Pesticides of the future – Part 2
In the last issue, Maxwell Amenity Technical Manager John Handley stated that the case in favour of chemical pesticides is that they have worked in the past. But, the inherent disadvantages that accompany their use - widespread toxicity, secondary pests, resistance and escalating costs, has brought us to the point where there is a need to use alternatives.
Integrated Pest Management is mandatory as part of UK law, whereby "biological, physical and other non-chemical controls must be preferred to chemical methods if they provide satisfactory pest control." In an amenity context, pests are defined as any organism harmful to plants - Microdochium patch, Microdochium nivale, a fungus affecting turf grasses, is a pest in just the same way as a leatherjacket larvae, Tipula paludosa or T. oleracea is considered a pest.
IPM and pesticides
Integrated Pest Management (IPM) is a philosophy of pest management - although a practical philosophy rather than a specific, defined strategy. Within modern horticulture it has been practised, in effect, for a century or more, in that it combines physical, cultural, biological and chemical control and the use of resistant varieties. IPM isn't a modern approach, Stern et al. (1959) calling it 'integrated control' defined it formally with a statement of principles.
One might imagine that the intention of pest control is total eradication, but this is not the general rule. Rather, the aim is an economic one: to reduce the pest population to a level below which no further reductions are profitable, i.e. below which the extra costs of control exceed any additional revenue (or other benefits). This is known as the economic injury level or aesthetic injury level for the pest. The economic injury level is the most basic of the decision rules; it is a theoretical value that, if actually attained by a pest population, will result in economic (or aesthetic) damage (Perdigo et al.).
The economic threshold (ET) differs from the economic injury level in that it is a practical or operational rule, rather than a theoretical one. Stern et al. defined the economic threshold as "the population density at which control action should be initiated to prevent an increasing pest population from reaching the economic injury level." Although measured in pest density, the economic threshold is actually a time to take action, i.e., numbers are simply an index of that time. If a control action is delayed until the point at which an economic injury is observed, then control costs are likely to be higher, costs may be incurred in repairing the injury and the pest may be more difficult to control due to the higher population density.
The relationship of the economic threshold to the economic injury level and action times is shown for a hypothetical pest in Figure 1.
System Modelling
These theoretical values help us develop models that can be used to identify optimum points in time to undertake a management action, e.g. apply a fungicide to control a disease. Sports Turf professionals commonly carry out this kind of assessment informally: a walk over the grass sward and the sight of some disease activity on an indicator area (the green or region of the pitch which commonly suffers first) might instigate the response of applying a fungicide. So why formalise this process?
As stated in the previous article, the nature of modern pesticides is that they will need to be more targeted on the pest and less harmful to the environment. They will contain less of the Active Substance than their predecessors and will be active for shorter periods. As a consequence, modern pesticides need to be applied in a more precise and considered approach which requires us to develop our understanding of how to use them in order to obtain the desired level of control.
The ET is a complex value that depends on estimating and predicting several difficult parameters. The most significant of these include:
I. Variables within the Economic Injury Level (this is because the economic threshold is based on the economic injury level).
II. Pest and host ecology and life cycles.
III. Pest (and host) population growth and injury rates.
IV. Time delays associated with the IPM tactics utilised.
The development of models will be increasingly required as we adopt new technologies such as biological control. Physical, cultural, biological and chemical controls are all interlinked and aren't used in isolation. Professional turf management has been a bit of a late arrival at the party, we've been pushed toward looking at biological controls rather than embracing them enthusiastically. When asked what practitioner's thoughts are relating toward biological controls, my experience is that there is a shuffling of feet, followed by "I haven't used them", when asked why, the muttered response is invariably "they're not very effective because you've got to get the environment right, temperature and moisture and timing." Contradictory statements, at odds with the results obtained in other parts of the horticultural sector whose members have been successfully utilising a variety of technologies for several decades. So what can we learn from them?
Biopesticides
'Biopesticide' covers a wide spectrum of potential products used as plant protection products, the Health and Safety Executive divide these into four broad categories:
- Products based on pheromone and other semiochemicals (for mass trapping or trap cropping)
- Products containing a microorganism (e.g. bacterium, fungus, protozoa, virus, viroid)
- Products based on plant extracts
- Other novel alternative products
Approvals are granted by HSE's Chemicals Regulation Division (CRD) on behalf of Ministers under a range of specific pesticide related legislation.
