Global trade routes have enabled the transport of people and goods, including pests, easier than ever before. As a result:

At the same time:

As a result of:

For a successful monitoring programme, a forester or nurseryperson needs to be adequately prepared.

Data collection is critical in monitoring and the management of pests and pathogens. The data can help to identify the pest/pathogen using the pest ID matrix. It can also be used to help predict the occurrence of future outbreaks.

Data should include:

As discussed, aquatic weeds fall into one of two categories – algae or flowering plants. Click here, for Working for Water Invasive Aquatic Plants poster identifying the key alien invasive and indigenous problematic aquatic weeds.

It is important to understand the threshold level for each species, i.e. the level beyond which the realised cost of pest damage outweighs the cost of control. For exotics, the threshold level is likely to be low, if not zero, as companies are required by legislation to protect South Africa’s natural biodiversity by removing these upon sight. However, for invasive indigenous species control thresholds will need to be developed that comply with legislation and certification compliance requirements as well as the companies own policies and procedures regarding controlling aquatic pests.

By far the simplest and most cost-effective way to control aquatic weeds is by preventing their establishment in the first place. To do this, you need to take the factors that aid aquatic weed establishment and look to prevent or reduce their impact:

• Reduce shallow areas by deepening the sides of ponds, dams, ditches, and drainage canals, reducing the ‘shallow’ or ‘clear’ areas where emergent plants can establish. Where shallow areas persist, where possible, apply flow level manipulations. It is recommended that water levels should be dropped significantly – if possible (by at least 0.5m) during winter months (May to September). Drawdowns will expose the shallow areas to winter weather and drying, and in doing so, eliminate weeds.
• Improve flow of water by ensuring upstream inlets and downstream outlets are clear of debris and sedimentation. 
• Reduce internal nutrient flow by:

  • Creating diversion ditches that are grass lined to divert animal waste, fence off the water body to stop livestock adding nutrients and churning the water (add an alternative water source).
    
  • Ensuring there is sufficient ‘no-spray’ buffer zones around the water body.

  • Ensuring the area around the waterbody is grassed over to prevent soil and sediment entering the waterbody.

  • Designing the waterbody in such ways that it is easy to maintain – keeping banks clear and clean to help mechanically weed them. 
• Reduce external nutrient flow by:

  • Continual dialogue with upstream neighbours and develop an action plan to reduce nutrients entering the system and warn of excess water being added or removed from the system (for example, opening of dams etc.)

  •  Add sediment basins and slurry dams upstream of the waterbody to trap sediment and nutrients preventing them from entering the waterbody from upstream.

  • Add reed beds and other natural water filters to help remove additional nutrients and other pollutants from the system.

Physical control

In general, physical control tends to be slower and more expensive, due to labour costs, than herbicides. However, for small freely moving waterbodies that are heavily utilised downstream it offers a plausible alternative.

• Manual removal
  Hand pulling shallow weeds and dredging deep submergent weeds are time consuming and relatively expensive approaches. They will need to be done regularly to remove new growth from the seedbank. This approach is mainly applied in small areas which are inaccessible for mechanical removal methods and where chemical control is not viable.
• Mechanical removal
  There are several motor-driven underwater weed cutters, which, when used consistently, can control aquatic weeds. The challenge when physically removing aquatic weeds is to ensure the complete removal of the weeds, as many species can spread and reproduce from fragments left behind from physical removal operations. Therefore, it is important to correctly identify and understand the growth characteristics of the target weed species. The key is to cut/uproot, collect and remove. If any of the plant is left in the waterbody, reinfestation will occur, resulting in further costs to remove.

Cultural control

Potential sources of nutrient enrichment include artificial feeding of trout, cattle, horses and overflow from septic tanks. If the weed problem is indeed caused by elevated nutrients, removing the weeds (for example by using herbicides) may simply lead to a problem of planktonic algal blooms, some of which can be highly toxic (eg the blue green algae Microcystis sp.).

Cultural control, otherwise known as habitat alteration, uses innovative techniques to discourage the establishment and growth of aquatic weeds. Like with prevention, it focuses on reducing factors that aid aquatic weed establishment.

• Clear water – artificial shading and dyes inhibit light penetration, reducing photosynthesis and inhibiting growth. The challenge, especially in the case of dyes, is maintaining an effective concentration that will persist for several weeks. Understandably, dyes are not an option for moving waterbodies and are costly. Black plastic sheeting could be spread out in strips from fishing bays and weighted down on the bottom. This plastic should be left in place for about two weeks in order to achieve temporary control. The plastic should be perforated with small holes to permit the escape of gases produced by decomposition and to prevent ballooning.

• Sediment removal – dredging (which also can remove weed seed banks) removes sediment required by rooted plants, as does the addition of bottom liners in the form of both black sheeting and mineral soils (chalk, clay, gravel). The latter is only cost effective in small waterbodies, while the cost of dredging makes it only feasible for large ones. Environmental Impact Assessment requirements will need adhering to.

