Mighty prob­lems: Spi­der mites

Ontario Gardener Magazine - - CON­TENTS - By Sharon Mof­fat

Ev­ery so of­ten, you may no­tice that a plant does not look as healthy as usual. At first glance the plant in ques­tion may seem slightly dried out and in need of a wa­ter­ing. If given a closer in­spec­tion, many small brown spots of dam­age can be seen on the leaves, you have a prob­lem. And the most tell-tale sign of the plant be­ing un­der at­tack is the pres­ence of fine web­bing all over, es­pe­cially be­tween the leaves and twigs. It is this web­bing that an­nounces that the plant is be­ing dam­aged by spi­der mites.

Spi­der mites are very tiny crea­tures in the sci­en­tific class Arach­nida, which also in­cludes spi­ders and ticks. Like spi­ders, they have eight legs and two body parts. They are sim­i­lar to aphids in the type of dam­age they do to plants. Spi­der mites have pierc­ing, suck­ing mouth parts which they use to in­jest the liq­uids in­side leaves and other soft tis­sues of plants. Left un­treated, th­ese mites will cause leaves to wilt and brown, and new growth to dieback. They are a fairly com­mon pest of in­door house plants and can in­fest out­door plants such as roses, rasp­ber­ries and spruce trees. Most mites are ex­tremely tiny and there­fore dif­fi­cult to no­tice on plants. They can be checked for by firmly shak­ing the af­fected plant or in­di­vid­ual leaves over a blank white sheet of pa­per. Mites will fall onto the pa­per and ap­pear as very small dark specks mov­ing around.

Spi­der mites can be very dif­fi­cult to con­trol as they are quite pro­lific and un­der ideal con­di­tions they can pro­duce a new gen­er­a­tion in less than a week. They pre­fer hot and dry grow­ing con­di­tions and are more com­mon in sum­mer months, both out­doors and in­doors. Spi­der mite prob­lems on house plants are of­ten best fixed by iso­lat­ing the af­fected plant and treat­ing it by reg­u­larly rins­ing it un­der run­ning water and ap­ply­ing in­sec­ti­ci­dal soap. Badly dam­aged ar­eas of the plant should be pinched off and dis­carded. They can also be killed off by in­creas­ing the hu­mid­ity lev­els around the plant by plac­ing it in a sealed plas­tic bag for a pe­riod of time.

Spi­der mites are al­ways present out­doors but their num­bers are usu­ally kept down by var­i­ous preda­tors, in­clud­ing preda­tory mites. Some­times though, weather con­di­tions are ideal for spi­der mites and they be­gin to rapidly in­crease in num­ber. How­ever, their pref­er­ence for hot, dry con­di­tions means they can be con­trolled to a cer­tain ex­tent by reg­u­larly hos­ing down the af­fected plant. Like aphids, they are soft bod­ied and can be knocked off plants with a strong blast from the gar­den hose. This will also help to re­move the un­sightly web­bing they cre­ate. Any re­main­ing spi­der mites can be sprayed with a va­ri­ety of avail­able gar­den in­sec­ti­cides such as in­sec­ti­ci­dal soap. Although it is im­por­tant to note that some­times out­breaks of spi­der mite pop­u­la­tions re­sult from overuse of in­sec­ti­cides that kill off their nat­u­rally oc­cur­ring preda­tors. As with most plants, it is a bal­ance of over­all ben­e­fi­cial prac­tices that en­sure op­ti­mal plant health.

Sharon Mof­fat has a Plant Sci­ence de­gree from the Uni­ver­sity of Man­i­toba and has worked for the City of Win­nipeg's In­sect Con­trol Branch for the last 24 years.

This plant is suf­fer­ing from a bad case of spi­der mites.

A close-up of spi­der mites.

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