GREENHOUSE GROWER NOTES August 2009

September 15, 2009
Written by Gillian Ferguson
Spider mites are extremely successful at surviving unfavourable environmental conditions such as winter temperatures and periods of heat or drought in the summer. This is because spider mites, like many insects, enter into a period of rest, called diapause, during which changes in their bodies and behaviour facilitate their survival.
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 Orange-red diapausing adult female two-spotted spider mite.
PHOTO COURTESY GILLIAN FERGUSON, OMAFRA
 
They are extremely successful surviving unfavourable conditions

Spider mites are extremely successful at surviving unfavourable environmental conditions such as winter temperatures and periods of heat or drought in the summer. This is because spider mites, like many insects, enter into a period of rest, called diapause, during which changes in their bodies and behaviour facilitate their survival.

Diapausing spider mites differ from the non-diapausing forms in several ways. One easily recognizable difference is the orange-red colour of the diapausing mites. Physiologically, diapausing mites have more fat deposits, and their oxygen consumption is considerably lower. They are also more tolerant to pesticides, and feed very little before leaving the host plant in search of hibernation or resting sites.

It is the adult female that goes into diapause, and she mates but lays no eggs prior to seeking her resting site. She also tends to move downwards towards the floor area and away from light.

38a
 
 Yellowish-green, non-diapausing adult female two-spotted spider mite with recently laid eggs.
PHOTO COURTESY GILLIAN FERGUSON, OMAFRA
 
THE KEY ENVIRONMENTAL FACTORS FOR INDUCING DIAPAUSE
As we approach the fall months, it is the immature stages of spider mites that are sensitive to the environmental changes occurring and begin the process of changing to the diapause form. Key factors in the environment for inducing diapause include daylength, temperature and food quality.

Effect of daylength – When the daylength is approximately 14 hours long, about 50 per cent of all the females will enter diapause. As the daylength decreases, increasingly more females will enter diapause. Although daylength is the predominant factor in influencing the diapause state, temperature and food also play a role.

Effect of temperature – Low temperatures combined with short days tend to promote diapause, whereas higher temperatures tend to suppress it. More specifically, it is the night temperatures that are critical. For example, when daylengths are about 12 hours long, if day temperatures are 25ºC and night temperatures 15ºC, about 98 per cent of female mites would diapause. If the conditions were reversed such that the day is cooler and the night warmer, then only about one per cent would diapause.

Effect of food quality – Food has a minor influence on the incidence of diapause. Only when daylength and temperature favour initiation of diapause does poor food quality (such as aging, yellowing leaves) have a major influence on promoting this condition. Interestingly, red forms of the spider mite can occur on leaves that are of poor quality, either due to aging or mite damage itself, during the summer when daylengths are long. Apparently, starving females are brick-red in colour and so resemble the hibernating forms. Such red, starving females are not diapausing forms.

Diapausing spider mites can hibernate in any crevices in the house structure, in hollow stems, irrigation equipment, door locks, and pipe fittings, and have been reported to withstand temperatures as low as -24ºC. After a period of a few weeks, the diapause condition ends and the mites remain dormant. They resume normal activities of egg-laying and feeding only when favourable conditions return.

IMPLICATIONS OF DIAPAUSE IN PEST MANAGEMENT PLANNING
Reduced incidence and damage during fall – Because diapausing mites feed very little and are mainly concerned with finding hibernation sites, their populations and associated feeding will decrease as conditions increasingly favour diapause. However, such a respite is only temporary as explained above.

A high late-summer population means a high spring infestation if their numbers are not sufficiently reduced before fall arrives. If controls for spider mite populations are delayed or inadequate during the late summer months, many mites will enter diapause and re-emerge during the spring crop, and in the same locations they infested during summer. It is especially important to aggressively reduce populations before the daylengths begin to favour diapause because of the more chemical resistant nature of the diapause forms, and also because some predators do not particularly favour feeding on diapause forms.

In summary, daylength is the primary factor that influences the diapause condition in spider mites and is modified by temperature and food quality. The physiological and behavioural changes in diapausing mites render them much more difficult to control because of their resistant nature and the difficulty in accessing them. Consequently, we need to reduce their populations as much as possible before daylengths reach 14 hours to minimize their incidence and problems associated with managing them in the new spring crop.

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