This year was a 'perfect storm' for California red scale; combine the drought with higher than normal average daily temperatures and pesticides that disrupt scales and you have a crisis.
Drought: Some insect populations increase when trees become water stressed and California red scale is one of those. It could be a direct response to the tree or it could be because increased dust reduces the effectiveness of natural enemies.
Higher than normal temperatures: Typically, San Joaquin Valley conditions produce 4 to 4.5 generations of California red scale per year. Usually, by November 1, the female scales stop producing crawlers and shut down for the winter. This year, the degree days (heat units) were higher than normal the entire red scale season (from March 1 - Dec 1). This produced a full 5th generation of scale and scale crawlers were still emerging in December. Many growers struggled to control California red scale because the problem appeared late in the season and this pest is very difficult to control with insecticides once it reaches the fruit. Degree day units were high not only this year, but have been higher than the 30 year average for the past 3 years, accumulating extra scales each year.
Insecticides: Research has demonstrated that systemic neonicotinoids such as imidacloprid can reduce scale on fruit and leaves but are not effective in controlling California red scale on woody tissues such as bark and twigs. These insecticides are also toxic to natural enemies. Using these products year in and year out builds scale on the wood, that eventually makes its way to fruit.
Below are the flights and five generations of crawler activity at Lindcove Research and Extension Center. Also shown are the degree day units accumulated at Lindcove through the season. See degree day units for California red scale for other locations.
Author: Cheryl Reynolds, UC Statewide IPM Program
An online course highlighting how pesticide resistance develops among pests is now available on the UC IPM Web site. Created primarily for pest control advisors and other licensed pesticide applicators, this course describes the mechanisms of resistance in pathogens, insects, and weeds and discusses ways to manage resistance within the different disciplines.
The online course is divided into three narrated presentations followed by a final test for each section. This course has been approved for 2 continuing education units in the “Other” category from the Department of Pesticide Regulation.
This course is based on a series of workshops held in Davis, Fresno, and at the Kearney Agricultural Research and Extension Center during the spring of 2014 presented by Dr. Doug Gubler (Dept. of Plant Pathology, UC Davis.), Dr. Larry Godfrey (Dept. of Entomology and Nematology, UC Davis), Dr. Beth Grafton-Cardwell (Lindcove Research and Extension Center and UC Riverside), and Dr. Kassim All-Khatib (UC Statewide IPM Program).
Check out the new course at http://www.ipm.ucanr.edu/training/pesticide_resistance.html.
On September 30, an in-depth workshop was held at the UC ANR Lindcove Research and Extension Center to discuss the biology and management of California red scale. Twenty seven participants spent the day viewing scales under microscopes and learning about Aphytis and Comperiella wasps. The hot topic (literally) this year, above and beyond the usual complexities of managing scale, was the much higher than average accumulation of heat units. Normally there are about 4 generations of scale per year in the San Joaquin Valley, but this year a 5th generation is developing. This is the third year in a row that temperatures have been above the 30 year average. We also discussed the fact that systemic neonicotinoid insecticide use is becoming very common place and repeated use of the systemic neonicotinoids results in a build up of scale on the wood that is difficult to control with natural enemies or insecticides. So, the combined problems of scale build up due to changes in insecticide use patterns and high average daily temperatures have resulted in outbreaks of California red scale in the San Joaquin Valley.
At a recent meeting in Exeter, I discussed the Fuller rose beetle management plan for 2014. Research from 2013 demonstrated that two insecticide treatments per season were better than one. These treatments could be ground treatments or foliar. The ground treatment helps to keep emerging beetles from climbing the trunks and the foliar treatment kills them if they do reach the foliage. If two treatments are planned, early August and early October are the best time periods for the San Joaquin Valley. Choice of insecticide depends on application target and concerns about MRLs. Remember that skirt pruning in June and weed control are essential components of the Fuller rose beetle program. Korea may allow blanket fumigation or fumigation of infested loads with Methyl Bromide this year, but they are counting on California growers to apply in-field treatments to lower FRB populations. Methyl Bromide is not likely to be allowed in future years. Treating blocks several years in a row with foliar and/or ground treatments will help to bring the populations down to very low levels.
California red scale crawlers emerge about 550 degree days (biofix in March, lower developmental threshold of 53oF) after the first flight of males. As you can see on the CRS degree day web page, Kern county passed that point last week, Tulare County crawlers are emerging this week and Fresno scale crawlers are soon to emerge. The first or second crawler generations are the best times to treat with most insecticides, because crawlers are most sensitive at this stage.
See the UC IPM Guidelines for Citrus for more information on treatments.