Herbicide resistance in agricultural weeds gets plenty of attention, but the same problem is developing in forest environments with less recognition and fewer management options. Invasive species that were once controllable with standard herbicide applications are now surviving treatments that should kill them.

How Resistance Develops

The mechanism is straightforward evolution. Herbicides select for individuals with genetic traits that confer tolerance or resistance. Those individuals survive, reproduce, and pass resistance genes to offspring. After repeated applications of the same herbicide or herbicides with similar modes of action, the population shifts toward resistant individuals.

In agricultural settings, this process can happen within a few years with annual weeds producing multiple generations per season. Forest weeds typically reproduce more slowly, so resistance takes longer to develop. But it does develop, and once established, it’s effectively permanent in the population.

The problem is compounded by how herbicides are used in forestry. Limited herbicide options mean repeated use of the same products. Budget constraints often lead to using the minimum effective dose, which is exactly the selective pressure that favours resistance development. And treatment success isn’t monitored as rigorously as in agriculture, so resistance can establish before anyone notices.

Which Weeds Are Affected

Blackberry (Rubus fruticosus aggregate) populations with resistance to glyphosate have been confirmed in several regions. This is a serious problem because glyphosate has been the primary tool for blackberry control for decades. Affected populations don’t die from standard application rates, forcing either increased doses (which creates other problems) or alternative control methods.

Bitou bush (Chrysanthemoides monilifera) is showing similar patterns in coastal areas. Populations that previously responded to herbicide treatment now require multiple applications or higher doses. Given how extensive bitou bush invasions are and how limited access is in many coastal forest areas, losing effective chemical control is problematic.

Bridal creeper (Asparagus asparagoides) has developed resistance to some herbicides in Western Australia. This vine is already difficult to control mechanically, so losing chemical options reduces management effectiveness significantly.

Lantana (Lantana camara) resistance hasn’t been definitively confirmed but is suspected in several locations where control has become markedly less effective despite no obvious changes in application technique or timing.

Testing for Resistance

Confirming herbicide resistance requires controlled testing. Suspected resistant plants are collected and grown under controlled conditions alongside known susceptible populations. Both groups are treated with the herbicide at label rates, and survival is compared.

This isn’t quick or cheap. Growing plants to testable size, conducting properly replicated trials, and analysing results takes months and requires facilities and expertise that most forest management operations don’t have. As a result, resistance often isn’t confirmed until years after it first appears in the field.

Quick field tests exist but aren’t reliable enough for definitive confirmation. Higher survival than expected after treatment might indicate resistance, or it might be application timing, environmental conditions, plant growth stage, or spray coverage issues. Distinguishing between these possibilities in forest conditions is difficult.

Management Implications

Once resistance is confirmed in a population, that herbicide is essentially useless for controlling that weed in that location. Increasing dose rates temporarily works but accelerates resistance development and increases environmental impacts. It’s not a sustainable strategy.

The standard recommendation is herbicide rotation – using products with different modes of action in sequence or combination. This works in agriculture where multiple effective options exist for most weeds. Forestry has fewer registered herbicides and fewer still that are practical for use in forested environments.

Integrated weed management becomes essential. Combining chemical control with mechanical methods, biological control where available, and cultural practices that prevent establishment. This requires more planning, diverse skills, and often higher costs than relying on herbicides alone.

Prevention Strategies

Preventing resistance development is far easier than managing it once established. This means rotating herbicide modes of action even when a single product is working well. It feels wasteful to switch from an effective treatment, but it’s insurance against losing that effectiveness permanently.

Using full label rates rather than reduced doses helps. Under-dosing creates strong selection for resistance by killing susceptible individuals while allowing slightly tolerant ones to survive. Full rates kill everything except truly resistant individuals, which are initially rare in most populations.

Monitoring treatment effectiveness systematically provides early warning of developing resistance. If control rates decline over time with consistent application methods, resistance should be investigated. Catching it early, when resistant individuals are still rare, allows management changes before the problem becomes widespread.

Biological Control Options

For some invasive forest weeds, biological control agents provide alternatives when herbicide resistance develops. The agents aren’t affected by herbicide resistance and can continue controlling plants that chemicals no longer kill effectively.

Blackberry has several biological control agents in Australia, though their effectiveness varies regionally. Rust fungi, leaf miners, and other insects can reduce blackberry vigour and seed production. They don’t eliminate infestations but can maintain them at lower densities, especially when combined with other methods.

Bitou bush has had successful biological control through a tip-boring moth and a leaf roller. These agents don’t replace herbicide use entirely but reduce the need for it, which could slow resistance development by reducing selection pressure.

The limitation is that biological control agents take years to establish and even longer to achieve meaningful impact. They’re long-term strategies, not solutions to immediate resistance problems. But in locations where herbicide resistance has eliminated chemical options, they’re often the only practical approach besides expensive ongoing manual control.

Economic Impacts

Herbicide resistance increases weed control costs directly through needing more applications, more expensive alternative herbicides, or labour-intensive manual control. It also increases costs indirectly through reduced effectiveness leading to larger weed populations that cause more damage to commercial and conservation forests.

Quantifying these costs is difficult because they accumulate over years and resistance often isn’t recognized as the cause of declining control effectiveness. But research from agricultural systems suggests resistance can double or triple the cost of managing an invasive species once it’s widespread in a region.

For forest managers operating on tight budgets, these cost increases often mean reduced control effort, which allows resistant weed populations to expand. This creates a negative feedback loop where resistance spreads because it isn’t being managed adequately, which then makes management even more expensive.

Research Gaps

We don’t know the extent of herbicide resistance in forest weeds across Australia because systematic testing hasn’t been done. Resistance has been confirmed in a handful of cases, but anecdotal reports of declining herbicide effectiveness are common for many invasive species.

Understanding the mechanisms of resistance in specific weed species could enable better management strategies. Some resistance mechanisms are overcome by adjuvants or application timing changes. Others are absolute and no amount of dose adjustment or formulation change will restore effectiveness.

Alternative herbicides with novel modes of action aren’t being developed rapidly for forestry markets. The market is too small to justify the development costs for entirely new active ingredients. This means managing resistance with existing tools and accepting that options will continue shrinking.

Looking Forward

Herbicide resistance in forest weeds is going to become more common and more problematic. This is predictable based on what’s happened in agriculture and how herbicides are used in forestry. Pretending it’s not happening or hoping it’ll go away isn’t a strategy.

What works is acknowledging the problem, testing where resistance is suspected, adjusting management practices to slow resistance development, and investing in integrated approaches that don’t depend solely on chemical control. None of this is easy or cheap, but it’s more effective than continuing practices that guarantee resistance spreading.

Forest managers who start adapting now, before resistance forces their hand, will maintain more options and better control than those who wait until their current approaches fail completely. Resistance isn’t coming – it’s already here. The question is what we’re going to do about it.