What are susceptible or resistant plants?

We often hear about resistant plants or plants that are susceptible but what exactly does that mean?

Resistant plants have properties that resist or repel an antagonist (usually a herbivore insect or microbe). Susceptible plants refer to the degree to which a plant is susceptible to another living organism. It’s the degree of attraction or the degree to which a plant is beneficial to another.

A third class known as intermediate plants and they have mixed (resistant and susceptible qualities) or diluted effects. Therefore a plant can be both resistant and susceptible at the same time.

However, no one plant can be described as ‘a susceptible plant or ‘a resistant plant’ or an ‘intermediate plant’. Susceptibility, resistance and intermediate effects are only ever in relation to a particular organism.

Susceptibility and resistance is a relationship and not a static state of being. Therefore a resistant plant is only resistant in relation to a particular antagonist.

Susceptibility and resistance fluctuate depending upon factors such as temperature, soil quality, soil microbial activity, plant health/stress etc. During these fluctuations they seem to become intermediate plants.


Simple plant chemistry for non chemistry people

All plants have metabolites.  Metabolites are chemical compounds.  All plants have primary metabolites. They are essential for growth and development.

Secondary metabolites are plant specific. Secondary metabolites are responsible for plant susceptibility and plant resistance. They mediate the interactions between the plant and other organisms. So the interaction between a plant and a bee is mediated by one or more secondary metabolites. For example: pollen and nectar.

An interaction between a plant and a herbivore, or a plant and a disease pathogen is mediated by secondary metabolites.  Some of these metabolites are glucosinolates and others are saponins.

Research shows that some secondary metabolites attract herbivores and feed them with no detrimental consequences to the herbivore. Those are susceptible plants such as most brassicas and the diamondback moth.

Some secondary metabolites repel or poison particular herbivores. For example, the brassica Barbarea vulgaris G-type (some Land Cress) attracts the diamondback moth and poisons the larvae.

Some, if not all plants have a mixture of both attraction and repulsive metabolites and all of them are species specific. This may be due to evolutionary processes where plants and herbivores evolve together causing a susceptibility and resistance over eons.

Some secondary metabolites are excreted into the root zone (the rhizome). These are called root exudates. Some of these root exudates attract and feed certain microbes. For example a legume attracts and feeds a particular bacteria (rhizobia) making that interaction ‘nitrogen fixing‘.

Other root exudates attract and poison certain microbes. Others simply repel certain microbes. For example, some cultivars of Marigolds (not all) attract some root feeding nematodes and poison them.

All plants secrete root exudates. Therefore, plants and the rest of the ecological community are constantly interacting.


Resistant plants and how they work

Generally, resistant plants have an effective defence mechanism against an antagonist. For example: glyphosate resistant plants have a defence mechanism against glyphosate.  All other plants exposed to glyphosate (applied under key conditions) will be susceptible and die.

Of course if we apply glyphosate just before it rains, that plant will not die. That’s not due to resistance. It’s because a key condition of susceptibility didn’t exist – dry weather.

Resistant plants may have medicinal/toxic properties the consumption of which makes it toxic to creatures that may graze upon them.

Therefore, some plants may be effective insecticides, herbicides and/or be effecting in killing microbes, generally. These are generally known as resistant plants. They have properties that are resistant to significant predation by specific herbivores. An example is some types of Barbarea vulgaris (Land Cress) with the diamondback moth.

Over eons antagonist herbivores become resistant to plant defences. Some plants adapt and produce better and more sophisticated defence mechanisms. Some researchers suggest that saponins are a more contemporary metabolite compared to glucosinolates. This is one theory behind the Barbarea vulgaris (G-type) and diamondback moth resistance relationship.

Other plants that don’t adapt simply become susceptible plants to that particular antagonist. Such as all other brassicas (except some Barbarea vulgaris) and the diamondback moth.


Susceptible plants and how that works

Susceptible plants produce metabolites that attract other organisms toward them. For example: a bee is attracted to pollen; birds and predatory insects are attracted to nectar.

Another example is with our legumes which are commonly referred to as nitrogen fixing. This nitrogen fixing capacity is because legumes are susceptible to rhizobia (a particular bacteria) and those organisms work together. Their relationship is mutually beneficial.

Some susceptible plant-herbivore relationships are totally antagonistic to the plant or at least they appear to be. For example our legumes are susceptible to some aphids or some leaf miners. This appears to be a relationship detrimental to the plant.

However, it is also a signal the that plant is distressed and/or growing in the wrong environment. A sick plant will produce sick seed and so it may be that these herbivore attacks are actually helping the plant to die so that the plant doesn’t set sick seed…

Susceptible plant-other organism relationships tend to foster the development and reproduction of the other organism. Therefore susceptible plants are also known as host plants. Our legume is a host plant to some aphids who consume the sap and grow in numbers and proliferate. Our legume is also a host plant to the rhizobia.


