Beyond Biodiversity: Growth-Defense Tradeoff

When describing a holistic approach to integrated pest management, I use this concept called the Everswarm: an abstraction of the various organisms that interact with plants, mainly pests, and their constant adaptability, mainly to simple management strategies. Mitigating individual pest species in isolation is often fairly straightforward, whether preventatively or reactively. Mitigating multiple pest species in tandem is chaotically complex, particularly when plant and grower solutions that work for one pest might have no effect or even increase susceptibility to another!

Pests are a multipronged threat: different species and even different subpopulations have developed physiological and behavioral resistance to natural chemistries, biocontrols, and even procedural countermeasures. Further, organisms that are usually beneficial can inflict antagonistic effects in the elicitation of the wrong immune signals for certain contexts or even beget lineages that become parasitic over time. This evolutionary force is responsible for the sophisticated tools pests use to overcome and exploit plant physiology as well as the development of these defenses themselves. Cannabis has specialists that have adapted to the species singularly, as well as pests that are shared with other plants, having developed without contact before human involvement. Healthy plants supplemented with optimized nutrients and microbiota may still be disadvantaged by a sufficiently incompatible suite of pest adaptations.

To put it in perspective: in 200 years, human cultivation and global trade significantly concentrated the selection pressures plants, and pests are exposed to and subsequently distributed them faster, longer, and in some ways safer than any avenue in the last 200 million and before.

At the strategic community level, pest management reliant on local, sustainable, varied, and rapid solutions disintegrates the “Everswarm” into manageable interactions, blunting the edge of any singular advantage and neutralizing what would otherwise be a quickly changing traveling problem. While ideally, there would be an arsenal of affordable options for each of the dozens of potential Cannabis pests, growers must, like their plants, compose a solution with finite resources. Safe compounds, microbial biocontrols or plant mutualists, banker plants that attract and sustain predators, physical barriers, and other lethal or sublethal influences add up to more than the sum of their parts when deployed synergistically.

At the localized grower level, the way to disrupt multiple pests reliably across time and space is an array of disparate, synergistic challenges to their physiology that exploit inherent weaknesses and deny the selection for strong adaptations in any one direction. The robust multidomain nature of ecosystem interactions is foundational and inspirational to that end. In the span of a human lifetime, ecosystems can retain a semblance of permanence, but they, too, change over short or long periods in discrete and significant ways. However, progress has been made by sustainability and ecologically oriented growers; there are selection pressures from conventional agriculture and other industries that strongly influence pest traits related to chemical resistance, symbiont acquisition, population reservoirs, and environmental resilience, among others. The more growers capable of synergistic, sustainable management the less these challenges persist. Since both pest and plant adaptations typically come at a metabolic cost, they are often suboptimal for contexts where they are unnecessary. For a common example, the production of detoxifying enzymes or changes in a receptor that limits the toxicity of a substance may severely reduce other traits like reproductive capacity, longevity, movement speed, and susceptibility to pathogens or biocontrols. This developmental reality underpins the efficacy of a holistic approach.

There are some important aspects of the plant-pest ecosystem that dynamic growers must understand for maximal effect, especially those that care about biodiversity as well as preventative and curative strategies. Some consequences exist for using certain microbes that can affect plant health negatively in certain contexts, like how some mycorrhizae can induce susceptibility to plant viruses or how beneficial microbes can lose their beneficial effect, become parasitic, or otherwise antagonistic over time through genetic mutations or as the net result during interactions with other organisms. Beneficial symbiotic relationships between plants and microbes like mycorrhizae also have energetic costs associated with attraction and resource “trading.” Because they must regulate their symbionts with an immune response, plants have evolved a sophisticated system of proportional balance towards the use of finite metabolic resources, referred to as the Growth-Defense Tradeoff, to engage mutualists, parasites, environmental influences, and general growth behavior. Knowing this allows growers to be precise with plant resources.

Nutrition is extremely important in plants because it powers primary (growth) and secondary (defense) metabolism as well as that of symbionts like microbes. However, if the defense mechanisms powered by nutrition aren’t relevant or effective against the threat through pest adaptation, plant mutation, or environmental interference, then the plant reaction will neutralize. Therefore, assuming adequate nutrition, if plant resistance is based on the effects of genes, then the absence of the right gene products or intensity of response to cope necessarily impairs plant health.

Pests are evolving at a rapid rate, too. Research shows that in addition to chemical resistance to even natural compounds derived from plants, resistance to microbial compounds and even insect biocontrols is much more widespread than previously realized, such as with budworm moths, two-spotted spider mites, Lygus, and various powdery mildew and aphid species which are found in Cannabis. They are also able to suppress, preferentially induce or neutralize plant immune response, which can disrupt interactions with beneficial microbes and other normal processes.

Viral entities common to other crops are being found in Cannabis at a growing but hard-to-quantify rate, including Beet Curly Top Virus, Cucumber Mosaic Virus, and, of course, Hop Latent Viroid, and these too have great mutation capacity despite being in some ways very simple.

Very basic evolutionary theory shows that population size matters *a lot*. Larger population of viruses means:

• More mutations are added to the gene pool.
• More opportunity for recombination and within-host evolution.
• Stronger natural selection on fit mutations.

Biodiversity affects pests and their ability to colonize healthy plants in the form of genetically diverse pest populations, symbiont consortia, and general ecosystem diversity. Pest adaptation to hosts is often faster than plant adaptation to pests because pests reproduce more times within the span of a single host generation time, and to understand pest-host-ecosystem relationships. It is important to realize these interactions have developed over hundreds of millions of years resulting in very intimate associations. Human cultivation has greatly quickened the exposure of pests to new environments, hosts, and hostile stressors, culminating in diverse specializations.

Insert photo credit: Matthew Gates 

Feature photo credit: Jake W.