day to nearly 2 days. Then it enters the resting stage. After a day or so it molts to a second active stage, which feeds Spider and and then becomes quiescent as in the first stage. The adult male emerges Resistance from the second quiescent stage, but the female passes through a third feed- Floyd F. Smith ing and quiescent stage before becom- ing an adult. Mating usually occurs within a few minutes after the female Spider mites, or "red spiders," at- becomes an adult. Only males develop tack nearly all kinds of field crops, veg- from eggs of unmated females. etables, orchard trees, weeds, and The developmental period varies greenhouse . The mites often v-^-idely with the temperature. The eggs confine themselves to one kind of hatch in 2 or 3 days at 75° F. or higher at first but move to other kinds when or after 21 days at 55°. The mites may injury increases and food becomes reach the adult stage within 5 days at scarce. 75° or in 40 days at 55°. Under aver- Several , of similar habits but age greenhouse temperatures of 60° to different morphological characters, arc 70°, the incubation period is 5 to 10 found out of doors. days and development to adult stage Of six species that infest green- from 10 to 15 days. One female lays a houses, the two-spotted spider is few eggs daily and a total of 100 to predominant. It is a general feeder 194 eggs during an average life of 3 to but it is almost constantly present on 4 weeks. , tomatoes, roses, carnations, One female can give rise in- one chrysanthemums, violets, sweet peas, month, through succeeding genera- snapdragons, fuchsias, and ageratums. tions, to a progeny of 20 mites at 60°, It is the major on roses and some about 13,000 mites at 70°, and well other greenhouse crops. If it is not con- over 13 million mites at 80° constant trolled it limits profitable production temperature. Multiplication therefore or makes the flowers unsalable. It is is rapid on hothouse crops, and control increasingly pestiferous in orchards and measures must be thorough and on other outdoor crops, especially if prompt. more susceptible related mites have The mites feed by piercing the epi- been removed from competition by dermis of the leaf and drawing the sprays or dusts. liquid contents from the cells. The leaf Adult female two-spotted spider turns pale and stippled around the in- mites are less than one-fiftieth of an jured part. When the infestation is inch long. The males are much smaller. severe the stippled areas coalesce and Bristles cover the oval body. The color cause the leaf to appear sickly, turn varies from green or yellow to orange rust-red, and then crumple and die. with dark spots. When the spots are Affected plants are stunted and may be close together the mites appear black. killed. The plant may become covered Some females are carmine or dark red. with fine silken webs, which the mites Mites generally become darker with spin as they move from place to place. age or in cool temperatures. Sometimes Mites sharply reduce the average the progeny of green mites may be red, production of crops or flowers. The but in at least one strain the adults are virtual elimination of red spiders with dark red in all generations. Spider one of the organic phosphates has in- mites have a well-protected respiratory creased production of roses fivefold system. during the summer, when mite injury The life history is complex. The is most severe. Rose growers have re- newly hatched mite feeds for part of a ported an increase of 20 to 40 percent 652 Spider Mites and Resistance 653 in production and better flowers v/ith quired dosages. But only certain crops the newer . Growers of hot- tolerated naphthalene , and house vegetables now can produce fall high temperature and humidity had crops of tomatoes and cucumbers and to be maintained. Sprays containing maintain spring crops in high produc- derris powder or rotenone extracts were tion until early summer or until field- followed by sprays containing a com- grown crops become available. plex selenium compound—(K-NH4- Spider mites in greenhouses have S)5Se, known as Selocide. The latter been hard to control because they at- was successful at first against spider tack so many plants. Their small mites but soon became ineffective de- sizc^ rapid reproduction, and protec- spite repeated applications. We do not tion beneath webs on the lower leaf know why that happened because no surface have increased the difficulty of specimens of the susceptible and resist- combatting them. ant mites were preserved for later Their well-protected respiratory sys- study. Another selenium compound, tem makes them resistant to the ordi- sodium selenate, applied in water to the nary contact sprays and fumigants that soil at the rate of one-fourth gram per their food plants tolerate. In