Plant Sickness – Why Do Pests Eat The Sick Plants?

Studying plant sickness is fascinating because we know pests only dine on sick plants.

What I have for you today is some info on why those plants invite pests when they’re sick, and how to avoid that.

Plant Sickness Transcript

Today I’m going to talk about plant sickness and specifically I’m going to be looking at a book that was written by a French guy named Francis Chaboussou – it’s called Healthy Crops and it’s really amazing.

What he did is he went and looked at a lot of different research about why plants get sick and why they get attacked by insects and I want to talk about that here.

So let’s say you have a plant. You have a plant and it’s getting attacked by insects and I’ve talked a lot before about how only sick plants get attacked by insects. A healthy plant doesn’t get attacked because it’s not food for insects – an insect cannot digest it.

But this research really got more interesting as to exactly what’s going on in the plant and what makes that happen, and he found out that if a plant has excess soluble nitrogen and I think also excess sugars then what’s gonna happen is insects are gonna wanna come and get those – especially nitrogen is my understanding.

As to what happens, there can be many reasons, but for some reason the plant is having a problem taking these things and producing proteins and carbohydrates. The synthesis of these things is stopping. Now this could just be a soil imbalance of minerals – you know, we have very imbalanced soil.

It could be a poor soil food web where we don’t have the beneficial microorganisms to help them out. It could be a water issue.

It could be that the plant got injured. It can be a number of things so on its own – it doesn’t really tell us that much but there are a couple of implications for us gardeners.


One is definitely that if we apply too much nitrogen, which is very common in the spring, especially soluble nitrogen if you use chemical fertilizers (pile them on the lawn in the spring to get that nice green up and even into the garden), what’s gonna happen is often you’ll see later in the summer the insects come to dine on the excess nitrogen.

So we don’t want to be applying too much nitrogen. It’s an important nutrient but we don’t want to apply too much.

Now if you’re applying compost, it’s more slow release so it’s not going to be as big of an issue, but you can apply too much compost and especially manure as well. My main point there is let’s not apply too much nitrogen at any one time cause that can cause a problem.

The other one interesting thing he found is that chemicals, especially pesticides, cause this to happen. They sort of stimulate this response where the plant can’t form protein or carbohydrates, so if we’re spraying an insecticide to kill insects we know that it hurts the plant. It causes this to happen to a plant.


So more research showing you how spraying any kind of pesticide, insecticide, fungicide – these things affect the plants too. They affect the whole soil food web.

So that’s the main thing to realize. I guess it’s really just more proof that only sick plants get attacked. If a plant is balanced and has the right amount of nitrogen and sugars and minerals to create protein and carbohydrates, it’s not gonna be food for the insects.

They simply won’t even find the plants. So that’s just what I wanted to talk about today about plant sickness. It’s a little more advanced and the details, the science of it, is not super crucial but some of the implications are: not applying too much nitrogen, not using pesticides and just making our plants healthy instead of fighting the pests.

Any questions?


  1. Barry hocking on May 6, 2012 at 9:37 am

    NO No the more sugars that are in the plant the more it will repel the insects. Remember your posts on brix readings?I think even you wrote somewhere that if you use molasses as a leaf spray it willhelp repel insects. Perhaps you should read some of your old posts. Regards, Barry. 

    • Phil on May 7, 2012 at 6:46 pm

      Hi Barry, you’re partially right. I didn’t go deep enough on this in this post, so thanks for making me explain further: First we have to remember that brix doesn’t measure just sugar. It measures sucrose, fructose, vitamins, minerals, amino acids, proteins, etc.The problem is when simple sugars like glucose aren’t converted into complex carbohydrates like starch and cellulose. That means trouble – the plant will be low brix and plant predators will let us know because they like to eat those simple sugars. When the plant has optimal nutrition and is able to turn simple sugars into complex sugars (and nitrogen/amino acids into proteins), it will be higher brix, healthier, and pest-free.So indeed a plant with excess simple sugars that aren’t being used properly will be low brix.

      • Barry hocking on May 10, 2012 at 7:57 am

        No I’m 100% right not partially right. A refractometer when used on plant sap is primarily a measure of the carbohydrate level in plant juices. Regards,Barry.

