Exploring poorly researched insect fauna: how plants amplify the net ecological benefit for inconspicuous invertebrate communities

Thursday 5th August 2021

The fourth in a short series of posts from Radoslav Valkov, born in Sofia, Bulgaria. In 2010 he won CIWEM’s prestigious British award “Young Environmental Photographer of the Year” with the highly artistic work “The Fortune Teller” that received wide international exposure. He studied Zoology at the University of Roehampton, London before returning to Byala Slatina, Bulgaria. A member of the Flemish Entomological Society, he is focused on plant-insect interactions and citizen science. His first article On the importance of inconspicuous flowering plants – how a “noxious weed” sustains valuable insects appeared in the Society’s journal, Phegea.

Assessing the actual value of individual plant species to beneficial insects that deliver ecosystem services is a topic of urgency. The array of complex interconnections between insects and plants, seen in natural pest control and pollination, for instance, is sometimes overwhelming. Here are a couple of intriguing, but largely overlooked areas of entomological knowledge that affirm the need to look at plant-insect interactions into more detail; these strike me as particularly appropriate to discuss.

As plants are vital ecological centrepieces, they are my primary reference point, the very shelter and food source to many insects, the places where I can learn more about what plants insects enjoy the most, why and how important this is to us. There is one simple fact: adequate plant diversity ensures more nutritional resources for insects, better survival and improved delivery of ecosystem services. Two seemingly discrete points have been drawing my attention to why ecologically worthwhile plants are so critically essential. In fact, these are fairly simple concepts that just require a closer look.

Small invertebrates and tiny flowering plants – the importance of their charming obscurity

The adult forms of many beneficial insects are generalists, i.e. they make use of several plant species to obtain food, like nectar and pollen. This is particularly useful when such insects can mediate pollination and natural pest control. These capacities remain poorly explored for many taxa, resulting in missed opportunities for us to reveal more about the benefit generated by small insects that are not obvious to the naked eye. I have been able to observe this generalist feeding preference in many insects, thanks to the excellent diversity of flowering plants in my garden.

My favourite genus of hoverflies is Paragus, well-known for its highly beneficial larvae that consume aphids. The adult forms are among the most diligent (and in my own view, visually astonishing) hoverflies to be observed in nature. In 2020 this notable genus caught my attention with its fairly consistent interest in consuming nectar from the flowering Polygonum sp. (Knotweed). The same genus of flies has been around for many years, because I remembered that Scarlet pimpernel (Anagallis arvensis) was among its preferred diet years ago; Paragus was also seen last year on Common field speedwell (Veronica persica).

Paragus spp. feeding on familiar arable plants and wildflowers. These delicately small plant species offer reliable food supply for this beneficial hoverfly from early spring until September.]

When I had a much better look at Polygonum sp., it revealed some incredible invertebrate fauna, such as micro-moths and spiders. Not surprisingly, the insects using it as a food source are well-adapted to feed on nectar and pollen from tiny flowering plants.

Polygonum sp

It did not come as a huge surprise when I saw Paragus hoverflies enjoying a small, but dense patch of Black medick I was very lucky to have at home this spring. This was a clear message that the life of minute invertebrates, dependent on small flowering plants, is no less ecologically resonant. I started inspecting small plants more often than usual and also considered other plants that I have never perceived as substantial ecological assets; it turned out some precious invertebrate fauna is thriving there. Since many insects co-evolved with the plants they utilise, it is clear to me that this is an “alternative”, minuscule and impressive world, with its own amazing particularities that deserve much attention. The beneficial organisms visiting such flowering plants basically deliver the same benefit to wildlife, but are just significantly smaller in size.

A micro-moth from the genus Scythris nectaring on Knotweed. Despite the small size of the plant, it offers space for spiders to settle and hunt, both passively and actively.

Black medick supplies generous amounts of food to solitary bees, beetles, butterflies and moths. When observed without bias, in all its ecological worth and numerous invertebrate inhabitants, this patch relays a very simple message: removing it means obliteration of a whole invertebrate community and destruction of habitat connectivity. By wiping out a seemingly unnecessary part of an ecosystem means we refuse to understand the simplicity of peaceful coexistence. This principle originates from the evolutionary mechanisms of interdependence between plants and insects, a fragile equilibrium we are continuously interfering with.

Gonioctena fornicata (1st photograph, top-left) is a leaf beetle that is not very welcome. However, the fact that small beneficial plants can attract insects that have negative impact on plants is not a valid reason to overlook other insects. It is self-evident that the benefit greatly outweighs potential damage here. Flower beetles, solitary bees, hoverflies (and other flies, like this intriguing Platystoma seminationis) make excellent pollinators that also aid nutrient cycles. Black medick aslo offers food to the micro-moth Pancalia leuwenhoekella (Violet cosmet) – not a common sighting in the UK

Sustaining beneficial insects requires some extra effort

Synergistic effects generated by plants can decrease due to the absence of flowering plants based on their specific phenology. As choosing appropriate plant species to assist pollination, for instance, requires deep and careful scrutiny, especially if used for the purpose of any agri-environmental objective, the range of choices offers options that are yet to be tested. Although leaving nature undisturbed can produce great results for insects, our critical mission is to ensure that the flow of sustenance for insects remains uninterrupted. This is how they can help us the most, which requires careful tweaks.

I have been applying a very simple solution by ensuring the availability of areas I call “transient nutritional adjustments”. They are small spaces that provide the much needed safe seasonal transition for insects when certain flowering plants cease to provide the much needed energy; such additional patches can even act as emergency backup at times of starvation due to phenological deviations that can negatively impact survivorship of particularly sensitive insect species. This can be done by simply planting an sowing well in advance or conforming to a specific regime of plant and soil maintenance. Since this timing could be difficult to predict, it is good to always have some space for creating additional “plant boosts”. It is not just diversity itself, the continuous availability of ready food is also vital. As this requires a certain degree of anthropogenic disturbance, our responsibilities are inevitably elevated.

The availability of different flowering plants throughout the year ensures a safe switchover for Bombylius major (Greater bee fly and Bombus hortorum (Garden bumblebee)

We neglect delicate plant-insect interactions by applying inadequate measures that typically have nothing to do with preserving the ecological value of insects, with the only wish to “tidy up”. Failing to understand the ecologically valid interpretation of the aforementioned examples will continue to undermine wildlife conservation. It is up to all of us to drive meaningful observations the right direction. One single plant species can sustain the energy requirements of many insect species. Many different plant species will offer a wide phenological spectrum throughout the whole year, a major trend in counteracting the damage to insects caused by climate change.