Pests don't attack healthy plants

I learned that 'pests don't attack healthy plants' from two different sources this week, a coincidence I can't ignore.

I heard it first during a lecture by Dominic Scanlon on 'How Trees Grow' hosted by Plymouth Tree Partnership. Dominic talked about it in the context of diseases as well as pests. He explained that for trees to succumb to either, they must be damaged in some way first, either through physical damage such as losing a limb or through nutrient deficiency such as low nitrogen availability. Insects, viruses, fungi, and bacteria, all effectively need a way in first, some sort of weakness in a tree's defences.

I found this concept quite thought provoking. I suppose this isn't something specific to plants. Thinking more widely, it seems to apply in the context of human health too. But I digress. 

While listening to Dominic's presentation, this idea made sense to me for diseases, but less so for pests. In my garden, for example, at times it feels like slugs gorge on everything. All those poor little seedlings there one moment and gone the next... sigh...

This is where my second encounter with 'pests don't attack healthy plants' comes in. I heard it again today while listening to an OrchardPeople.com podcast in which Dr Thomas Dykstra explained how to use the measurement of leaf brix to determine the susceptibility of your orchard trees to pest attack.

I know what you are thinking, what the heck are 'leaf brix'? I must admit, before today I would have asked the same thing. Thomas, using Dr Dykstra feels just too formal, explained that brix are units of measurement for dissolved solids in a liquid. In the context of leaves, Thomas said you can think of brix, simplistically, as a measurement of dissolved sugar. There you have it, leaf brix.

According to the science, a leaf brix measure of 12 or above for a plant is highly desirable. Farmers ideally aim to hit or exceed this for their crops. This is because high sugar content indicates the plant is photosynthesising efficiently and as such is healthy. Take this one step further, you already know what I'm about to say, 'pests don't attack healthy plants'. In other words, leaf brix are a proxi-measure for a plant's natural pest resistance. Plants with a leaf brix measure of 12 or above have enough resources to invest heavily in metabolising anti-pest chemical defences.

Interestingly, leaf brix are relatively cheap to measure, which surprised me. You can buy a decent hand-held refractometer that'll do it for £20 to £30 online with ease. 

I must admit, however, after listening to Thomas' talk, I'm not convinced going to this expense is even necessary. It seems to me, the best thing to do is read the pests themselves.

If you notice pest damage on a plant, the leaf brix measure must be low, right? As soon as you notice, you can ask yourself why, implement changes, and watch the plant resolve its own pest problems as its health naturally improves. For example: Is it too shady? Replant in the sun. Is the soil too acidic? Amend with lime. Is the plant root bound? Repot it. This is what I'm now thinking.

I could have used this advice with my basil recently. I had two pots, one happily growing in my greenhouse and the other happily growing outside. Both were thriving. A few weeks ago, I needed more greenhouse space so I moved the greenhouse basil outside right beside the other. To be honest, I sort of forgot about them after that. When I next looked, I was crestfallen. The former greenhouse basil was gone, no more than skeletonised stems. Utterly annihilated by slugs. Just look!

How striking is that? One pot ready to harvest and the other non-existant... What happened? What I can surmise is brix. The former greenhouse basil must have gone into shock with the temperature drop, its photosynthesis suffered, its defenses then plummeted, and the slugs moved in to reap the harvest as it were. Fascinating!

I don't think I will ever look upon pest management the same way again.