292-Understanding Brix and How It Affects Plants and Pests

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Brix is a measure of sugar content, and when it comes to plants, the higher the Brix value, the healthier and more pest and disease resistant the crop. To give a rundown of the science and significance of Brix and how gardeners can raise the Brix of their plants, my guest this week is entomologist and Brix expert Thomas M. Dykstra, Ph.D.

Tom grew up in Rochester, New York, and went to college at Cornell University in Ithaca, New York, before he earned his master’s degree and Ph.D. at the University of Florida. All three of his degrees are in entomology. “So on paper, it looks like I’m an entomologist, entomologist, entomologist, and that’s all that I know,” he jokes, explaining that the truth is there is always another discipline associated with entomology. In his case, it was neurophysiology for his master’s and bioelectromagnetics for his Ph.D. 


Entomologist and Brix expert Thomas M. Dykstra, Ph.D., runs Dykstra Laboratories Inc. in Gainesville, Florida.  (Courtesy of Tom Dykstra)

Entomologist and Brix expert Thomas M. Dykstra, Ph.D., runs Dykstra Laboratories Inc. in Gainesville, Florida.  (Courtesy of Tom Dykstra)(Courtesy of Tom Dykstra)


Since 1997, Tom has had his own lab, Dykstra Laboratories Inc., in Gainesville, Florida, and is still heavily involved in bioelectromagnetics and insect olfaction research. He also consults for attorneys, the U.S. Department of Agriculture and academics.

If you ever do research about Brix, you’ll find Tom. He has pushed forward the agricultural science on Brix, having discovered that various Brix values have different effects on what pests show up on plants and if they stay. 

Before proceeding with my conversation with Tom about Brix, I want to take a second to remind you that I have a new book out, “The Vegetable Gardening Book: Your complete guide to growing an edible organic garden from seed to harvest.” It’s chock full of insider tips and new-to-you information that will help you step up your gardening game and tackle challenges. 

And on tap for 2023 is my new Online Gardening Academy premium course, Organic Vegetable Gardening. Sign up for the waitlist here.

 What Is Brix?

As far as gardeners are concerned, Brix is a measure, in essence, of sugar. Tom says that, to split hairs, Brix is measuring more than just sugar but practically, it’s just looking at sugar. If there are some solutes also present on a solution that contribute to the Brix value, they are minor. 

Brix is named after 19th century German mathematician and engineer Dr. Adolf Brix, who came up with a mechanism for measuring sugar in beer. Today, Brix is determined using a Brix refractometer, which measures the bending of light. 

Brix data is used in the Florida and California citrus industry as well as the grape industry. “Some of the guys who are out there looking at the grapes will have a Brix refractometer in their back pocket,” Tom says “It’s kind of one of those indispensable things that they carry around with them just to give them an idea of what’s going on.”

A wealth of valuable information about a plant can be gleaned from a Brix refractometer, and it’s not expensive at all. We’re talking less than $20. Leaf Brix, stem Brix, flower Brix and root Brix can all be measured with a Brix refractometer.

The results will reveal the amount of sugar being produced by photosynthesis at that moment and the storage of sugar.

“It gives you a window into the inside of the plant saying, how well is it photosynthesizing? And once you determine how well it’s photosynthesizing, that is a direct result of how healthy it is,” Tom says.


digital Brix refractometer

Tom’s digital Brix refractometer. (Courtesy of Tom Dykstra)


Understanding Brix Value

A Brix value is the amount of dissolved sugar in a solution. Each degree Brix is equal to 1 gram of sucrose in 100 grams of solution. 

The school of thought about Brix had been that 12 was the only number that mattered. The idea was that when a plant has more than 12 Brix, it won’t have any insect issues, and when the Brix is less than 12, everything under the sun will attack, Tom says. But as a scientist, he didn’t believe it could possibly be that simple.

Every pest known to attack a certain plant doesn’t show up all at once when the Brix value drops below 12, he observed. “The plant was either edible or wasn’t edible,” he thought. But then a lightbulb went off: Pests show up in succession, he realized.

Tom found that there are various levels of Brix that different pest insects can tolerate. “The insects are on the plant for a very specific reason to eat the plant at a certain, let’s call it, health level,” he says. “They do very well at that level. They need certain nutrients, they need certain secondary plant metabolites. They need certain sugar — high, low, whatever it is. They’re eating it because it is edible and digestible to them.”

