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Six spotted mite

Eotetranychus sexmaculatus or six spotted mite are found on the underside of leaves. They prefer to feed adjacent to and beneath a protective webbing by the midrib and large lateral veins. The damage caused by feeding removes chlorophyll from the leaves and produces purplish irregular patches. This almost invisable pest gets a lot of notice because heavy feeding on avocado leaves can cause severe defoliation

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Six spotted mite adult is about 0.3 mm and hard to spot without a glass 10x or more

This spider mite is found in all the major avocado growing regions in this country, but  populations are greatest in the Whangarei and Far North regions.  Although 6SM was first  found in New Zealand in 1953, it began to be of concern only relatively recently, rising to  become a major pest in the last few years.  Although there are no clear patterns, Dr Philippa Stevens* suggests "the most likley cause of outbreaks of 6SM is the high levels of natural enemy mortality following the application of broard spectrum insecticides used for controlling primary pests (eg leaf rollers, amoured scale, or thrips). The disruptive effects of copper fungicides may be a contributing factor resulting in high levels of natural predator insect mortality. Research investigating the effects of copper based fungicides on ladybird predation levels and and increased predator mortality when compared to non copper fungicides are detailed in Lo and Blank 1992; Lo 2002.  Dr Stevens states that controlling mites is difficult with chemicals which can lead to residues and resistance to the chemicals resulting in a collapse of the natural ways of controlling populations.

  Apparently overseas 6SM rarely requires intervention on avocados, with numbers being successfully  contained by a range of predator insects (natural enemies).

There is an urgent need for more understanding of the bigger picture and unfortunatley the avocado industry is putting most of its its considerable funding into miticide research - a slippery slope in the wrong direction.

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Typical mite damage and eggs on the leaf under side
Plenty of growers we know, as well as our orchard (which is in one of the worst areas for mites) have no mite problem. Mites are  certainly present, but established beneficial predator insects expand to limit defoliation damage and 6SM cause no problems. Buying predator insects to apply in an attempt to create a balance is becoming popular but as there are no commercially available predators specifically of six spotted mites in NZ this practice is of dubious benefit as well as completely unnecessary. For example, if worms are not present in the soil that is because conditions are not right. Adding worms to hard ground will not correct these conditions but if the conditions for worms are right they will multiply rapidly.  The same goes for the kind of beneficial predator insects required to keep 6SM under control. You can't expect to get them into your orchard from a bag, this is not what happens in nature. If conditons are right the beneficial predator insects will thrive without any attention.  When Dr Stevens inspected our orchard and spent a few hours sucking up insects and identifiing them she said she found less problem insects and more beneficial predator species than any other orchard she had visited. She commented that we must be doing something right. That somthing is nothing. We do not spray in our orchard with any sort of spray"organic" or otherwise.

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Unidentified insects and a Stethorus ladybird cleaning up mites

*NZ Avocado Growers Annual Research report Vol 2 2002 

References

Avocado source is a free world wide searchable database of Avocado information

http://www.avocadosource.com/ 

A useful lot of pest id photos from the williams and kettle crop scout team

http://www.wilket.co.nz/cms/pestkey.htm

What they get up to in California with predator mite

http://www.rain.org/~sals/avo.html 

 and http://www.biocontrol.ucr.edu/mite1.html

Read about Avril, the filthy muck most growers use at the Pesticides action Network