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 Mites 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 plants. The mites often v-^-idely with the temperature. The eggs confine themselves to one kind of plant 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 species, 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 mite 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. cucumbers, 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 pest 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 pesticides. Growers of hot- tolerated naphthalene fumigation, 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 host 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 poisons fumigating with burning sulfur 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 Agriculture. 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 acaricide, 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, hexaethyl tetraphosphate, be- Since about 1929 many acaricides— 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 aphids, whiteflies, and available by manufacturers. Fumiga- roealybugs without injuring foliage or tion with naphthalene flakes was the flowers. first new development, and special Parathion, 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-oxon 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.