        • Phil on May 10, 2012 at 4:55 pm

          Yes Barry, we’re in generaly agreement about that, but the point still stands that insects go after plants which have excess simple sugars that aren’t being properly converted to complex carbohydrates.

  2. Suzanne A Tilton on May 6, 2012 at 2:49 pm

    Another very compelling reason not to use insecticides!

  3. garden storage on May 7, 2012 at 5:17 pm

    Wow! Smartly written post. Thanks a lot.

  4. Veggie on May 10, 2012 at 3:42 am

    I guess you are saying too much nitrogen causes more growth but this growth is less nutrient rich or complex tissue. I wonder if eating nitrogen stimulated veggies will taste sweeter than slower organic growht?Also, wouldn’t spraying the leaves with molasses compost tea attract insects to the sweet coating?

    • Phil on May 14, 2012 at 2:58 pm

      Yes, too much nitrogen causes fast, lush growth, but it is often too fast and then plant predators are attracted. And actually, veggies grown with chemical nitrogen are often less sweet.Molasses may attract some insects, but they don’t tend to be plant-feeding insects

  5. Barry hocking on May 10, 2012 at 7:42 am

    Hi Veggie,You would you not spray your leaves with molasses Why?  Molasses has been proven by many university and field studies to be one of the best bug deterrents around. We conducted tests over a 3 Year period. In Australia, South Africa and China. Not only does it make some plants inadvisable to pests. It also gives the plant a shot in the arm as it were.  It is not widely used as the big chemical company’s cant make any money off it. When a plant becomes stressed or some other problem the first thing to drop are the sugar levels. I use the word sugars in a broad term. There are many forms of sugar.Still the old saying stands tall. Look after the soil and the plants will look after themselves. Regards,Barry.        

  6. Rainer Buell on June 6, 2012 at 2:53 pm

    Hi Phil,My family has planted our first organic garden and just this week we have noticed some leaves being eaten by some type of insect. Prior to planing we prepped the soil with manure, ag lime, seed meal, bone meal and kelp meal. After reading this post and the comments it is evident that there is some kind of imbalance. What kind of immediate intervention do you recommend to stop the insects from eating the leaves? We have found your blog and Academy very valuable, thank you!Rainer

    • Phil on June 9, 2012 at 11:13 am

      Hi Rainer, it’s a very complicated process and I can’t get into much detail here. With some insects, there are homemade recipes that work that include garlic and mineral oil, or there are purchased products such as insecticidal soap or horticultural oil, but I don’t use any of these much.If I have insect problems, I will spray with a mix of effective microorganisms and a biostimulant such as sea minerals or liquid kelp, in order to hopefully improve the health of the plant enough so that the insects move away.But if they stay, I let them eat the plants with the knowledge that I need to improve my soil or plant source or maintenance methods for the next time.

      • BarleySinger on October 17, 2015 at 7:09 pm

        If I know it will take time to diagnose a sickly plant and I want my plant to survive my “learning process”, then I go for cayenne sprays & neem oil (DIY using safe liquid soap). Neem kills the insects you don’t really want in your garden and leaves to good ones alone.Sidenote – in Australia, when Neem oil hit the market in a big way and farmers started turning to it instead of dangerous agro poisons, the chemical corporations reacted by contacting their friends in the government regulatory group for such things… and THEN the government put a rating on NEEM that claimed it was more dangerous than DDT – and it took over a year of serious political action to get them to stop lying for their friends. After the dust settled, Neem plantations became a new cash crop here. The revolving door (and golden handshakes) between entrenched businesses and government can be annoying.

  7. Bill_G on June 12, 2012 at 2:00 pm

    My advise to Rainer is do nothing. Observe the plants. Try to determine what is dining on them. It may be just snails or slugs that you can pick off by hand. It takes time to achieve a garden that can withstand the occasional munch. The plants attract insects. The insects attract predators. The predators maintain the balance. Chasing off the insects chases off the predators leaving you to personally manage and protect the garden. That’s a lot of work. Suffer through the bitten leaves. Wait for the birds and wasps and other friendlies to arrive. Your garden will end up feeding more than just yourself. 

