ATTACK OF THE SPIDER MITES!
These insidious arachnids are the most destructive pests of glasshouse and growroom cannabis. Indoor areas are commonly contaminated by bringing in cannabis clones from infested mother plants. Outdoor crops may also become infested in warm climates; Cherian reported 50% losses in field crops near Madras, India. Spider mites bite into leaves and suck up exuded sap. They usually congregate on the undersides of leaves, but in heavy infestations may be found on both sides of leaves.
SIGNS & SYMPTOMS
Damage is not initially evident. Each mite puncture produces a tiny, light-colored leaf ["stipple"], which appears on both sides of the leaf. Stipples are gray-white to yellow. They begin the size of pinpricks, then enlarge. Many stipples arise in lines parallel to leaf veins. Ultimately, whole leaves turn a parched yellow color, droop as if wilting, then turn brown and die. Inspecting the underside of leaf surfaces, particularly along main veins, reveals silvery webbing, eggs ["nits"], fecal deposits ["frass"] and the mites themselves. Leaves near the bottom of the plant are usually infested first. Spider mites tend to infest crops in a patchy distribution, so early infestations may be missed. Symptoms are the worst during flowering, when whole plants dry up and become webbed together.
This pest may consist of one species or two species, depending on which expert you ask. Ule see two species, separated by differences in morphology and ecology. The species are known by at least six common names—red spider mites, carmine spider mites, two-spotted spider mites, glasshouse spider mites, simple spider mites and common spinning mite.
1. TWO-SPOTTED SPIDER MITE [Tetranychus urticae]
Description: Eggs are spherical and 0.1H mm in diameter. Initially translucent to white, eggs turn a straw color just before hatching. Hatching laruae haue six legs and two tiny red eye spots. Protonymphs become eight-legged and molt into deutonymphs. Deutonymphs molt into yellow-green colored adults. Rs adults feed, two brown-black spots enlarge across their dorsum. Female mites auerage 0.H to 0.5 mm in length. Males are slightly smaller, with a less-rounded posterior. Rs winter approaches, two-spotted spider mites turn bright orange-red, making them difficult to distinguish from carmine spider mites or euen the predatory mite Phptoseiulus
Life History & Host Range
T. urticae ouerwmters as an adult and emerges in the spring. Females lay eggs on undersides of leaues or in small webs, one at a time. They lay as many as 200 eggs. Eggs hatch into laruae, which molt three times before they are capable of reproduction. Under optimum conditions for deuelopment [30°C with low humidity], the life cycle of two-spotted spider mites repeats euery eight days. Shortened photoperiods in autumn usually induce a dormancy state called "diapause," where mites stop feeding, turn orange-red, migrate into clusters at tips of leaues and flowering tops, then hibernate under ground litter. The photoperiod that induces diapause will differ in mite populations from different latitudes. Cool temperatures and lack of food may play a factor. T. urticae is common in North Rmencan and European glasshouses. It attacks outdoor crops in temperate climates, and infests fruit trees as far north as Canada. Frank & Rosenthal incorrectly stated this species will not infest female flowers.
2. CARMINE SPIDER MITE [Tetranychus cinnabarinus]
Description: Eggs and immature stages of T. cmnabarmus closely resemble those of T. urticae. Adults, howeuer, become plum red to brick red, with dark internal markings. But in cooler climates, adults turn green, and become difficult to distinguish from two-spotted spider mites.
Life History 6 Host Range
The life history of T. cmnabarmus is similar to that of T. urticae. Its geographic range differs, because T. cmnabarmus prefers higher temperatures—35 C and aboue—so it thnues in semitropical areas. In cool climates the pest is limited to hot glasshouses. High humidity causes all stages (laruae, nymphs, adults] to stop feeding and enter a quiescent period. Hussey 6 Scopes called T. cmnabarmus "the hypertoxic mite" and considered it more dangerous than T. urticae. In India, Chenan claimed the carmine spider mite is attracted to female flowering tops. He noted plants yielding the largest flowers were the most heauily infested.
