Center for Invasive Species Researches the Mighty Mite
Every 60 days, California gains a new and potentially damaging invasive species. (UC Riverside)
This rate of invasion, on average, results in six new species establishing in California each year. Economic loses to California from invasive species are estimated at $3 billion per year.
The unique climate and geography of California provides diverse ecosytems that are perfect for the establishment of a diverse variety of new pests. UC Riverside’s Center for Invasive Species Research (CISR) researchers lead the way to determine how pests enter California, where invading populations came from and why these pests are successful in establishing California as their home.
Ricky Lara, a UC Riverside graduate student researcher with Mark Hoddle, Ph.D., Biological Control Specialist and Principal Investigator, is focusing on updating and reinforcing the integrated pest control program against the non-native persea mite that infests Southern California avocado orchards.
“High persea mite populations cause premature leaf drop and defoliation. Defoliation leads to sunburned bark and fruit, aborted or dropped fruit, and severely stressed trees, which ultimately reduces yields,” said Lara.
My first objective,” began Lara, “is to further develop a presence/absence sampling plan for growers so they can make keep track of pest densities throughout the growing season to guide spray application decisions. This sampling method will prevent misuse of pesticides and for PCAs to be able to provide growers with better information.”
“Because counting mites on avocado leaves is so difficult, we use a presence/absence method, or binomial sampling, by choosing 30 random leaves per tree, on orchard trees located where the mites prefer.” The sampling simply requires the numbers of avocado leaves infested with the persea mite and the numbers of clean leaves with no persea mites. This ratio of infested leaves to clean leaves is used to estimate the average number of persea mites per avocado leaf. Thus, binomial (presence vs. absence) sampling is fast and simple, and allows large areas of orchards to be surveyed quickly.
“Next, I will figure out where the persea mite comes from and find and examine its natural enemies,” explained Lara. “The logic is that if a pest is not native to the area, its natural predators aren’t here either.”
Lara remarked, “Furthermore, I plan on assessing the risk that novel pesticides being developed for persea mite control pose to beneficial predatory mite populations that attack persea mite. By reducing pesticide use and conserving the presence of predators, we expect to enhance the avocado orchard ecosystem’s capacity for self-regulation of persea mite by making better use of natural enemies for pest control.”
The persea mite infests 99% of avocado acreage in California (There are no records of this pest in the San Joaquin Valley but it has been reported from avocados growing in San Francisco.) This mite is sensitive to high temperatures (>95oF) and low humidity when experienced over several consecutive days, and abrupt population crashes in the field have been observed under these conditions. The persea mite most likely originated from Mexico and arrived in California on smuggled plants.
Scientists at UCR have investigated the efficacy of releases of predatory mites for persea mite control. A highly effective natural enemy, Neoseiulus californicus,is commercially available and has been shown to be very effective, but is cost prohibitive. Seven commercial cultivars of avocado have been screened for resistance to persea mite feeding, and a new cultivar, Lamb Hass, is quite resistant to this pest.
Several species of predators occur naturally in California avocado orchards, and they have been observed to feed on persea mites. Yet, none of these natural enemies provides effective control of the mite. Nonetheless, their presence in orchards is desirable because they probably lessen the severity of persea mite infestations and will feed on other pest species.
Work is currently in progress monitoring persea mite populations, assessing predator quality after an imported shipment arrives, and refining release methodology, rates and timings of these predators.