Sacrificial plants and trap plants

I’ve noticed that other blogs and writers tend to not differentiate between the terms ‘sacrificial plants’ or crops and ‘trap plants’ or crops. However, I’m differentiating between them. I’m doing so because it appears to me that the mechanisms are based on susceptibility and resistance. Likewise, the outcomes to herbivores are very different.

With susceptible plants, we can select a plant to sacrifice. We sacrifice one plant to herbivores and in doing so entice those herbivores away from other plants more valuable to us. For example we might sacrifice our nasturtiums to aphids rather than our roses to those same aphids.

Typically the sacrificial plant or crop doesn’t poison the herbivore, it simply satisfies their hunger away form another plant or crop.

When the herbivores are feasting on our susceptible, sacrificial plants: we can remove those infested leaves to reduce reproduction. However we need to be vigilant with this as most herbivores (like aphids) reproduce very quickly.

By the same token, an aphid infested nasturtium will also attract predatory insects. The flowers and the aphids are the attraction here. So in setting up that nasturtium to sacrifice we are also increasing the populations of predatory insects in our garden.

Likewise, sacrificial plantings don’t tend to work so well with soil microbes and possibly other herbivores. For that we probably want a trap crop.  These are resistant plants with toxic metabolites. For trap crops we want consumption of the toxins. Often the amount consumed is very little and the plant is not ‘sacrificed’ in the process.

Of course, there can be a mixture of the two. That would make it a sacrificial trap crop! That would be a plant that attract and then poisons.

Plants that have metabolites that merely repel can’t be called trap crops because there is no trap. However, these plants can be handy in companion planting and they are classified as resistant plants.


So what do we do about herbivore and ‘pest’ problems?

We need to create healthy soil appropriate for our plant species. Our plants need to be suited to our climate. Our plants need to have access plant specific natural resources like light, shade, rain, air etc.  If we tend to these conditions, we’re much less likely to have a herbivore or ‘pest’ and disease issue.

I cover most of this in my blogs on creating healthy soil and also on organic methods. Overtime I’ll be posting blogs that are plant specific so keep an eye on the Plants menu and category as they’re being posted all the time.

We also need to observe some basic rules. One of these is crop rotation. The pathogenic reason for crop rotation is based on the principle of susceptible plants. If we grow susceptible plants of particular types, we need to observe and practice crop rotation rules.

Failing that best practice form of horticulture…

If we have a herbivore problem in our garden then we need to identify that particular species. If it’s a nematode issue, we need to identify the specific type of nematode. We can’t just go and get a Marigold and hope it works on a root feeding nematode. That type of Marigold might be susceptible to the type of nematodes we have in our gardens…

If it’s a caterpillar issue, we need to identify the specific type of caterpillar. Then once we’ve identified the herbivore, we then go about identifying the particular type of plant specifically resistant to that herbivore.

Sometimes we think this is all to hard, and instead rely on pesticides. However, in pretty much all countries including the USA and Australia, we need to identify the herbivore before using a pesticide. This is particularly the case with insecticides.

If we don’t do that and we use the pesticide on the wrong species – ie a species not identified on the label: we have just broken the law. Many of us don’t realise this but that is the law in most countries. Therefore to identify that we have mites for example, is not enough. What kind of mites do we have? What is their lifecycle? We need this information to comply with the law and horticultural best practice.


Herbivore identification is a global issue

Many herbivores (for example species of aphids, mealy bugs, mites, thrips and many moths and their larvae) are becoming increasingly resistant to many synthetic insecticides.  For example, the diamondback moth is resistant to pretty much every insecticide. Apparently, it only took 7 years for the diamondback moth to form a resistance to DDT. I’ll cover insecticides and insecticide resistance in another blog.

I’m going to stress this point: We need to understand that each time each and every one of us use synthetic chemicals, the more we contribute to global ‘pest’ resistance.  Insects are increasingly becoming resistant to insecticides.  Bacteria are increasingly becoming resistant to antibacterials.  This is a global problem caused by local use and misuse of synthetic chemicals.

In contrast, even though resistance is a natural process, that process takes eons of time, rather than 2-20 years as reported by the Insecticide Resistance Action Committee.

That’s just another good reason to garden organically and mindfully.


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Beverley · June 30, 2018 at 2:49 pm

Oh sorry- I didn’t ‘log-in’ with my comment – so I am ‘Anonymous’!

Anonymous · June 30, 2018 at 2:45 pm

What an amazing and informative article! I feel as though I am now a ‘chemistry person.’ So much easier to understand! – although I must admit I did read the entire thing twice…! Thank you, Amber.

    Amber Hall · July 5, 2018 at 8:17 am

    Thanks Beverley, yes there’s a lot of information in that article and I’m glad you found it informative 🙂

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