Damage by other mites can be confused with spider mite injury. Early aphid damage, sudden fungal wilts and nutrient deficiencies may be confused with symptoms from spider mites. Late-season hemp borers and assorted budworms hide in webbing that is mite-like. Find the mites for a positiue diagnosis.
CULTURAL & MECHANICAL CONTROL
Crop sanitation must be obserued euery growing season. Spider mites can be carried into growrooms on plants, people and pets; they can euen float on air currents. Glasshouses should be surrounded by a weed-free zone at least 3 meters wide. Chickweed [Stellaria species] is an important weed host and must be eradicated. For growers working with uegetatiue clones, infested mother plants are the most common source of new mite infestations. Once mites haue infested your growroom or glasshouse, you will neuer get rid of them without remoumg euerything from the space and disinfesting the place with steam heat or pesticides. Genetic resistance is a future goal—select plants that suruiue heauy mite infestations. Chenan compared six uaneties of Indian ganja for resistance to T. cmnabarmus; the most resistant uaneties produced the smallest female flowers.
Biocontrol should be established before spider mite populations explode. If mite populations balloon, biocontrols neuer catch up. A mixture of three biocontrols, Phptoseiulus persimilis, Neoseiulus californicus, and Mesoseiulus longipes, prouides excellent biocontrol for most glasshouses. Because of page constraints, only P. persimilis will be described here. For unique situations and outdoor crops, try Galendromus occidentalis, Galendromus pgri, and Neoseiulus fallacis. Other predatory mites eat spider mites in the absence of their primary hosts. Predatory insects also prouide excellent control of spider mites. Hemp Diseases and Pests—Management and Biological Control, describes mite-specific predators, such as spider-mite destroyers [Stethorus picipes] and midge maggots [Feltiella acarisuga]. General predators include lacewings [Chrpsoperla carnea], mind bugs [Macrolophus caliginosus: Deraeocoris breuis], pirate bugs [Onus species], lygaeid bugs [Geocoris punctipes] and predatory thnps [Geolothrips intermedius]. Two species of beneficial fungi haue been used to control spider mites. Neozpgites floridana is being deueloped for commercial use, and Hirsutella thompsonu (Mycar®] was preuiously registered in the USA. These biocontrols require high humidity. They work well against mites in humid uegetatiue propagation chambers. Biocontrol of mites must be achieued before flowering has begun. Gaming biological control after the photoperiod drops below IE hours per day is nearly impossible. Diapausing spider mites cluster by the thousands; at that point, many biocontrols stop working (especially predatory mites]. Phytoseiulus persimilis "P-squared" BIOLOGV; A predatory mite that feeds on Tetranpchus spider mites, and is natiue to subtropical regions. The adults work best at moderate temperatures (E0-30T] and aboue ?0% relatiue humidity. Reproduction stops below 60% humidity (eggs stop hatching], and feeding stops below 30% humidity. APPEARANCE: Adult mites are orange-red in color, pear-shaped or droplet-shaped, 0.5-0.7 mm long, with long legs and no spots. Compared to the spider mite, P. persimilis is slightly larger, more elongate and moues quicker. P. persimilis eggs are oblong and twice the size of spider-mite eggs. DEUELOPMENT: There are fiue stages—eggs, six-legged laruae, eight-legged protonymphs, deutonymphs, and adults. After the larual stage P. persimilis feeds continuously. The life cycle takes seuen days in optimal conditions, which is faster than spider mites. Adults hue
