Everything counts in large amounts

What are collembolans doing in your orchard? By Anna Mouton.

Collembolans are among the most common animals on earth — they occur on every continent and in almost all terrestrial habitats. You can probably find somewhere between 10 000 and 100 000 collembolans in every square metre of your orchards. What are they doing there? Fresh Quarterly put this question to Dr Charlene Janion-Scheepers, invertebrate ecologist in the Department of Biological Sciences at the University of Cape Town.

What are collembolans?

Collembolans — also called springtails — are six-legged animals related to insects. Collembolans are wingless. Most of them have a special tail-like appendage called a furca that can snap open to propel the collembolan away from danger — a bit like having a built-in ejection seat.

Most collembolans are only 2–4 millimetres long. “But some can get really big,” says Janion-Scheepers, “up to 6 millimetres.” The world record is just over 1 centimetre for a forest-dwelling species in New Zealand.

Collembolans can live on plants, or in litter, mulch, compost, and soil. The majority of collembolans eat algae, fungi, and decomposing plant material. “They’re important for nutrient cycling because they speed up the decomposition rates of litter, thereby providing nutrients to plants,” explains Janion-Scheepers.

While feeding, collembolans break up organic material, exposing it to microbes. This enhances carbon cycling, and has been shown to make nitrogen, potassium, calcium, and magnesium more available to plants. Collembolans also excrete nitrogenous waste in forms that plants can take up. Research has shown that collembolans can contribute to improved soil structure.

Other intriguing research found that some species of collembolans feed on fungi that cause diseases in plants.

“Studies abroad have shown that predator populations survive on collembolans during the times when there aren’t any pests around,” says Janion-Scheepers. “Then, when the pests arrive, the abundances of predators are high enough to control them. That’s why we encourage the conservation-agriculture approach — to keep predator levels high.”

The impalas of the soil

“Collembolans are like the impalas in the Kruger,” says Janion-Scheepers, “they carry a wide range of predators, including mites that also prey on other, plant-eating, mites.”

Predatory mites are key to the control of pests — notably red spider mite — in orchards. Hortgro is funding a new project, led by Prof. John Terblanche of the Department of Conservation Ecology and Entomology at Stellenbosch University, to examine the importance of collembolans in supporting predatory mite populations in South Africa.

The project will take two approaches. One is to collect predators and prey from orchards, and to study them under controlled conditions in the laboratory. This will provide a window into the preferred menu options for predatory mites. According to Janion-Scheepers, not all collembolans taste good. “There’s a group that has a repellent in their skin, so they won’t always be eaten by predators.”

The second approach is to conduct stable-isotope analysis on samples collected from the laboratory experiments, as well as on samples collected from orchards. The researchers will examine various components of the food web — plant material, plant litter, predators, and prey.

Isotopes are different forms of the same element. They occur in specific ratios in different food sources, and therefore in the animals that consume those food sources.

For example, ¹³C is an isotope of carbon that contains seven neutrons in its nucleus — most carbon atoms have six neutrons in the nucleus. Different types of plants contain different amounts of ¹³C, depending on their method of photosynthesis, and this in turn affects the amount of ¹³C in the organisms that feed on them. So, determining the amount of ¹³C in collembolans and mites from orchards can help identify their primary food source.

Nitrogen isotopes, on the other hand, provide clues as to how high up the food pyramid an organism finds itself. A predator will have a relatively higher ratio of ¹⁵N to ¹⁴N than its prey.

Just as the number of lions depends on the number of impalas, and the number of impalas depends on the amount of grazing, so the size of the predatory mite population hinges on the food chain to which they belong. A better understanding of this food chain could open opportunities for growers to boost their biocontrol.

Collembolans as bio-indicators

Another Hortgro-funded study, led by Janion-Scheepers, is characterising collembolan populations from orchards under different management conditions. Abdul Ghaseeb Jacobs is the master’s student on the project. He has been looking at collembolans from apple orchards, some of which are mulched and some of which aren’t. One surprising finding so far is that the communities sampled from the tree rows are very similar to those from the work rows.

Preliminary data show that collembolan communities differ in sites with and without mulch. Jacobs found a significant decrease in the total number of collembolans in sites without mulch over the course of two seasons. This suggests that mulch is important in maintaining collembolan populations.

The researchers will identify the collembolan species from the different samples, and see whether they are indigenous or introduced. “About 25% of South Africa’s collembolan species are introduced or invasive,” clarifies Janion-Scheepers. “Invasive collembolans are usually more tolerant of heat and desiccation than indigenous species, so if you think about a very barren soil without mulch, those invasive species can better tolerate the dryness and the heat.”

Jacobs has also been sampling collembolans from orchards under shade-netting, to learn more about how the shade-netting boom could impact the species living under them.

The ultimate goal of this research is to find collembolans that can be used as biological indicators of the soil health in agroecosystems, says Jacobs. Measurements based on the identification and enumeration of nematodes are already well-established methods for monitoring soil health. Collembolans can expand the bio-indicator tools available to growers.

This might, for example, give you an indication of whether your mulch treatment is successful in getting the right species composition to enhance predators,” says Janion-Scheepers.

Preliminary results from cover-crop trials conducted by Matthew Addison, crop-protection programme manager at Hortgro Science, showed that collembolan numbers tended to be higher in treatments with more diverse cover crops, compared to treatments with no cover crop, or treatments with a grass-only cover crop. This suggests that enhancing collembolan biodiversity may be one of the mechanisms by which cover crops could improve soil health and pest control.

You want to create an environment where collembolans can thrive, and you want the right mixture of species in your system,” says Janion-Scheepers. Collembolans are a vast unpaid workforce, labouring tirelessly to support your orchards. So, investing in their well-being makes good ecological and economic sense.