the qui- square foot to such crops as carnations escent stages the mites are highly re- and chrysanthemums, however, is still sistant to most chemicals and until toxic to some strains of the spider mites. recently no safe treatment was known It is inefiPective on roses or other plants that killed the eggs. with woody stems. To meet those difficulties^ greenhouse Sodium selenate, a systemic insecti- operators attempted to destroy all mites cide, is absorbed by the roots and trans- by cleaning out plant material at the located with the sap to the foliage and end of the cropping season and by flowers of herbaceous plants. It fumigating with burning before the mites as they feed. The material is the new crop was planted. Sprays and highly toxic to humans. It should not dusts containing sulfur were applied to be used on any food crops and is not the young crop to reduce the mites to a recommended at all by the Department minimum in the fall. When the crops of . were grown at cool temperatures, the mites did not increase markedly until WHEN AZOBENZENE was found to be late winter or early spring. Then pro- an effective , many rose grow- ductiveness of the crop was prolonged ers and others fumigated with it at by spraying with i ounce of dry limíÍT monthly intervals to control spider sulfur in 3 gallons of water at lo-day mites even though for several days the intervals. Some used a spray contain- flowers developing from treated buds ing I ounce of common salt in a gallon were faded in color and had to be dis- of water. Spraying with glue in water carded. Despite such losses, the fumi- to stick the mites to the foliage was gations controlled 95 to 99 percent of recommended. Syringing with water to the mites and more salable flowers were wash the mites from the leaves was a produced than had been obtained with general practice, but it damaged tender any previous treatment. Its use quickly foliage and encouraged spread of such declined, however, when the first of the diseases as blackspot on roses, mildew aerosols containing the organic phos- on cucumbers, and carnation diseases. phate, , be- Since about 1929 many — came available in 1947. It eliminated mite-killing compounds—have been re- or greatly reduced the spider mites and ported on by research workers or made also controlled , , and available by manufacturers. Fumiga- roealybugs without injuring foliage or tion with naphthalene flakes was the flowers. first new development, and special , which became available lamps were used for vaporizing re- in aerosols early in 1948, gave the same 0701íí4°--52 43 654 Yearbook of Agriculture 1952 Comparison of Toxicity of Several Or- that mixed populations of susceptible ganic Phosphates and Other Chemicals in Methyl Chloride Aerosols to Two Strains and resistant mites were not present of Two-Spotted Spider Mites in those greenhouses. Because the houses had been repeatedly treated Perceni mortality with parathion or hexaethyl tetraphos- Nonre- phate, it is possible that all susceptible sisiant Resistant Ingredient mites mites mites, if formerly present, had been Parathion 99-9 5 eliminated. Para- loo 6 The three infestations of resistant Methyl parathion i oo i mites in Pennsylvania and New Jersey, ?EP7.V.;.-.;:;.;:;:: ^,r' ?f separated from one another by 50 to Tetraisopropyl pyrophos- 100 miles, and the one in Connecticut phate 98 38 apparently did not have a common Sulfotepp I bo 59 origin through exchange of infested DMG 9*7 22 Aramite 100 2 roses or other stock. Although one firm in Pennsylvania propagated roses for NOTE : The following names and symbols sale each year, none of the other three were adopted in 1951 : Para-oxon, for oxy- firms with resistant mites had procured gen analog of parathion; methyl parathion, stock from that source in recent years. for methyl homolog of parathion; HETP, The results indicated that in the for hexaethyl tetraphosphate; TEPP, for tctraethyl pyrophosphatc ; sulfotepp, for area surveyed in February 1949 re- tetraethyl dithiopyrophosphate; DMC, for sistant mites were restricted to the four di ( />-chlorophcnol ) methylcarbinol ; Aramite firms. In June, however, for the first for 2-{p-tert-huXy\ phcnoxy)-l-mcthyl ethyl- time, resistant mites were encountered 2-chIoroethyl sulfite. in areas where only nonresistant mites had been previously collected, but they high degree of control of spider mites were definitely traced to introductions in most commercial greenhouses with on rose plants recently purchased from less frequent appHcations than were re- the Pennsylvania firm. Thus the dis- quired with hexaethyl tetraphosphate. tribution of resistant mites was greatly Rose growers in Connecticut, New extended in the late spring and sum- Jersey, and Pennsylvania, however, di- mer of 1949 by these infested