• Reducing nutrient input.

• Maintaining good sod and grass cover around the waterbody to prevent run-off and erosion.

• Ensuring a suitable spray buffer zone is in place around the waterbody.

• Do not allow livestock access to the waterbody as they increase turbidity, add nutrients and ruin the banks. Add drinking water stock tanks for livestock below the waterbody.

• Maintain septic tanks to ensure no run-off into the waterbody. Ensure when constructing a septic tank that it’s not positioned close to waterbodies.

• Do not permit run off from animal waste, e.g. dairies, poultry farms and piggeries.

Biological control

There are numerous biocontrol agents that have been shown to be effective against aquatic weeds, most notably grass carp Ctenopharyngodon idella – note of warning to anyone thinking of introducing grass carp: ensure they are sterile to prevent them from becoming a problem. Insects, ducks, geese, tadpoles, fish, snails and turtles have all been used as biocontrol agents, the challenge of using herbivores as a biocontrol method is that often the bodily waste produced by the control agent negates their effectiveness.

Biocontrol agents can be incredibly effective and have gained huge support over the past decade, however, a note of caution as the long-term effectiveness of biocontrol agents is difficult to predict and past experiences of control agents have themselves becoming pests and therefore due diligence is required before introducing a new biocontrol agent.

Chemical control

READ THE LABEL – the importance of this can not be overstated, when applying agro-chemical control measures it is your responsibility to ensure all legislative and label requirements have been met and any restrictions have been abided by.

The rule of thumb is to never treat more than 25% of the affected area, and leave about 2-3 weeks before the next treatment. Restricting use – chemical control should always be the last resort when all other avenues have been explored and found wanting.

Chemical control is a delicate balance – too low and it will be ineffective, too much and it can be too effective, leading to mass plant deaths that can dramatically reduce the water’s oxygen content and lead to indirect fish deaths. To reduce the risk of this, chemical applications should:

• Be applied in spring when organic matter is minimal, and growth is just starting.
• When water and air temperatures are lower, slowing the degradation of organic matter and reducing the activity of the oxygen removing bacteria.

Chemical control of aquatic weeds can either be via systemic or contact herbicides and can be subdivided into zones where they are applied: surface; total water volume (although only ¼ of the volume should be treated at any one time); bottom 30 cm – 1m, and; the bottom soil.

Applications can take the form of granular applications and spray mixes.

• Granular applications – need to be applied via a slinger or spreader, trying to ensure even coverage. The benefit is they can provide a slow release of the active ingredient which provides extended control. It also enables them to be administered at a lower concentration reducing toxicity. This is the primary form of control to treat the bottom soil.
• Spray mixes – the herbicide is mixed with water and applied so it disperses easily either in the form of a water-soluble powder, a wettable powder or an emulsifiable concentrate. This is the primary form of control for submergent plants.

Floating and emergent weed species
Controlled via contact applications on foliage applied using a boat or around the shoreline.

Submersed weeds and algae species
Those in shallow water can be controlled by spray mixes, as long as a good level of dispersal of the active ingredient is achieved. Submersed weeds will require a granular application.

Limitations

• Static waterbodies – because the water will not move downstream, any environmental or health implications of the chemical control method are contained and therefore more easily mitigated against.
• Limited flow waterbodies – can result in downstream contamination, as some downstream movement of water does occur. It is therefore important to understand how downstream neighbours use the water and ensure they are informed of any control measures being applied.
  
• Moving water – there will be temporary downstream contamination, requiring stakeholders to be informed in advance and mitigation steps in place to minimise any human health or environmental implications this may cause. Granular application methods will not be suitable.
  
• Restriction period – after any chemical treatment, a restriction period for various activities will be stated on the label. This must be adhered to, and stakeholders need to be informed.

Dosage

• Ensuring the correct dosage, as provided on the label, is adhered to also cannot be overstated. Dosage can be worked out by multiplying surface area by average depth.
• Over application can lead to quick plant die-off, the breakdown of this organic material by bacteria will reduce oxygen levels in the water and cause subsequent fish deaths.

Timing

• Late spring is the best time to apply chemical control methods as this is when young plants are actively growing and therefore most susceptible. Summer applications are not advised as these can result in as serious risk of fish deaths due to warm conditions leading to a rapid decomposition of plant material, which depletes oxygen levels in the waterbody.

Temperature

• Most chemical control methods become ineffective for aquatic weed control if the temperature is below 15.5 degrees Celsius. An ideal temperature would be around 19 degrees Celsius, as this aids active plant growth.

Follow-up treatment

• Most aquatic weeds have large seed banks that can remain dormant for many years. This means annual treatment is often required to keep weed populations below the threshold level.