The Agriculture and Horticulture Development Board (ADHB) funded a project called AMBER (Application and Management of Biopesticides for Efficacy and Reliability), a 5-year project with the aim of identifying management practices that commercial nurseries can use to improve the performance of biopesticide products within IPM. Ideally, funding would be organised and available within the sports turf sector to undertake similar research but, in the absence of this resource, there is still plenty that can be applied from the outcomes of this project. Further articles will focus on developing an integrated approach to resolve specific problems but, in this article, it is helpful to highlight some of the key findings from this project.
Knowledge gaps
Existing knowledge about biological control mechanisms has been collated to identify areas of knowledge clusters as well as highlighting any knowledge gaps. Further research efforts can then be directed into the most constructive areas. Research relating to biopesticides in the commercial nursery sector is currently developing understanding of the impacts of pest survival rate, fecundity, stage length, initial numbers, biopesticide spray timing, efficacy, age class susceptibility, population structure and persistence. It is also seeking to utilise technology effectively to assist with decision making and knowledge of the parameters that are relevant to Economic Thresholds.
Technology
Computer models have been created that simulate detailed pest and disease populations accurately based upon environmental conditions within a commercial nursery setting. The models are location specific but the data can be grouped regionally or nationally to enable commercial horticulture production managers to receive advance warning of potential problems.
Product Application
Product application is a key aspect of success for both biopesticides and for chemical pesticides. Research is focusing on optimum water volumes, including studies looking at retention of substances on the leaf, efficacy and longevity.
The variables tested were nozzle type, forward speed, pressure, nozzle flow rate, applied volume, boom height, nozzle angle and nozzle configuration. The spray volumes applied covered the typical range of water volumes recommended for biopesticide products, from 500 to >1000 litres per hectare. Contrary to expectation, lower volumes were the most efficient at depositing spray liquid on the plant, as it resulted in a greater proportion of the spray volume adhering to the plant foliage. The data suggest that the most efficient application strategy is to apply a higher concentration of biopesticide product in a lower volume of water (Ellis), and that there is a false perception among managers that increasing the water volume gives better spray application to the target.
There is a significant opportunity to improve all areas of biopesticide application, including product storage, product preparation and mixing, spray equipment set up and maintenance, optimising application volume and tank cleaning. Given some of the findings, this
is likely to impact existing practices and could significantly improve product effectiveness, demonstrating how important research is, and how this can undermine existing assumptions.
Relevance to sports turf management and amenity horticulture
With so much to learn, yet with no approved products, why should we engage with biopesticides at this stage? A small number of biopesticides have been available to UK growers for some time, though no product crop approvals currently exist within amenity grassland or managed amenity turf, however, an increasing number will be entering the market in the next few years. Biopesticides now represent over 50% of new active substance applications and have done so since the beginning of 2016, so it is no small issue. Within ten to twenty years, the number of biopesticide products available is likely to exceed the number of conventional synthetic chemical pesticides (Cary).
Commercial nurseries have been successfully using biological controls for several decades (Mizell). Knowledge of pests, lifecycles, monitoring systems, sanitation, pheromones and lures, and the conservation of competitors, antagonists and beneficial organisms (natural enemies) by judicious use of pesticides is an already accepted practice. But how similar is commercial horticultural production and professional sports turf management?
Ornamental plant production nurseries utilise different growing media, the pests and diseases are far more comprehensive because commercial growers produce an extensive range of plants. There is large-scale use of enclosed systems or glasshouses where lighting, humidity and irrigation can be closely monitored and controlled. Is sports turf a different situation? Where surfaces are open to the elements with no control of the environment, the number of pests and diseases are more limited, and the growing media is at least partially dependent on local conditions.
Within the sports turf sector there has been little commercial impetus to engage in this pursuit because the products that utilised broad-spectrum synthetic chemistry were cost-effective and available, but the rules of the game are changing and as professionals we need to be prepared to change too. As turf managers we understand and control the substrate: by selecting the appropriate dressing, we determine the ability of both water and nutrients to move through the surface and into drainage systems. Most facilities have the capacity to apply water through irrigation systems allied to evapotranspiration levels - the total sum of the evaporation of free water from surfaces and transpiration by plants of soil water over time. We also have models of moisture flows, gains and losses, that can help us decide when and how much water to apply.
We know that the form, quantity and timing of nutrition is one of the key determinants of the composition and playing quality of the sward, as is our approach to irrigation and aeration. To argue that we're not controlling or managing the environment is evidently delusional, but we may need to adjust our thinking to account for a world we've not been considering. We currently have a good understanding of the physics and chemistry of the environment in which the host and pathogen exist; however, biology is currently a 'black box' into which we've only just started to look.