Once the plant switches physiologically and biochemically to a different sugar level — a different Brix value — the specific pest disappears from the plant. “Then another insect might come in right after it,” Tom says. “And so the concept of succession started to creep into the lexicon.”

Tom decided to sit down and make a chart of leaf Brix and four categories of insects that favor specific Brix values. He originally had just two categories, sucking insects and chewing insects, but he broke that down further because he discovered that insects that suck phloem tissue had a different reaction in changes to Brix value than other suckers, and there were also differences among chewers.

The four groups Tom has defined are: 

Grasshopper group – long-horned grasshoppers (Tettigoniidae), crickets, and related insects. They will target a plant when the Brix falls below 12 but will lose interest in it when the Brix sinks below 10.

Chewing insects – beetles (Coleoptera) and butterflies and moths (Lepidoptera) and related insects. They lose interest in a plant when the Brix value reaches as high as 9 to 11. Below that range, a plant is vulnerable to chewing insects.

Sucking insects – leafhoppers (Cicadellidae), froghoppers (Cercopoidea), planthoppers (Fulgoromorpha), stink bugs (Pentatomidae) and thrips (Thysanoptera). They begin to lose interest in a plant when the Brix value reaches between 7 and 9.

Aphid group – aphids (Aphidoidea) and scale (Coccoidea). The insects in this group prefer a really low Brix value, below 6 to 8, because they can’t tolerate too much sugar at once.

Tom says when he sees a plant infested with aphids and he measures the Brix, sure enough, the Brix value is between 3 and 5. The aphids can tolerate as high as 7 Brix, but at 8 or higher, aphids are absent. 

All insects are indicators of Brix levels — not just some, Tom says. So if you know the Brix tolerance of a pest and you see it on a plant, you have an idea of what the Brix value of that plant is without having to use a Brix refractometer. 


The four insect groups that Tom has defined lose interest in plants as the Brix level rises.


Spreading the Word About Brix

Over the past 25 years, Tom has looked at Brix in many different plants, from household plants and crop plants. But because he is an industry entomologist and not in academia, he can’t publish studies and research papers. Still, Tom wanted the information he learned to get out there, so he started talking more publicly, predominantly to farmers. “They’re the ones that were producing most of the food that we are eating in this country, and I wanted to make sure that they got this information,” he says. “And so I would go to the farmers, and it was very well received.”

A little over a year ago, Advancing Eco Agriculture, a company that promotes regenerative agriculture, asked Tom to share his findings on a webinar. Rather than reaching up to a thousand farmers at a time, he reached an audience of hundreds of thousands of webinar viewers on YouTube

“This has opened up a whole span of discussion across the internet and across email space,” Tom says. “And when I go to conferences, this also spawns many conversations as well, because so many people have seen the presentation on Brix and they’re now really starting to have a discussion about it.”

Now, universities around the world have reached out to Tom asking for his blessing to study Brix in the way he has and to publish academic papers and scientific articles. He encourages further study.


Can a High Brix Value Make a Plant Insect-Proof?

In entomology, talk of “inducement” is common, such as a caterpillar chewing on a plant inducing that plant to produce secondary plant metabolites to possibly deter feeding by the caterpillar or to advertise to wasps who prey on those caterpillars, Tom says. Since caterpillars only end up on a plant if a moth lays her eggs on it first, he wondered what told the moth that a plant was a good place to lay eggs in the first place. 

For a brassica grower like myself, I wonder if this means I can prevent cabbage butterflies from laying eggs on my broccoli without having to use row cover. Tom has good news:

“You can reach optimal health, and you can prevent the white cabbage butterfly from laying her eggs on your cabbage,” he says. “The answer is unequivocally yes.”

A cabbage butterfly is looking for a plant that her young can chew through and actually get nutrition from. That means a plant without too many secondary metabolites to deter her children from eating the plant, and of a particular health level. But if the health of the plant increases — if the Brix value increases — after the mother lays her eggs, her young will starve to death.

Likewise, a mass of fall webworms on pecan trees will not eat a healthy leaf just inches away, Tom says. “Over time they will starve to death. and they will fall off one by one. They will no longer make a webbing so they’re incredibly vulnerable to birds at that point.”


floating row cover as pest protection

I often use row cover to protect my plants from egg laying insects, but Tom days row cover becomes unnecessary when the Brix value of the plants is high enough.


