Broad Mite in lemons has been episodic, following a pattern of warm, humid summers.This past two years along the coast have been a real problem, and has been even worse and earlier because of the mild winter. The mite was posing significant problems in Somis as early as this January, when winter cold should have knocked it out. The problem has been significant enough that a new spray trial is being organized to determine what current materials might bring it under control. I thought it would be interesting to read the experience of PCA Tom Roberts who wrote of the outbreak in 2014, forecasting 2015. Here it is:
Why So Much Trouble With Broad Mite?
Some thoughts on why there has been a resurgence of broad mite Polyphagotarsonemus lotus (Banks) in
coastal lemons the last few years and possible ways to improve control.
My experience with the current acaricides registered for California citrus, showed that none controlled
broad mite for more than 8 weeks in 2013 and 2014. It is interesting to note that Dr. Robert Brown in the
July-August 1983 issue of California Agriculture stated "None of the acaricides tested in the laboratory or in
five field studies (in San Diego, Orange, and Ventura counties) seemed to give more than six to eight weeks'
control when applied during the worst broad mite period (July and August)". Dr. Joe Morse, who succeeded
Dr. Brown at U. C. Riverside, found in a 1983 field study in Ventura County that the best materials were
Morestan and Plictran (effective at 44 days post treatment). Thiolux (sulfur), Lorsban with oil, and Vendex
with oil were effective to 30 days post treatment. Based on my experience the past two years, I consider
abamectin (Epi-Mek) to give the most consistent control of broad mite.
So why the increase in broad mite? One reason would be more summer heat but it's likely that less winter
cold and rain contributed more to the mite increase. We have had spring like weather the past two winters
(Jan-Feb 2015 and Jan-Feb 2014). This leads to earlier broad mite treatments due to more flushing of leaves
in lemons and lots of food for broad mite until new fruit sets for them to move to. Lack of rain played a
part by not washing enough dust off the trees. Dust hinders beneficial predator activity but more
importantly it interferes with good spray coverage. Broad mite prefers the inside of trees and the shady
side of young fruit and with the reliance on 500 gallon per acre treatments, it is difficult to get adequate
coverage for broad mite control on mature lemon trees. Higher gallon per acre dilute type sprays that get
better inside coverage should be considered where broad mite has been difficult to control in the past. This
is especially true with higher density plantings or where trees have not been pruned recently. Other
. contributing factors for broad mite increases may include the close proximity to lemons of alternative host
plants such as weeds and crops like basil, peppers, and tomatoes that are planted nearby. Row crop
operations also contribute dust along citrus borders. Additionally, good lemon prices may have given
picking a priority over treating in some locations, thus delaying applications that led to increased broad mite
populations and corresponding feeding injury. Labor shortages have the same effect by delaying picking
and pruning.
For 2015, we should expect another year where broad mite influences pest control decisions. Try to keep
dust down around orchard borders. To reduce injury risk, if an orchard has a history of broad mite and live
broad mite is found in the late winter or spring then timely treatment with good application coverage
should be considered a priority. Frequent inspections are critical to finding and maintaining broad mite
below damaging levels. The female broad mite lays an average of 5 eggs per day and lives about 10 days. It
doesn't take long to go from a trace to an economic level.
Tom Roberts
Integrated Consulting Entomology