Author: Frank Byrne, Joseph Morse and Robert Krieger

CAC-funded Research Examines Avocado Thrips Management Options

Since avocado thrips were first identified in California in 1996, they have become established throughout California avocado growing regions. Thrips are primarily attracted to young foliage on avocado trees, but as the leaves mature the thrips — which find the mature leaves less appetizing — begin to feed on immature avocados leading to scarring that reduces fruit quality and has an adverse economic effect on California avocado growers. Tree health also can be affected by premature leaf drop caused by feeding and egg laying on immature leaves.

The most commonly used foliar-applied insecticide for avocado thrips management is abamectin. While abamectin has proven effective in controlling thrips populations, it can be cost-prohibitive to apply for those growers whose groves are located on steep hillsides and thus require aerial applications. In addition, there are increasing concerns that reliance on abamectin will eventually result in thrips’ resistance to the insecticide. Another concern is that foliar applications of insecticides can have adverse effects on thrips’ natural enemies, as well as honey bees.

In light of these concerns, the California Avocado Commission funded research by UC Riverside researchers to identify other insecticides that may be effective in thrips control, the most effective means of chemical application and the optimal timing of applying the insecticide.

In order for an insecticide to be considered a viable option for avocado thrips management, the insecticide must protect the fruit when they are the most vulnerable — and considered the most appetizing by the thrips — in the early weeks during and after fruit set (spring flush). To determine whether the insecticides being tested were present in toxic concentrations during this particular time period of fruit development, researchers tested sampled leaves to measure insecticide residues.

Results from early field trials indicated that trunk spray applications of the neonicotinoid insecticides imidacloprid and clothianidin were an ineffective means of controlling avocado thrips due to poor uptake of the chemicals. Soil uptake also wasn’t deemed effective, primarily due to the heavy layer of organic matter in the soil that interferes with the sorption of pesticides.

Researchers then examined whether systemic application (trunk injections) of neonicotinoid insecticides would be effective in avocado thrips pest management. As part of the project, researchers tested trunk injections of the organophosphate, acephate, as well as two systemic neonicotinoides — imidacloprid and dinotefuran. In addition, researchers focused on determining the best timing of injections and tested for residues in the fruit to determine whether trunk injections would present a greater risk of fruit contamination than other insecticide application methods.

Data indicated that acephate and imidacloprid are the strongest candidates for trunk injection. Because acephate moved rapidly within the tree and remained effective for weeks even after new foliage emerged it was determined the insecticide could be injected at any stage of flush and would be effective as a response to sudden outbreaks. While ineffective during the early flush period, imidacloprid was found to be most effective when injected during the mid-flush period. Although dinotefuran was taken up by the tree quickly, data indicated that the chemical didn’t reach the concentration levels needed for avocado thrips control. In addition, residues of dinotefuran were detected within sampled fruit and would need to be monitored in order to ensure the chemical level was below established tolerances.

Researchers noted that trunk injections were the most effective means of delivering these insecticides and this process could be utilized to treat hot spots within avocado groves. However, they caution that an economic analysis should be conducted to determine if this is a viable strategy.

Download the entire article, "Evaluation of neonicotinoid, organophosphate and avermectin trunk injections for the management of avocado thrips in California avocado groves" :