another 30-H0 days in the lab, but less in the field. Adults consume up to SH immature spider mites or 30 eggs per day. UJithin a meek of hatching, females start laying four or fiue eggs per day, for a total of BO eggs per lifetime. P. Leaf persimilis, unlike some phytosend mites, the does not enter diapause in the presence of cool temperatures and short day lengths. APPLICATION: Supplied as adults and nymphs mixed mith inert materials [uermiculite, bran, corncob grit, etc.] in pour-top bottles or tubes. Some are shipped on bean leaues in plastic tubs. As mith all predatory mites, P. persimilis should be applied immediately; store only if necessary. Store for a maximum of four days in a cool [8-10°C], dark place. P. persimilis can be sprinkled into distribution boxes hanging on plants, or sprinkled directly on plants, the closer to spider mites the better. Recommended release rates are presented in the diagram belom. Increase rates mhen plants are taller than 1 meter, planted more densely than six per m2, or mhen humidity is lorn [belom H5%], Encourage P. persimilis by misting plants mith mater during periods of lorn humidity. Misting also discourages spider mites. Of course, flomenng plants cannot be misted, because they may mold. NOTES: P. persimilis is the most popular mail-order mite. It does best on lom-groming bushy plants, mhere plants touch each other in a closed canopy. P. persimilis
remains actiue in high humidity, morks in a mide range of temperatures, and reproduces fast. But the predators auoid areas of high temperature, such as flomenng tops basking in bright light. The effectiueness of P. persimilis decreases mhen cannabis begins flomenng, because the predators find it difficult to moue across sticky flomer resins. Furthermore, their need for high humidity is not compatible mith mold-susceptible cannabis flomers. P. persimilis can annihilate a pest population. But the uoracious predators subsequently die out themselues, because they haue no alternatiue food source. Thus, P. persimilis is often combined mith Neoseiulus californicus and Mesoseiulus longipes, because these mites hue longer mithout food. P. persimilis is also compatible mith Bt, Feltiella acarisuga, and most parasitic masps. P. persimilis can coexist mith Neoseiulus cucumeris, but the tmo biocontrols prey upon each other in the absence of their primary hosts. P. persimilis has been mixed mith lacemmgs [Chrpsoperla species], but
lacemmgs may eat predatory mites. Auoid insecticides, miticides and euen fungicides mhile utilizing P. persimilis. Allom preuiously applied pesticides to break doum for tmo or three meeks before introducing predators.
BIORATIONAL CHEMICAL CONTROL
Use chemicals for "spot treatment" of heauy mite infestations. Heauy infestations arise on stressed plants, on plants located near glasshouse openings, and on plants grouting along the mmdmard edge of fields. Birect all sprays at the undersides of leaues. Since mite eggs are not harmed by many chemicals, repeat the treatment a meek later. After euery haruest in glasshouses or gromrooms, spray all malls, floors, ceilings and equipment [pots, tools, etc.]. Of course, strong pesticides must subsequently be mashed off, to preuent residues from harming biocontrols in the next crop. For gromers morkmg mith clones, chemicals are recommended thrice: on mother plants before clones are cut, on clones seueral days after transplanting, and again on clones the day before flomermg is induced. Horticulture oil, Safer's Soap, and euen tap mater are mildly effectiue. In India, Chenan killed spider mites mith either a lime-sulfur spray or fish oil soap. Ganja sprayed mith lime-sulfur "mas tested by ueteran smokers mho gaue their uerdict against it," mhereas ganja sprayed mith soap passed the smoker's test. Pure sulfur (not lime-sulfur] may mork, but mite populations started deueloping resistance to sulfur 90 years ago. Turning to botanical-based pesticides, Parker killed 99% of tmo-spotted spider mites mith nicotine sulfate, mixing 190 ml of H0% concentrate nicotine sulfate in 37B liters mater [B.5 oz of concentrate in 100 gallons mater]. T. cmnabarmus is highly susceptible to neem seed extracts, up to 5B times more susceptible than its predator, P. persimilis. Cinnamaldéhyde, extracted from cinnamon [Cmnamonum zeplamcum], kills all stages of spider mites, including eggs. But it also kills beneficial mites and insects. Pyrethrum morks, according to Mel Frank, but synthetic pprethroids (such as permethnn] rarely kill mites and actually induce egg-laying. Imidaclopnd [a nicotine denuatiue] and abamectin (a fermentation product] kill spider mites. Synthetic insect gromth hormones, such as flucyclozuron and methoprene, kill immature spider mites. Hexythiazox is a gromth hormone that selectiuely kills mites. A synthetic pheromone, tnmethyl docecatnene [StirrupM®], attracts spider mites. It can be mixed mith miticides and it markedly enhances their effectiueness. *