Meet the insects

A photo gallery and introduction to insect monitoring methods from Münster intercropping trials 2017-2019…

When listing plant traits, the first that come to mind are morphological or production related. However, ‘indirect’ traits such as crop ecological interactions are also key aspects to be aware of when assessing plant teams. With this in mind, diversity of the surrounding insect community is a valuable indirect trait which can be used as an indicator of beneficial ecological interactions and thus probable resilience of the cropping system. However, compared to other plant traits, insect communities are highly variable and…mobile! So, how to monitor these little fellows?

During three years of summer-season experiments in Münster (2017-2019), we monitored the insect community hosted by our plant teams. We used traditional (e.g. visual plant inspection, pollinator observations, pollinator exclusion tests and pan- or pitfall traps) as well as modern and creative monitoring methods (e.g. camera installations) to track down which and how many insect pollinators, herbivores and carnivores we could find in our trails. Not only that, we also had fun observing their feeding behaviour (predation rate monitoring), followed them around the plots (beetle movement monitoring) and asked the plants what they thought about their herbivorous insect neighbours (collection of plant volatile organic compound). This was done thanks to the work of many curious and motivated researchers and students in our working group in Münster.

This picture gallery gives a quick but clear overview of the methods we applied in our crop diversification trails to monitor insect community and crop-insect interactions. But these methods are not only for ecologists: the aim would be to bring them out on the farm scale. What do you think? Any farmers interested in giving this a go?

1. Pan- and pitfall traps and identification

A classic method to find insects is…to set them a trap!

Here we used pan traps and pitfall traps to capture lower vegetation fauna (Figure 1). Pan traps are set on the ground (or higher) and can be used in different colours to attract different insect groups (yellow traps, for instance, are great for flying insects). Instead, pitfall traps, are placed into the ground and are effective on ground dwelling species. Rain shields help to keep extra rain water out. Both type of traps are filled with water and a small drop of soap to break the water surface tension, so the insects cannot escape by climbing up the trap. Among the methods this is unfortunately the most ‘violent’…After field work, this method also requires some lab time to identify the insects under a microscope. Coffee, music and nice insect identification books are great lab companions in this phase!

2. Look who’s there (visual arthropod monitoring)

Direct observation of insects in the filed is a more peaceful method to find out who’s living on your crops. As this consists of seeing insects directly on plants, it has the advantage that you can be sure of which insect was found on each plant type (Figure 2).

All you need is some experimental plants, a bit of knowledge on the species you might find, a scoring sheet and a lot of patience! The waiting pays off however: this method can be sometimes close to watching a wildlife documentary live! You can see the insects busy in their daily life: eggs hatching, foraging for food, predator and prey battles, parasitism in action or, perhaps, just a nap in the sun.We also developed an alternative to this method – which we called the “box-monitoring” (Figure 2c) – to cope with the extreme 2018 summer temperatures: instead of monitoring insects directly under the sun, we brought our insects into the shade by harvesting small subplots and carefully placing every plant in separate transparent plastic boxes according to species. We then brought the boxes to our work station (a table under a nice shady tree nearby) and opened the boxes one by one to visually screen each plant. Nothing of this small harvest was wasted: after insect monitoring, harvested plants were used for herbivory, disease and biomass data collection.

3. Man v machine: eye and camera pollinator monitoring

Pollinators such as honey bees, bumble bees, hoverflies can also be easily monitored by direct visual screening (Figure 3). This consists of screening subplots of interest and recording every flower visitor landing on a flower within the studied subplot. Observation time can be from 6 to 15 min! This relatively easy method comes however with a few downsides: as flying time of most pollinators is limited to a short time span of day and to weather, and, because in many cases plots cannot be monitored simultaneously, monitoring conditions (and results) between replicates can be sometimes very different! To circumnavigate the problem we thought of two solutions. To monitor replicates simultaneously, we carried out team monitoring in 2018 field trail (Figure 3a). Whereas, in 2019 we additionally installed cameras to capture pollinators on videos and photos (Figure 3b). This method definitely produces more data compared to human-visual monitoring. It also enables storage and re-use of the information over time. Be ready, however, to spend some time in front a screen to identify pollinator species!

4. To bee or not to bee (pollinator exclusion test)

How efficient is insect-pollination compared to self-fertilisation? In some crops, such as fava bean, both mechanisms can lead to successful reproduction. But which is higher yielding? You can find this out with a pollinator exclusion experiment (Figure 4). Just before flowering time, select a number of random plants in your plots, cover them with crispac ‘bread’ bags (these bags still allow gas exchange and do not heat up too much). Let them grow bagged until fruit development and senescence. Select randomly a similar amount of control plants and leave then un-bagged. Then compare fruit set of bagged and un-bagged plants.

5. Invite a spider over for lunch (predation rate monitoring)

Can you ask a spider how hungry it is and if finding a prey is easier in a monoculture or in a mixed plot? You could offer it some lunch to find that out…Shape some green plasticine into little fake caterpillars (Figure 5a) and place them in the experimental plots (Figure 5b). The hungry predatory insects will soon get close, thinking to bite on a juicy caterpillar. After one or two days re-collect the little shapes back and identify and quantify the damage you find on them. The spider might still be hungry after this meal though…

6. The beetle run (beetle movement monitoring)

Finding and identifying insects is not the only interesting thing you can do when you want to know more about these little creatures on your crops. You could also…follow them!We monitored ground beetle movement in our 2019 intercropping trial (Figure 6). Beetles were collected from a spontaneous population in our area. Racing beetles were labelled with a tiny number on their back and left freely moving inside a gridded subplot. Every time the beetle moved, it´s steps were hand-scored on an equivalent gridded chart. The final result is a map showing how ground beetles find their way through monocultures vs mixed plots.

7. Listen to what plants say (collection of plant volatile organic compounds)

Our insect story becomes even more challenging when we try to ask our crops what they think about it. When a plant is attacked by an insect, it produces highly volatile compounds which are quickly released in the surrounding environment. These compounds can be detected by surrounding plants as well as passing nearby insects, acting almost as a local newspaper: soon the whole small insect-plant neighbourhood (1.5 m distance) will know that a plant is under insect attack. Predator and parasitoid insects (so called ‘beneficial insects’) can find this information quite appealing: a plant under herbivore attack likely means that a prey is available! Due to their potential to attract beneficial insects, plant volatile organic compounds are becoming increasingly interesting in sustainable pest management. Within our DIVERSify 2018 experiment we set up a trail to test how these compounds are produced and work in intercropping systems. We induced herbivore damage by infesting focus fava bean plants and collected air samples using a self-built pump system (Figure 7). The method is relatively new and comes with a number of technical field and lab challenges which are nevertheless great fun to unravel. It’s all a matter of finding the right way to translate this chemical language!

Watch this space as we complete our analysis and reveal the results from our adventures meeting insects in the field…

| AUTHOR: Jana Brandmeier, Hannah Reininghaus, Silvia Pappagallo |