A Fresh Look At Insects

Dr Tom Dykstra is an American agriculturalist who specialises in entomology. Many farmers see pollination as the main
benefit of the insect population, but as he describes, there are many more. Insect damage is often an indication of an
unhealthy crop, and this is caused by crop stress which results from heavy applications of N, pesticide damage to soil
microbes, and mechanical soil damage. Through his laboratory and talks to farmers he helps them move away from chemical farming. In 2018, with John Kempf he published a seminal podcast with Regenerative Agriculture. The hour long broadcast explores the direction of his research, and this article brings out the points of greatest relevance to practical farmers.

Photosynthesis thoughts

Dr. Dykstra says “Photosynthesis can be increased substantially, and is most easily measured through spectrometer analysis using the Brix scale.” The system determines levels of sugar in plant sap and the reading is compared with standard figures for each plant species to produce a Brix measurement. Most plants score between Brix 4-8. At this level plants are productive and produce crops which most will consider satisfactory. Yet they are susceptible to disease and damage. Plants with higher Brix levels are less prone to damage and when the Brix level reaches around 12 serious insect damage stops. Higher scores in the range of 14 show a genuinely healthy plant. The Brix scores suggest that plants are not working up to anywhere near their potential and achieving this is a matter of getting the photosynthetic rate up. The problem is that farming methods particularly pesticides create blockages in the plant which cause weakness to disease.

Doing an experimental fallow

“How much land can you afford to leave fallow for a year or more?” asks Dykstra. He suggests that 5 acres from 1000 would be possible, and he wants farmers to choose some high ground which won’t get pesticide residue run-off. “let weeds grow and spray a sugar solution of 5lbs per acre as often as feasible. Planting cover crops is another useful method of getting rid of pesticide residues.” The purpose of sugar or molasses is to feed the important microbes in the soil. Microbes including fungi, insects and bacteria feed nutrients to plants and the population of microbes is heavily reduced by fungicides, insecticides and other spray products. Soil without microbial action becomes fluffy and is easily eroded.

How insects smell

In the podcast John Kemph asked Tom “what topic have you been puzzling over for a long time?” and the answer was “How insects smell”. He explained that insects have highly tuned receptors that can detect suitable food over distances of a mile or more. But insects have poor digestive systems and so look for food which is deteriorating. “The bad apple, the over-ripe plum in the basket is the one which the flies go for, and in the field insects go for plants which are weaker with poorer cell walls.” For more than 10 years Tom’s research yielded little, but by Nov 2016 his lab had worked out the mystery of an insect’s sense of smell. The signal comes from both antenna and palps.

Certain insects are tuned into certain smells – the ones which are easily digested, that are rotting and unhealthy. In the same way some plants advertise themselves as unhealthy. This might explain why insects attack the crop in one field while ignoring that in the next, even though both look much the same. Locusts are a classic and devastating example, swarming and stripping plants in a specific field. For Tom Dykstra the question is “why?” And the follow up is “how can we use this to help protect vulnerable crops?” What are some of the compounds that serve as insect attractants we could manage and monitor? Ethanol is a universal odorant which advertises plants as unhealthy – a lot of plants will release some sort of alcohol which is attractive to many insects.

These in turn have a Brix cutoff beyond which they won’t attack the plant but instead go looking for something easier and tastier. Dykstra’s dogged in-field research has shown that the Brix number is not static, but will vary with weather conditions. In particular, plants will lower their defences shortly before a storm hits, causing insects to become increasingly active and feed. Insect olfaction is the science of how insect’s smell. Although the predominant theory has revolved around a “lock and key” mechanism for multiple decades, direct evidence for it was and is lacking. Dykstra’s own research, along with that from many other laboratories, has made it more certain that the insects actually smell “wavelengths”. In 2016 the mechanisms involved became clearer and many of the details have been worked out to give a firm grasp on not only how insects smell, but how we can prevent them from doing so, and how we can simulate insect olfaction with uncanny precision.

Two discoveries are of considerable interest. The first is that nitrates and ammonium are indirectly attractive to certain insects and this increases as the level of N is raised. The plant advertises this stress making it more susceptible to insect attack. The second is that the detection distances for these insect signaling compounds are extraordinarily great.

His work concludes with the statement:

“Insects are only attracted to unhealthy plants”

The consequence is that if healthy plants don’t attract insects they don’t need pesticides for their protection. Plant protection then comes not from a can but from the conditions under which it is grown. And what is a healthy plant? One with a Brics of 12 or more. Bric scores are raised by soil microbes, and they are damaged by pesticides and soil damage.

The research and logic of its meaning is considerable for farming who have largely considered the benefits of organic farming to be with the consumer, and with the higher price they pay for the product. Tom Dykstra is telling the farmer that he benefits as much as anyone through the use of natural controls and the abandonment of pesticides. That chemicals weaken plants and cause them to be unhealthy and therefore open to disease and attack. He goes on to say that raising the Bric score produces crops and product which have greater flavour and longer shelf life, because they are inherently healthy, and this is translated to further benefit for the grower. If his fruit tastes better, it sells better.

Though technical as well as conversational, the podcast is well worth taking the time to listen to. Scan the QR Code below to listen to the whole podcast now.

Above top to bottom: Damaging soil microbes weakens plant health and invites insect damage ; Microbes in this soil increase the health of crop which makes it less vulnerable to insect and other damage ; Sward diversity lifts health of plants and livestock Image credit: Practical Farm Ideas

Author: Mike Donovan