How to Take a Leaf Brix Measurement

For an accurate measurement of a plant’s leaf Brix, take a healthy-looking leaf at an optimal time of day when photosynthesis is going strong, in the afternoon. Extract liquid from inside the leaf and put it into the refractometer, according to the manufacturer’s instructions. 

The leaves are where plant sugars are produced through photosynthesis, and where most of the sugar is stored. As far as gardeners are concerned, leaf Brix measurements are what you need. Scientists may learn a lot from root or stem measurements, but obtaining root or stem samples typically involves the destruction of the plant — and we don’t want that.

To perform a Brix measurement with a Brix refractometer, you need enough leaves to give you two to three droplets of liquid. Tom uses 30 leaves, but you don’t necessarily need that many. The leaves go into a garlic press — laboratories use a hydraulic plant press — to get the liquid out. 

The liquid goes into a Brix refractometer. There are types that the user holds up to the light to get a line reading, and there are digital Brix refractometers.

Tom points out that fruit Brix will be higher than leaf Brix by the time the fruit is ripe. Apples, oranges, strawberries and things of that sort can all easily break 20 Brix, Tom says, while leaves almost never break 20.

“The goal of the plant, of every plant, is to put more sugar into the fruit because it wants it to be eaten, because that’s where the seeds are located, and they want the seeds to spread,” Tom says. “So you want the sweetness to be in there, and so that’s where the plants put most of the sugar.”

This can take months. Plants store as much sugar as they can in their leaves before fruiting begins. When they set fruit, they start sending sugar to the fruit.


hydraulic press

Gardeners can use a garlic press to extract liquid from leaves, but labs use a hydraulic press.(Courtesy of Tom Dykstra)


How Fertilizer and Pesticides Negatively Affect Brix Values

Avoid using nitrogenous fertilizers and pesticides, Tom advises. Nitrogen-heavy fertilizers will, in fact, lower the Brix. 

“They are almost all universally going to lower Brix, or they will do nothing to it,” Tom says. 

High-salt fertilizers are not good for soil, Tom adds. (In rare instances, when the conductivity in the soil is so low to begin with, there can be a benefit, he notes, but he emphasizes that this is an exception.)

“Many farmers have gotten to the point where they’ve now got dead zones on their property from the nitrogenous fertilizers — the high-salt, NPK fertilizers — where nothing will grow anymore because the conductivity is too high,” Tom says.

Pesticides will also damage soil health. Tom says insecticides and herbicides will kill microbes in soil as well as the insects or weeds they were designed to target, and fungicides designed to kill a fungal pathogen will also kill beneficial fungi in the soil. 

Because some pesticides persist for weeks, months or even years, and fertilizer salts also last, attempting to replenish microbes by inoculating the soil won’t help because the soil is toxic to the microbes, either by salt toxicity or chemical toxicity


aphids on plant stem

Repeated use of nitrogenous fertilizer and pesticides destroys soil health, lowering the Brix value of the plants grown in the soil and leading to more pest issues.


How to Raise Brix Levels

To raise Brix levels, there is no silver bullet. That is to say, there is no one thing you can give all ailing plants to fix them. Plants have low Brix levels when they are deficient in specific nutrients, so only those specific nutrients will help.

Tom says to use nutritional addendums in solid or liquid form, rather than nitrogenous fertilizers. Some may not have an effect because they aren’t providing what the plant has been missing, but others can increase the Brix value rather quickly. 

Measure the Brix value again 24 hours after adding nutrients to the soil. Tom looks for a jump of half a Brix, but he’s seen some nutritional addendums result in a 2-point jump in a day. “There may have been a deficiency in the plant, which is now being supplied by the particular addendum that you’re putting in,” he says.

He compares this to how scurvy was addressed on ships. Sailors were dying of scurvy, which occurs in people suffering from a vitamin C deficiency. But when the sailors were given oranges or other citrus fruits to bring on their journeys, scurvy was no longer a problem. Giving people — or plants — a nutrient that they were deficient in can be life-saving, Tom says. 

There are a number of ways to identify which nutrient is lacking.

