Spider Mites and Resistance

Floyd F. Smith.

Spider mites, or "red spiders," attack nearly all kinds of field crops, vegetables, orchard trees, weeds, and greenhouse plants. The mites often confine themselves to one kind of plant at first but move to other kinds when injury increases and food becomes scarce.

Several species, of similar habits but different morphological characters, are found out of doors.

Of six species that infest greenhouses, the two-spotted spider mite is predominant. It is a general feeder but it is almost constantly present on cucumbers, tomatoes, roses, carnations, chrysanthemums, violets, sweet peas, snapdragons, fuchsias, and ageratums. It is the major pest on roses and some other greenhouse crops. If it is not controlled it limits profitable production or makes the flowers unsalable. It is increasingly pestiferous in orchards and on other outdoor crops, especially if more susceptible related mites have been removed from competition by sprays or dusts.

Adult female two-spotted spider mites are less than one-fiftieth of an inch long. The males are much smaller. Bristles cover the oval body. The color varies from green or yellow to orange with dark spots. When the spots are close together the mites appear black. Some females are carmine or dark red. Mites generally become darker with age or in cool temperatures. Sometimes the progeny of green mites may be red, but in at least one strain the adults are dark red in all generations. Spider mites have a well-protected respiratory system.

The life history is complex. The newly hatched mite feeds for part of a 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 and then becomes quiescent as in the first stage. The adult male emerges from the second quiescent stage, but the female passes through a third feeding and quiescent stage before becoming an adult. Mating usually occurs within a few minutes after the female becomes an adult. Only males develop from eggs of unmated females.

The developmental period varies widely with the temperature. The eggs hatch in 2 or 3 days at 75 F. or higher or after 21 days at 55 . The mites may reach the adult stage within 5 days at 75 or in 40 days at 55 . Under average greenhouse temperatures of 60 to 70 , the incubation period is 5 to 10 days and development to adult stage from 10 to 15 days. One female lays a few eggs daily and a total of 100 to 194 eggs during an average life of 3 to 4 weeks.

One female can give rise in one month, through succeeding generations, to a progeny of 20 mites at 60 , about 13,000 mites at 70 , and well over 13 million mites at 80 constant temperature. Multiplication therefore is rapid on hothouse crops, and control measures must be thorough and prompt.

The mites feed by piercing the epidermis of the leaf and drawing the liquid contents from the cells. The leaf turns pale and stippled around the injured part. When the infestation is severe the stippled areas coalesce and cause the leaf to appear sickly, turn rust-red, and then crumple and die. Affected plants are stunted and may be killed. The plant may become covered with fine silken webs, which the mites spin as they move from place to place.

Mites sharply reduce the average production of crops or flowers. The virtual elimination of red spiders with one of the organic phosphates has increased production of roses fivefold during the summer, when mite injury is most severe. Rose growers have reported an increase of 20 to 40 percent in production and better flowers with the newer pesticides. Growers of hothouse vegetables now can produce fall crops of tomatoes and cucumbers and maintain spring crops in high production until early summer or until field-grown crops become available.

Spider mites in greenhouses have been hard to control because they attack so many host plants. Their small size, rapid reproduction, and protection beneath webs on the lower leaf surface have increased the difficulty of combating them.

Their well-protected respiratory system makes them resistant to the ordinary contact sprays and fumigants that their food plants tolerate. In the quiescent stages the mites are highly resistant to most chemicals and until recently no safe treatment was known that killed the eggs.

To meet those difficulties, greenhouse operators attempted to destroy all mites by cleaning out plant material at the end of the cropping season and by fumigating with burning sulfur before the new crop was planted. Sprays and dusts containing sulfur were applied to the young crop to reduce the mites to a minimum in the fall. When the crops were grown at cool temperatures, the mites did not increase markedly until late winter or early spring. Then productiveness of the crop was prolonged by spraying with 1 ounce of dry lime-sulfur in 3 gallons of water at 10-day intervals. Some used a spray containing 1 ounce of common salt in a gallon of water. Spraying with glue in water to stick the mites to the foliage was recommended. Syringing with water to wash the mites from the leaves was a general practice, but it damaged tender foliage and encouraged spread of such diseases as blackspot on roses, mildew on cucumbers, and carnation diseases.

Since about 1929 many acaricides; mite-killing compounds have been reported on by research workers or made available by manufacturers. Fumigation with naphthalene flakes was the first new development, and special lamps were used for vaporizing required dosages. But only certain crops tolerated naphthalene fumigation, and high temperature and humidity had to be maintained. Sprays containing derris powder or rotenone extracts were followed by sprays containing a complex selenium compound (K-NH₄- S) ₅Se, known as Selocide. The latter was successful at first against spider mites but soon became ineffective despite repeated applications. We do not know why that happened because no specimens of the susceptible and resistant mites were preserved for later study. Another selenium compound, sodium selenate, applied in water to the soil at the rate of one-fourth gram per square foot to such crops as carnations and chrysanthemums, however, is still toxic to some strains of the spider mites. It is ineffective on roses or other plants with woody stems.

Sodium selenate, a systemic insecticide, is absorbed by the roots and trans-located with the sap to the foliage and flowers of herbaceous plants. It poisons the mites as they feed. The material is highly toxic to humans. It should not be used on any food crops and is not recommended at all by the Department of Agriculture.

WHEN AZOBENZENE was found to be an effective acaricide, many rose growers and others fumigated with it at monthly intervals to control spider mites even though for several days the flowers developing from treated buds were faded in color and had to be discarded. Despite such losses, the fumigations controlled 95 to 99 percent of the mites and more salable flowers were produced than had been obtained with any previous treatment. Its use quickly declined, however, when the first of the aerosols containing the organic phosphate, hexaethyl tetraphosphate, became available in 1947. It eliminated or greatly reduced the spider mites and also controlled aphids, whiteflies, and mealybugs without injuring foliage or flowers.

Parathion, which became available in aerosols early in 1948, gave the same high degree of control of spider mites in most commercial greenhouses with less frequent applications than were required with hexaethyl tetraphosphate. Rose growers in Connecticut, New Jersey, and Pennsylvania, however, directed our attention to the poor results they got with parathion when they first used it in October 1948.

Comparison of Toxicity of Several Organic Phosphates and Other Chemicals in Methyl Chloride Aerosols to Two Strains of Two-Spotted Spider Mites.

NOTE: The following names and symbols were adopted in 1951: Para-oxon, for oxygen analog of parathion; methyl parathion, for methyl homolog of parathion; HETP, for hexaethyl tetraphosphate; TEPP, for tetraethyl pyrophosphate; sulfotepp, for tetraethyl dithiopyrophosphate; DMC, for di (p-chlorophenol) methylcarbinol ; Aramite for 2- (p-tent-butyl phenoxy) -1-methyl ethyl-2-chloroethyl sulfite.

We learned that there are two strains of the two-spotted spider mite with respect to its resistance to parathion. No mites could be found in many greenhouses where parathion and hexaethyl tetraphosphate had been used to control them. In 33 other greenhouses in three States mites were collected on rose and other plants and were treated experimentally with parathion aerosols. Practically all the mites from 3o greenhouses were killed. Mites from one greenhouse in New Jersey and two in Pennsylvania survived the treatment. Twenty-four collections were made from various parts of the three establishments and the mites were found to be equally resistant to parathion when tested an indication that mixed populations of susceptible and resistant mites were not present in those greenhouses. Because the houses had been repeatedly treated with parathion or hexaethyl tetraphosphate, it is possible that all susceptible mites, if formerly present, had been eliminated.