plants. rected our attention to the poor results By 1950 a large percentage ■ of rose they got with parathion w^hen they first growers in the East, as far west as Illi- used it in October 1948. nois, had encountered resistant mites. We learned that there are two In many greenhouses, in which all strains of the two-spotted spider mite roses are propagated from existing with respect to its resistance to para- stocks and where other crops have been thion. No mites could be found in many treated regularly with parathion or greenhouses where parathion and hexaethyl tetraphosphate for nearly 5 hexaethyl tetraphosphate had been years, the progeny of surviving mites used to control them. In 33 other show no increased resistance. That greenhouses in three States mites were finding does not support the theory collected on rose and other plants and that increased resistance generally fol- were treated experimentally with para- lows repeated applications of the same thion aerosols. Practically all the mites . The evidence wc had in from 30 greenhouses were killed. Mites 1952 was that resistant mites were from one greenhouse in New Jersey limited at first to a few greenhouses and two in Pennsylvania survived the and extended their range through treatment. Twenty-four collections commerce following the use of highly were made from various parts of the effective acaricides, which eliminated three establishments and the mites nonresistant strains of the same species were found to be equally resistant to and other nonresistant species as well. parathion when tested—an indication In commercial greenhouses resistant Spider Mites and Resistance 655 mites have been found on roses, car- water obtained good control of resist- nations, chrysanthemums, cucumbers, ant mites but with considerable stunt- tomatoes, snapdragons, ageratums, ing and hardening of plant growth. china asters, strawberries, beans, can- Others went back to azobcnzene, but nas, dahlias, and at least nine species discontinued it after obtaining poor of weeds, all of which are common control of resistant mites and injury to hosts of nonresistant mites. Resistant the flowers and succulent growth. mites have not been found on outdoor Many syringed the plants with water crops or weeds close to infested green- and reduced the population, but foli- houses. But resistant mites transferred age was severely injured, and black to strawberries in the field in November spot was encouraged. 1950 survived winter temperatures of In experiments on the resistant and 8° F. and continued the infestation the nonresistant strains of mites, each of following spring. nine materials killed a higher percent- Resistant mites from roses were age of the latter. It is evident that the transferred to beans or the other host one strain is resistant not only to para- plants I mentioned and reared for 3 thion but to all other materials tested. years. Samples taken from the colonies In further experiments, I discovered and tested periodically with parathion that aerosols containing /?-chloro- apparently did not lose their original phenyl p-chlorobenzene sulfonate were resistance. These mites also did not lose toxic to hatching young of resistant their resistance when reared for several mites but at a higher dosage than is re- months on any of the hosts. Perhaps quired to kill nonresistant ones. The the mites in the original collections material can be used safely on roses from single-plant colonies on roses were only during the spring and summer be- homozygous for resistance and their cause when days are shorter it is likely progeny did not lose this resistance to cause the plants to drop their leaves. through an estimated 50 generations The material in sprays has also been regardless of the food plant. This effective against resistant spider mites thought, though, is not in agreement but it causes leaf drop or other injury with statements of other investigators. during short days. The material is also I believe that their colonies were mixed volatilized from heating pipes. with nonresistant strains and that the Sprays, aerosols, and smokes contain- latter became dominant in the biologi- ing di ( p-chlorophenyl ) methyl-carbi- cal competition. nol are highly effective against active In view of the difíference in resist- stages of resistant mites. It has not ance of mites from various sources, been widely used because of the tend- entomologists compared nonresistant ency for plant injury due to improper mites with resistant mites and found no formulations and unavailability of the differences in the morphological char- chemicals. acters. All were considered to be typical Sprays containing 2-(j&-í^rí-butyl two-spotted spider mites. I have ob- phenoxy)-i-methyl ethyl 2-chloroethyl served living mites for a long