Exploring the Environmental Impact of Pesticides within the South African Forestry Plantations

Pesticides are an important management intervention used by forest managers to control pests and pathogens/diseases. Pesticides have however been under scrutiny in past and recent years and have been restricted for use within forest plantations, especially forest plantations that subscribe to forest certification standards. This presents a challenge to the forester who has to manage a wide array of destructive pests and diseases.

What makes this worse is that the process of approving or restricting pesticides to be used within forest plantations has shortcomings. The forest certification schemes base their decisions on conclusions derived from laboratory-based studies and/or field studies conducted in environments that are not reflective of the South African (SA) forestry landscape and operational practice. Both are not ideal. The laboratory studies might overestimate the risk whereas field studies carried out elsewhere might misrepresent the risk that would occur under SA forestry conditions.

There is a lack of SA forestry relevant data regarding the impact of pesticides to the environment.

This data gap presents a couple of challenges:

1. the industry has no data to refute or confirm restrictions imposed by forest certification schemes;
2. the industry cannot ascertain whether SA forestry pesticide-use poses a risk to environmental values; and
3. high risk areas cannot be prioritized and mitigated.

A field study was therefore implemented at NCT’s Ingwe Plantation in 2019-2021 to investigate the ‘environmental behaviour’ of key pesticides used within SA forest plantations, and the risk posed by their use to non-target organisms (such as earthworms in the soil, aquatic organisms in streams and rivers, and to human health).

Pesticides applied on the site included:

• RoundUp (active ingredient (a.i.) glyphosate);
• Triclon (a.i. triclopyr);
• Claw (a.i. metazachlor);
• Kemprin (a.i. cypermethrin);
• Custodia (a.i. azoxystrobin and tebuconazole).

Over the 26-month study period pesticides were applied as per standard operational practice. Soil samples on the site, and water and sediment samples in nearest stream (Figure 1 – page opposite) were thereafter collected at pre determined intervals to answer the questions:

1. at what concentrations do pesticides occur in soil, water and sediment post pesticide application;
2. how long the pesticides remain/persist in soil, water and sediment; and
3. Whether pesticides were moving down the soil profile.

To determine the risk posed by pesticides to non-target organisms and human health, data collected from the field study was then compared to drinking water standards and lethal values derived for standard aquatic and soil ecotoxicity test species.

The most significant result that came out from this study was that glyphosate, which equates to 97% of herbicide use within SA forest plantations, posed a low risk to human health and non-target soil and aquatic organisms. This was attributed to the rapid dissipation of glyphosate in soil and water and/or the low concentrations recorded at the trial site relative to concentrations known to cause adverse or toxic effects.

Moreover, the results from this work are encouraging as they showed that apart from cypermethrin and metazachlor, all other pesticides applied, when used according to product label recommendations and standard operating procedures, pose a low risk to human health and non-target soil and aquatic organisms. These pesticides can be safely used as a part of an integrated pest and disease management programme.

The reader is encouraged to use these results with caution as they were derived from a single study, in one site, and over one growing season. Additional studies, similar to this work but with more comprehensive sampling strategies, covering all environmental components (like insects and bees, avian species and terrestrial species) should be implemented to confirm the findings this study.

Written By: Noxolo Ndlovu – Research Scientist

Source: NCT Forest – (Pages 18 – 19)

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