They say timing is everything in life, and that’s certainly true of the Asian citrus psyllid, which has devastated Florida agriculture for the past decade by transmitting citrus greening disease, also known as huanglongbing or HLB.
To reproduce, this small, flying insect must lay eggs on citrus “flush” – the tender new leaves and shoots that citrus trees produce several times each year in Florida. Flush is only suitable for egg-laying the first 15 to 20 days after it appears, then the tissue toughens and will no longer support the feeding activities of newly hatched psyllid nymphs.
Of equal to concern to growers is citrus trees are at increased risk of becoming infected with HLB when they produce flush, because adult psyllids are attracted by flush and feed on it. If the pests have previously fed on infected trees and acquired the bacterium responsible for HLB, they may transmit the pathogen to healthy host trees.
Complex though it is, the psyllid’s reproductive timetable may provide growers with a new option for managing the insect and, thus, reducing the possibility of new HLB infections occurring in their groves, according to researchers with the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS).
The scientists believe they can lure psyllids to their doom with a technique called flush manipulation, and simultaneously help growers use insecticide more efficiently.
“Flush is the key to the HLB disease cycle,” said horticulture expert Christopher Vincent, an assistant professor at the UF/IFAS Citrus Research and Education Center in Lake Alfred. “We want to capitalize on the fact that it’s possible to control when the flush appears.”
Although the research team has mainly conducted studies in greenhouses so far, here is how Vincent hopes the system will work:
First, a grove is treated with a plant growth regulator, which is a compound that either stimulates or inhibits plant growth. Assuming that all trees in the grove are approximately the same age, they should begin to produce flush simultaneously, attracting male and female psyllids ready to mate. Just before the flush is expected to appear, the grower sprays the trees with one or two applications of a contact insecticide. Ideally, psyllids attracted by the flush make contact with the insecticide and receive enough exposure to die within a few hours.
“It’s unlikely that we can kill all of the psyllids that come to a grove, but we believe we can eliminate a large percentage of that group, and also reduce the number of eggs they lay,” Vincent said. “Lots of details need to be worked out before we can optimize the approach and recommend it to growers.”
Since August 2017, the team has conducted three greenhouse trials on plant growth regulators and is midway through a fourth. They have already demonstrated that the compound naphthalene acetic acid will delay flush emergence for about 14 days when applied to soil, whereas applications to foliage will delay flush by only 10 days.
The researchers also determined that gibberellic acid-3 induces flush when applied in concentrations of 20 parts per million, and that cytokinin did not appear to be a strong flush inducer.
Field trials began at the Citrus Research and Education Center in early 2018, and initial results will be available soon, Vincent said.
The team has begun addressing one real-world consideration, a phenomenon called the “age effect.” It refers to the idea that there is an ideal time window when the tissue in the tips of mature branches will respond vigorously to plant growth regulators. Branches that are too young or too old to fall inside that window may respond more slowly. The research team is determining how age effect may alter the timetable needed to induce flush.
A related part of the project investigates the possible effects of flush manipulation on adult psyllids. Entomologist Kirsten Pelz-Stelinski, an associate professor at the Lake Alfred center, has already collected baseline data concerning the effects of four plant growth regulators and one adjuvant on psyllid egg-laying activity as well as the survival of eggs and newly hatched nymphs.
Next, Pelz-Stelinski and her colleagues will investigate whether plant growth regulators impact transmission of the HLB pathogen from psyllids to host citrus trees.
The team should be in a position to make initial recommendations in a year, Vincent said.
Funding for the project totaled about $130,000 and was provided by the UF/IFAS Citrus Initiative to Vincent and Pelz-Stelinski for their respective investigations.
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