Soil biology test – Pitfall traps
Written by Deborah de Groot, revised by Yanniek Schoonhoven and Clara Bosch
The pitfall trap is a useful method to monitor soil biology in your fields. Having healthy soil biology helps plant growth, pest control, disease suppression, nutrient cycling and improved water-holding capacity and soil structure.
A pitfall trap is a little cup in the soil that catches ground-dwelling insects like ants, beetles, spiders that hunt on the ground, millipedes, centipedes and more (figure 1). These insects are important for breaking down organic matter (detritivores and plant feeders) and controlling insect population by predation (thus preventing pests). The method is especially useful when repeated over time (every 1-2 years) to see how the populations are evolving and how they are impacted by your management practices. At the end of this article, you can find the links to download this manual and the identification sheets.
Figure 1: Here you see an installed pitfall trap. As you can see, the pot is in the soil until ground level. The lid that covers the container will protect the container from rainfall and animals that may eat the content of the trap (e.g. birds). If you are in a windy area, make sure the nails are strong enough to prevent the lid from blowing away.
- Repeat these steps every 1-2 years at the same time of the year and at the same locations to explore how your populations evolve over time.
- If you expect it to freeze, add 100% alcohol to the soapy water. You can find this in normal supermarkets in the pharmacy department. A ratio of 20:80 alcohol to soapy water will reduce the freezing point of the water to -7 °C. -Try to avoid days when rain is likely. Rainwater may run into the traps and overflow them.
- If you expect rain: avoid placing the trap at the bottom of a slope to prevent water from flowing into the traps.
- Note down your findings carefully. This will help you compare the results over time and draw conclusions from them.
Below you can find an example data sheet that you can use to keep track of your data. After filling in the data, it is interesting to look at
- The total number of insects you found (total abundance)
- Number of different types of insects you found (diversity
- The number of insects you found for each different type of insect (abundance of insect types)
You can compare this data between samples, between fields/management practices, and over the years. The weather influences the activity of insects (after the rains there is more activity, after a dry period activity may be lower), so it is useful to note down the weather conditions.
You can download this manual and identification sheet here:
Identification sheet: https://drive.google.com/file/d/1rFA5LyLfh0CRAk7Oog2Gwg7MGtUnjXsl/view?usp=sharing