There is recognition that biology is important: catalogues are full of biostimulants such as seaweeds, sources of carbon, humic, fulvic and amino acids as well as elicitors. We would benefit from developing our understanding of how these link with the microscopic world of bacteria and fungi and the physical, cultural and chemical management we utilise. For example, the same broad-spectrum fungicide we apply to control Microdochium nivale might prove detrimental to the population of beneficial microbes we have been attempting to support. That can be absolutely appropriate but the important thing to recognise is that it is an informed, conscious decision where we know what we are doing and our decisions determine the outcome.
Biorational product solutions
Ideally, as managers we need to predict the outcome of the operations we undertake and the products we use. This is becoming increasingly important if we are undertaking contradictory actions without thought or understanding, we are not going to get anywhere, i.e. aerating to stimulate bacteria but then applying ferrous (iron) sulphate to target another problem which then kills the bacteria we've encouraged to assist with the original concern. Use of biorational products will be a key determinant of our ability to effectively develop a best practice integrated pest management approach which enables us to deliver nutrients and control pests without negatively affecting non-target species or the beneficial microorganisms within biocontrols.
The term biorational describes substances or processes that, when applied in a specific system or ecological context, have little or no adverse consequence for the environment and non-target organisms, but cause lethal or other suppressive or behaviour modifying action on a target organism and augment the control system. Regardless of origin, these agents might be developed from natural or synthetic models, and generally exploit the evolutionary divergence of physiological systems in the target organism from non-target species, including humans. If properly designed and deployed, a biorational agent should be nearly fully compatible with biological controls as envisioned for 'selective insecticides' by Stern et al. (1959) (Horowitz et al.).
The term biorational is not restricted to the product being applied to control a pest or disease. If we are looking to support a complex ecosystem that can support beneficial microbes, we need to know that the fertilisers and biostimulants we use will also achieve these ends, otherwise this isn't an integrated strategy.
Dangers
As with any new technologies, the ability to utilise them effectively will be related to our understanding of how they work. Biological controls and biopesticides are potentially a safer option than the alternatives which they replace, both for the people who apply them and for the environment.
In an attempt to identify potential solutions, managers will be tempted to try biological solutions; products that are not approved are already being imported into the UK and being sold for people to use. It would be interesting to know how much support is being offered to customers who have no experience or training in how to use these types of products. As already indicated, biopesticides operate in a different manner to conventional chemical pesticides: there is a need to account for more factors which requires greater understanding of many of the things discussed in this article. If managers don't comprehend what is required to obtain effective control, there is the risk that a failure will result in the perception that these technologies don't work, compounding the foot shuffling views expressed earlier. For any type of solution to be successful; chemical, cultural or biological; implementation of an appropriate, integrated strategy in which relevant factors have been taken into account and best practices have been applied is necessary.
It was heartening to see what the AMBER project was achieving: recognition that there are gaps in knowledge and real research has been undertaken to fill those gaps. This research was well communicated, at an appropriate level for nursery growers, the respective target audience, to be able to apply what was being discussed and improve their ability to use an array of solutions.
There's a lot that we have to learn, the key question is; what is the best way of going about this and then practically implementing the findings?
References
Cary, D. Address to the European Parliament, Copa-Cogeca and IBMA High Level Symposium on Sustainable Plant Protection: Expanding the Farmers' Toolbox, Brussels, 7 February 2019.
Ellis, C.B. Improving the performance of biopesticides in the production of ornamental crops (AHDB CP158). 26th February 2019, Kenilworth, Warwickshire.
Horowitz, A & Ellsworth, Peter & Ishaaya, Isaac. (2010). Biorational Pest Control - An Overview. 10.1007/978-90-481-2316-2_1.
Mizell, R.F. III and D.E. Short. Integrated pest management in the commercial ornamental nursery. 2006. University of Florida Electronic Data Info. Serv. Bul. ENY-336.
Pedigo, L. P., S. H. Hutchins, and L. G. Higley. 1986. Economic injury levels in theory and practice. Annu. Rev. Entomol. 31:341368.
Stern V, Smith R, van den Bosch R, Hagen K. 1959. The integration of chemical and biological control of the spotted alfalfa aphid: The integrated control concept. Hilgardia 29(2):81-101.