some form for their control. Up to this time the writer has not found it practical to use a combina- tion of sulphur dust and 40 per cent nicotine sulphate against the insects of the avocado. iSil^iiM ,i.. LIME-SULPHUR CONCENTRATE; In using lime-sulphur concentrate spray on the avocado a number of strengths were tried, e. g., 1 gallon of concentrate to 40 gallons of water, 1 to 60, and 1 to 75. In spraying with lime-sulphur solu- tion it was found through actual count that nothing is to be gained in applying too strong a solution of lime-sulphur to red spiders on the avocado. A strength of 1 gallon of the concentrate to 60 gallons of water proved to be the most efficient, generally killing 99 per cent of the spiders and producing sufficient body as a spray on the foliage to remain effective during the dry season against later hatching young. Under certain conditions applications of 1 to 40 and 1 to 60 were too strong, and considerable damage resulted to the foliage from burning, especially on the south side of the trees. When the tem- perature is above normal during the winter season, or when the trees do not attain a thoroughly dormant condition, a strength of 1 gallon of the concentrate to 75 gallons of Avater was found satisfactory. It was also ascertained that it is not necessary to incur the extra expense of adding a spreader of lime-suli)hur spray, such as flour paste, glue, or fish-oil soap, because good results were obtained without it. COMMERCIAL SODIUM SULPHID. Commercial sodium sulphid was used at a strength of 2 pounds to 50 gallons of Avater. It was ascertained that this spray killed approxi- mately 95 per cent of the red spiders present. Because of its chemical composition, however, the spray did not dry thoroughly on the foli- age. The hydroscopic condition so formed permitted the spray cover- ing the eggs and foliage to be readily washed off by succeeding rains, and nothing remained to destroy the hatching young. NICOTINE SULPHATE CONTAINING 40 PER CENT NICOTINE. At times the red spiders make their appearance during the fall before the fruit is picked. At this time it is not advisable to use any of the sulphur sprays, as they adhere to and discolor the fruit. By using 40 per cent nicotine sulphate at the rate of 1 part to 900 parts of water, with the addition of 2 or 3 pounds of fish-oil soap to each 100 gallons of the diluted spray as a spreader, satisfactory re- sults were secured. The spray, however, proved to be effectiA^e only THE RED SPIDER ON THE AVOCADO. 13 temporarily, as none remained on the foliage to destroy the young on hatchinfr. SPRAY ROD VERSUS SPRAY GUN. In making a comparison of the spray rod (fig. 9) and the spraj^ gun (figs. 10 and 11) it was found that the latter gave better satis- faction against the red spider. Where an orchardist is short of help considerable benefit can be derived by using the spray gun, as the spray operator can cover the ground much faster than when using Fig. 9. — Spraying in an avocado gi-ove witli spray rods. the spray rod. As the red spider works on the top of the foliage the operator can stand off a short distance from the tree, and with the use of the spray gun can spray from the bottom to the top of the tree by turning the handle of the gun which changes the sj^ray from the fan or fog spray (fig. 10) to a long-distance spray (fig. 11). The writer has found that growers in using the spray gun often neglect to change the disk in the nozzle frequently enough and won- der why they can not maintain sufficient pressure while spra3ang. The operator should watch the opening of the disk in the nozzle of the gun, and should replace it when it is worn. 14 BULLETIN 1035, U. S. DEPARTMEXT OF AGRICULTURE. CULTURAL METHODS IN THE GROVE. Clean culture does not play an important part in the control, as this species does not infest weeds or plants in or about avocado groves. Orchards mulched in various ways in southern Florida were found to be less infested with the red spider as a rule than those Avhere clean culture w^as practiced. The avocado seems to thrive better where mulching is practiced and the moisture is conserved. Red spiders generally like dry conditions such as are afforded in S ijfc Mj&k r |f\jl Ql ■Q^^E^-^-^JtfaB oHH H ^^^^Bj^^^^a^^^^^^^^^ ^ ^||a^|aMfl| w mm. m Fig. 10. — Spraying in an avocado gi'ove with spray gun, using the fan or big fog spray. groves where clean culture is followed. This is especially evidenced in seasons of drought during the year. One factor which influences greatly the abundance and appearance of the red spider in a grove is the vitality of the trees. Nothing is to be gained by allowing trees to suffer from w^ant of proper attention in the way of mulching, plant foods, and culture. Where growers are in doubt about the proper procedure to use in caring for their avocado trees, they should get in touch wdtli either the Plant Introduction Garden maintained by the United States Bureau of Plant Industry at Miami, Fla., or their particular county agent. THE RED SPIDER ON THE AVOCADO. 15 RECOMMENDATIONS. The presence of the avocado reel spider on the trees in considerable numbers while the foliage is still green should be a sufficient indica- tion of impending injury to cause the grower to begin immediate application of control measures. The grower should not wait until the foliage attacked becomes noticeably brown prematurely and begins to drop. During the winter after the fruit has been picked use the liquid lime-sulphur 1 to GO. When the temperature is above the normal and the trees do not attain a thoroughly dormant condition the liquid lime-sulphur should be reduced to 1 to 75. ^4-4- I'm. 11.-- Spiayiu^ in an avocado ^rove witli spray guu, using tli. long distance spray. If the red sj^iders are present while the fruit is still unpicked in the fall, 40 per cent nicotine sulphate 1 to 900. with the addition of 2 or 3 pounds of fish-oil soap to each 100 gallons of the diluted spray, will give temporary relief and will not discolor the fruit. Thorough application is a most essential point in combating the red spider. — If haphazard work is performed and much of the foliage left unsprayed, such infested foliage serves as a source of reinfestation to the tree, and the mites will be more in evidence after application than when thorough work has been done. ADDITIONAL COPIES OF THIS PUBLICATION MAY BE PROCURED FROM THE SUPERINTENDENT OF DOCUMENTS GOVERNMENT PRINTING OFFICE WASHINGTON, D. C. AT U) CENTS PER COPY LIBRARY OF CONGRESS DDDDflfi'=]D377