    • Phil on June 15, 2012 at 1:56 pm

      I agree, there’s usually not much point in spraying to get rid of predators. But insects do tell us a lot about our soil/garden ecosystem, and they actually give us a clue as to what we need to do to improve things. So I do encourage learning about these clues and taking steps (e.g. fertilizing, soil food web improving, watering) to improve the system.

  8. Kathy on June 23, 2013 at 7:35 am

    My Italian plum tree bloomed and had a plethora of plums, then I saw it was totally infested with aphids. It killed most of the plums. What do you do about aphids? The leaves are curled and of course I won’t spray.

    • Phil on June 24, 2013 at 5:32 pm

      That’s a big question. For short term control, neem oil or hort. oil or insecticial soap can help and are not too harmful. In the long run, improving soil and plant health are key, as well as providing food/water/habitat for aphid predators like ladybugs.

      • Kathy on June 25, 2013 at 10:50 pm

        I have a hard time believing that insects only dine on sick plants. Why would they be different from animals? I had an Aussie who knew the difference between organic cabbage and commercially grown and refused the commercially grown, he would only eat organic cabbage, and he was very possessive of his cabbage. He knew it was good for him. It cleared up his cloudy eyes. Why would a looper moth lay eggs on sick plants so that the loopers would have to eat sick plants? It makes more sense that the moth would choose the best and tastiest plants for her offspring.

        • Phil on June 26, 2013 at 2:09 pm

          I agree entirely – it does make sense if you don’t know much about insects, as I didn’t when I started studied organic farming and gardening. But insects go after free amino acids and reducing sugars which only manifest in plants that are imbalanced.

          • Kathy on June 26, 2013 at 5:59 pm

            oh, so they are more like alcoholics! LOL Thanks for your time.

  9. martha on May 25, 2014 at 3:10 pm

    What do flea beetles tell you about soil? Most of my vegetables are very healthy, but the eggplants are consistently riddled with flea beetles, despite application of beneficial nematodes, molasses, em1 and compost tea. Any suggestions about what I can do to improve my soil?.I just sent for a reams soil test, but that might take a while to get the results. Any quick fix?

    • Phil on May 26, 2014 at 12:02 pm

      The only info I have on flea beetles and eggplants is the usual deficiency of calcium and phosphorus. The N-P-K fertilizer I use could be a quick fix (… ). If your eggplants are the only ones suffering, I wonder if there’s another issue causing them to not be able to get those nutrients, such as cold soil, which they really don’t like.

      • martha on May 27, 2014 at 11:57 am

        Thanks Phil. I’m committed to organic gardening, but this points me in the right direction. Another source suggested a calcium, phosphorous and selenium deficiency (Arden Andersen’s “Energy in Agriculture”) so I will experiment with some solutions.Have you ever tried making Water-Soluble Calcium Phosphate (WCP) which I learned about from Korean Natural Farming? I’ve tried it, but it’s too soon to measure results. This is made from charred cattle or pig bones soaked in brown rice vinegar (BRV). A charcoal grill, long handled tongs, charcoal and clean, boiled bones are needed. Get the charcoal hot, then place the bones directly touching them, turning a few times so they blacken evenly. This will take about 45 minutes on low heat. When they are done – they will be gray evenly on every surface. Black is underdone, white is overdone.Let them cool a bit, and weigh them. The proportion of bones to BRV should be 1:10. Place the charred bones into a wide-mouthed, glass container which is almost filled with BRV. The bones will give off bubbles, sizzling like ginger ale. This is the phosphoric acid being released. Leave 1/3 empty space on top for air. Store in the dark at 72 degrees. The liquid should stay fairly clear. Strain after a week and store in a glass container.To use, dilute with water 1:1000. Spray on leaves during flowering stages or growth periods. Use also when buds have weak differentiation or growth is poor.

        • Phil on May 27, 2014 at 7:23 pm

          I’ve made something similar and I think it’s a useful thing. Doesn’t supply all that much calcium or phosphorus (unless you have a small garden), but sometimes just a little can help a lot.

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