time, but sulfite are also effective against active have found no differences in coloration stages of resistant mites and appear to or other characters that will positively be safe on greenhouse plants. This is separate the two strains. Controlled one of several materials used success- tests have not been made to determine fully in commercial orchards in 1950 whether the two strains will interbreed. and greenhouse operators used it against resistant mites in 1951. IN EFFORTS to reduce the damage All three of these materials remained from resistant mites, some greenhouse toxic to mites for i to 2 weeks after operators returned to the use of some application. They are also sufficiently older remedies. Those who applied volatile to act as fumigants. These ma- sprays containing derris extracts in terials in aerosols or smokes are more 656 Yearbook of Agriculture 1952 convenient to the greenhouse operator The new, highly effective insecti- than spray formulations. cides have nearly eliminated most in- Tetraethyl dithiopyrophosphatc as a sects and mites as well as their preda- fumigant in aerosols kills a higher per- tors and parasites from many green- centage of adult resistant mites than houses. Freed of their pests, plants have any of the organic phosphates and even greatly increased their production and more of the active stages of young quality of flowers or fruits. The higher mites. It was first used commercially standards thus established make it im- in 1950 to combat this pest^ but it has perative to continue with the chemical not been entirely satisfactory because methods of in greenhouses. of the frequency of applications re- Because of the saving in cost of labor, quired to keep resistant mites in check the aerosol method of applying insec- and occasional leaf distortion that fol- ticides is preferable to spraying or dust- lows continued usage. ing. Spraying and dusting leave con- In other experiments, the systemic spicuous residues, and spraying encour- octamethyl pyrophosphor- ages the spread of serious fungus and amide and one of the trialkyl thiophos- bacterial diseases. phates were applied in aerosols, in To combat the resistant spider mite, water as foliage sprays, or as soil ap- the propagator of roses or other plant- plications. They killed both strains of ing stock could assure against dissemi- mites in the active stages but killed the nating resistant mites by treatments nonrcsistant mites i or 2 days earlier with octamethyl pyrophosphoramide than the resistant ones. The materials or other systemic acaracide that would resulted in the most satisfactory con- the sap to feeding mites, and trol of resistant mites in experiments by killing the mites in all stages by conducted in commercial greenhouses. fumigating with methyl bromide. They arc absorbed by the plant and The commercial greenhouse opera- render the sap toxic to feeding mites tor may keep resistant mites in check and aphids for 2 to 4 weeks or longer by repeated applications of aerosol con- after they have been applied. Octa- taining tetraethyl dithiopyrophosphatc methyl pyrophosphoramide has no con- or carefully timed applications of p- tact action of value for killing mites chlorophenyl jf?-chlorobenzene sulfo- or aphids. nate in aerosols, sprays, smokes, or Outdoors, where spraying is practi- fumigants to avoid plant injury. Sprays cal and the crops have a higher toler- or aerosols containing 2~{ p-tert-hutyl ance than ornamentals, mites (includ- phenoxy)-I-methyl ethyl 2-chloro- ing those that may be of the resistant ethyl sulfite or di(/?-chlorophenyl)- strain) are being controlled by p-chlor- methylcarbinol, or smokes containing ophenyl /;-chlorobenzene sulfonate, di- the latter, are effective alternates. The (/?-chiorophenyl)methylcarbinol or 2- octamethyl pyrophosphoramide aero- (p-tert-hutyl phenoxy)-i-methyl ethyl sols or sprays or even one of the newer 2-chiorocthyl sulfite. In greenhouses systematic insecticides promise to give until recently only a partial kill of the desired control of the resistant mites was obtained by the inefficient spider mite as we recognize it in 1952. acaricides if any were used at all. Pred- ators and parasites could always be PYoYD F. SMITH, a senior entomolo- found and in some instances they prac- gist in the Bureau of Entomology and tically killed out heavy infestations of Plant Quarantine^ has devoted 28 mites but not until after severe dam- years to the study of affecting age had been caused to the plants. The greenhouse and ornamental plants. He predaceous mite Iphidulus sp. will de- has published more than 1^0 articles stroy resistant as well as nonresistant on the biology and control of these mites but not rapidly enough to pre- pests and on insects as vectors of plant vent extensive plant feeding. diseases.