The spring of 2023 in California was abnormally cool, making avocado thrips abnormally populous. Thrips are tiny insects that chew on avocado fruit as the fruit is small, and this chewing becomes brown scarring.
The thrips damage can result in large patches that some people call “alligator skin.” But if the chewing is minor, the result is only faint brown lines.
Thrips like it cool
The avocado trees in my yard in the foothills of San Diego County are usually not an attractive feeding ground for thrips. In the past decade, I had seen almost zero thrips damage. In 2023, I saw the most ever.
At an avocado farmer event at Cal Poly San Luis Obispo in August of 2023, Brett Chandler of Associates Insectary said that he was seeing the worst thrips damage in 20 years, and he said it was because of the long, cool spring.
Heat “burns out” thrips, said Chandler. Inland locations (such as mine) see less thrips damage than coastal locations, in general, because “thrips numbers are suppressed by warm, dry conditions,” according to the page on avocado thrips by the University of California.
Before I learned about why, I had always noticed that avocados trees close to the beach had worse thrips scarring on their fruit compared to avocado trees farther inland. Chandler said that even a few miles can make a difference: In Ventura County, Fillmore gets almost no thrips damage whereas Santa Paula gets 40% damage because it’s just enough cooler (at around eight miles closer to the ocean).
What to do?
So move inland. Oh, that’s not a viable option? Sorry. Then warm up the spring weather. Not a helpful suggestion either?
Before thinking about what to do, note that brown skin on avocados can be caused by things other than thrips and sometimes it’s hard to discern the cause.
Other causes of brown skin on avocados
Two types of skin damage on avocados that are most easily confused with thrips scarring are rub scars and russeting.
Rub scars are caused by the fruit rubbing against something as it hangs on the tree. The fruit usually rubs against branches or other avocados.
Russeting, also sometimes called corking, appears on the skin of some varieties as the fruit reaches maturity. It’s not caused by any external force; it’s just an indication of maturity. Varieties that I’ve seen russet a lot include Edranol, Thille, Nabal, and Nowels.
(For more pictures of scarring on avocados, see this page of photos from the University of California.)
What to do to prevent thrips scarring: Spray?
For thrips scarring on avocados, some farmers in California spray their trees in order to kill the thrips and minimize their damage because scarred avocados fetch a lower price. Farmers and pest control advisers tell me that the standard spray for non-organic farmers is Agri-Mek, which is produced by Syngenta, a company owned by the China National Chemical Corporation. And the standard spray for organic farmers is Entrust, which is produced by Corteva, a public company that had been owned by DowDuPont until 2019.
Both of these insecticides are toxic to many other insects besides avocado thrips, including bees, according to their safety data sheets (found in links above).
Encourage thrips predators
What can be done to minimize thrips damage without spraying? One idea is to develop a yard or farm that is hospitable to avocado thrips predators.
When I say avocado thrips I’m referring to Scirtothrips perseae. There are many other species of thrips; there are even other species that eat avocado thrips. (See photos and descriptions of “good” thrips species here.)
So you want habitat for other thrips that prey on avocado thrips, and you want habitat for the other natural enemies of avocado thrips, such as green lacewings. That would mean not spraying pesticides and having nectar and pollen available, which means either planting flowers or allowing weeds to flower, or both.
Encourage spring leaf growth
Another idea is to encourage spring leaf growth. At the Cal Poly SLO event, Brett Chandler explained that thrips start out feeding on succulent new leaves in the spring, but as those leaves harden, the thrips move onto tiny avocados to feed. If you can induce your tree to continue growing many new leaves while the setting fruit is still small, then “more of the thrips population will remain on tender foliage,” as it is put in Avocado Production in California (Book 2, Chapter 5, page 112).
I saw evidence to support this in one of my Hass trees during 2023. On certain (girdled) limbs, the tree set so much fruit that few leaves grew during spring. Thrips damage on the avocados of these limbs was widespread. In contrast, the avocados on the other (non-girdled) limbs on the same tree had noticeably less thrips damage; those limbs had grown many more new leaves throughout spring.
But what to do with this observation? How to encourage more spring leaf growth? Don’t girdle too large a portion of a tree’s canopy. And make sure the tree has enough soil fertility to support both fruit growth and leaf growth.
But the bottom line is that we’re stuck with our location’s climate and each spring’s weather, and these factors make a big difference. So we must tolerate a certain amount of scarring from thrips on our avocados.
Luckily, the damage is usually only skin deep. Only if the damage is extensive does it affect the avocado’s eating quality or limit the ability of the avocado to develop normally.
Here we are in the middle of winter 2024. Will late winter and spring 2024 be wet and cool like 2023 was? Nobody knows.
What is known is that the California Avocado Society is offering a seminar next week just in case. “How to manage avocado trees in a wet year,” is the seminar’s title. It takes place in Ventura (and on Zoom) on February 15. Thrips will be one of the main topics covered. The seminar is free and all are invited to attend. I hope to learn more about thrips at the seminar and update this post accordingly afterward.
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