“When we’re looking at crop plants, we do like to test rather than guess,” Tom says. “It’s just better. Being a scientist, it’s always nice to have numbers, especially with as many significant digits as possible.”

Scientists may use tissue analysis, plant sap analysis, soil analysis and leaf Brix readings with the refractometer.

With the data in hand, it’s easy for Tom to zero in on what the problem is and how to resolve it. 

“Many times people don’t realize that the answer is microbes, and they simply just need to feed the microbes,” he says.

The microbes need sugar and may need certain nutrients that the plants themselves don’t necessarily need. Healthy plants will put sugar out through their roots into the soil, so healthy plants will lead to more microbes in the soil, which in turn will lead to even healthier, more vigorous plants. It’s a virtuous cycle.

“Suddenly everything starts to improve. Your Brix readings will go up, the insects will lose interest in the plant, disease will not take hold of the plant anymore, and you’re there,” Tom says.

And don’t worry about having the exact ratios of nutrients in addendums. Tom says the microbes in healthy soil will pick off the necessary minerals from the rock dust you added to the soil and deliver the nutrients to the plants. 

When plants have low Brix, adding sugar from the grocery store, diluted in water, to your soil will create an immediate increase in soil microbes, according to Tom. He says there can be enormous benefits to low-Brix plants in less than 24 hours.

Molasses is far superior to a bag of sugar but more expensive, he says. “You’ll get much greater benefit because not only do you get sugar from the molasses, but you get a lot of micronutrients from the molasses too.”

The results are temporary — the microbe levels will be improved for about a week, after which the microbes have eaten up all the sugar. Gardeners with a short growing season could keep applying sugar weekly, Tom says.


Applying liquid fertilizer

Applying missing nutrients to soil can have a rapid effect on Brix values.

Overcoming Brix Skepticism

Some skeptics have told Tom that raising the Brix value of their crop above 7 or 8 simply could not be done, so Tom went into the woods where there had been no human intervention and took leaves off trees that have never been fertilized by anyone but nature — and the tree had a leaf Brix of 15.5.

He drives home the point by noting that if high Brix values really are unachievable, then all of the insects on Earth would have eaten all of the vegetation by now. But in reality, insects only prey on some vegetation. 

I hope you enjoyed my conversation with Dr. Tom Dykstra. If you haven’t listened yet, you can do so now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title. 

Have you used a Brix refractometer in your garden? What did you learn? Let us know in the comments below.

Links & Resources

Some product links in this guide are affiliate links. See full disclosure below.

Episode 028: The Role of Minerals in Making Great Soil

Episode 116: Understanding the Soil Food Web, with Dr. Elaine Ingham

Episode 270: Plant Science for Gardeners, with Robert Pavlis, Part I

Episode 281: The Chemical Age: How Tools of War Became Agricultural Chemicals

Episode 282: The Vital Role of Soil Bacteria in the Garden, with Jeff Lowenfels

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joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.

joegardener Online Gardening Academy Growing Epic Tomatoes: Learn how to grow epic tomatoes with Joe Lamp’l and Craig LeHoullier. 

joegardener Online Gardening Academy Master Pests, Diseases & Weeds: Learn the proactive steps to take to manage pests, diseases and weeds for a more successful garden with a lot less frustration. Just $47 for lifetime access!

joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.

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Growing a Greener World®  


Advancing Eco Agriculture YouTube: Why insects do not (and cannot) attack healthy plants | Dr. Thomas Dykstra | Regenerative Ag

Brix refractometer

Digital Brix refractometers.

Garlic press

Disclosure: Some product links in this guide are affiliate links, which means we get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, AeroGarden, Milorganite, Soil3, Greenhouse Megastore, PittMoss, Territorial Seed Company, Earth’s Ally, National Wildlife Federation and TerraThrive. These companies are either Brand Partners of and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.

About Joe Lamp'l

Joe Lamp’l is the creator and “joe” behind joe gardener®. His lifetime passion and devotion to all things horticulture has led him to a long-time career as one of the country’s most recognized and trusted personalities in organic gardening and sustainability. That is most evident in his role as host and creator of Emmy Award-winning Growing a Greener World®, a national green-living lifestyle series on PBS currently broadcasting in its tenth season. When he’s not working in his large, raised bed vegetable garden, he’s likely planting or digging something up, or spending time with his family on their organic farm just north of Atlanta, GA.

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