| Forside | | Indhold | | Forrige | | Næste |

Evaluation of Possibilities for Reduced Use of Pesticides in Fieldgrown Vegetable Crops

Summary

This report describes methods to reduce the use of pesticides in fieldgrown vegetable crops. The report also reviews existing knowledge of the extent to which the surrounding environment is affected by pesticides used in Danish fieldgrown vegetable crops. The report was prepared for the Kirsten Jensen Committee for use in their evaluation of the consequences for the Danish horticulture industry of introducing additional restrictions on the use of pesticides in plant production.

Danish fieldgrown vegetables are primarily sold on the domestic market. Green pea is the vegetable most often exported, mainly as frozen products. Danish production is insufficient to meet domestic demand. The most important fieldgrown vegetable crops are pea, cabbage, carrot, onion, leek and lettuce. In total, 10,500 ha are cultivated with fieldgrown vegetables. The quality of the vegetables is crucial for their market value, regardless of whether production is sold domestically or exported. EU quality standards stipulate that firstgrade vegetable products must be pestfree and undamaged by pests. Production and marketing are subject to strong competition from other countries.

About 10% of the total area is cultivated organically, and carrot is by far the largest organic crop. Of the remaining 90% with conventionally grown crops, most is used to produce vegetables in accordance with the principles for integrated production that the growers have established. For quite some time, efforts have already been made to reduce pesticide dependence in vegetable production, as demonstrated by the increased organic production and the establishment of integrated pest management systems.

This report deals with the possibilities of implementing alternative methods for the prevention and control of weeds, diseases and insect pests. In preparing the report, we have drawn on experience gained in organic cultivation systems, ongoing research projects on alternative methods of weed, pest and disease control, and documented and published knowledge within these areas. The report also describes current pesticide use in fieldgrown vegetables.

Effects on the environment

Based on pesticide sales figures for the period 1996-99, estimated insecticide treatment frequency in vegetables varied from 1.1 to 3.0, and fungicide treatment frequency varied from 1.1 to 2.8. A qualified estimate of herbicide treatment frequency is that it varied on average for the entire area cultivated with vegetables from 1.0 to 1.9. There is, however, great variation in pesticide use among crop species. Seeded onion crops can be treated up to five times with herbicides, whereas cabbage and lettuce are treated, on average, considerably less than once.

From available knowledge, row crops (e.g. onion, leek, potato and beet) are expected to pose a greater risk of pesticide transport to surface and ground water than broadsown crops (e.g. peas and cereals). This is because row crops are more open and have a limited leaf area for a longer period of time, and the need for weed control is thus greater and herbicide use more intense. At the same time, there is no vegetation to intercept spray droplets in row crops, and the risk of percolation and runoff is therefore higher.The flora and fauna of vegetable fields have not been documented for most crops. Studies in beet and cabbage have shown that fields support a richer invertebrate fauna when some weeds are present. Due to predation, weedy fields have often been found to be less favourable habitats for insect pests like aphids. It is fair to assume that this applies generally to all row crops. These conclusions do not apply to broadsown vegetable crops such as green pea.

Vegetable seeds are often treated with insecticides, which may be poisonous to many different animals and birds. To prevent birds from feeding on treated seeds, it is important to ensure that seeds are completely covered with soil during sowing. The insecticides may still be harmful after seedling emergence if birds feed on young plants contaminated with the chemical.

Prevention in general

There are a number of possible preventive measures – both at the overall farm management level and in the management of individual crops – that can be used as a starting point to counteract severe attacks of insect pests or diseases. Generally, it is advantageous to employ integrated strategies of prevention and control, because single measures are rarely sufficient to stop pests appearing and subsequently attacking crops. Efforts should focus on strategies that (either in the specific crop or in the production system in general) strengthen the growth of crop plants and their ability to withstand outside influences. It is also important to maintain a reasonable balance between harmful and beneficial organisms in the soil.

When planning crop rotation, it is important to alternate between pathogen starvation strategies and strategies that promote pest antagonists. A wellbalanced crop rotation, using species that are not closely related to one other, will impede soilborne pathogens and insect pests. Depending on the host specificity of the insect pest or disease and on its persistence, a six to tenyear crop rotation can limit pestinduced yield loss in most crops.

"Hidden" host plants, such as weeds that are wild relatives of crop plants, can allow insect pests or pathogens to survive crop rotation. A varied crop rotation, including alternation between row and broadsown crops, will indirectly reduce a number of specific weed problems.

Crop rotation can to some extent be designed to include plant species that have a beneficial, antagonistic effect on insect pests and diseases. Manipulating the natural content of microorganisms in the soil by incorporating green manure or adding organic matter may hinder certain soilborn diseases. However, a severe attack of root pathogens cannot be overcome by this method alone. The strategy of increasing soil organic matter content is one of many that can have preventive effects in crop rotations with low infection rates from root pathogens.

In addition to separating closely related species in time in crop rotation strategies, it is also appropriate to grow crops at a large geographical distance from areas where the same crop was previously grown (spatial crop rotation). In this way, the spread of pests with limited mobility can be reduced.

Diversified, wellplanned and welltimed crop rotation is assessed as having potential in vegetable cultivation to prevent damage caused by weeds, diseases and pests. The preservation of soil fertility in crop rotation systems is just as much a question of preventing an increase in soilborne diseases, as it is of securing a suitable nutrient balance.

Other potentially beneficial preventive measures include the liming of fields to optimise soil acidity and soil biological activity, the drainage of waterlogged fields, the planting of windbreaks on areas exposed to soil drift and the deep cultivation of compacted soil to improve root development. However, windbreaking hedges can have both negative and positive effects, as they provide habitats for pests and beneficial insects.

Alternative methods for the prevention and control of weeds

Most fieldgrown vegetables are sown or planted in rows between which tractors can be driven in order to provide necessary crop management treatments throughout the season. The use of mechanical weed control in many crops has therefore increased in recent years, partly in order to reduce pesticide use.

In transplanted crops, which possess considerable competitiveness against weeds, it is judged that existing rowhoeing techniques and other everdeveloping supplementary methods of mechanical weed control provide a relatively broad spectrum of alternatives that could be implemented immediately. This applies to crops like cabbage, lettuce and leek where methods such as false seedbeds, row hoeing and brush weeding are appropriate. However, hand weeding will be necessary when no herbicides whatsoever are used.

In seeded vegetable crops (e.g. onion, leek and carrot), which possess weaker competitiveness against weeds, there are a number of alternative methods that can either be implemented directly or after minor adjustment. These include heat treatment (with steam or flame) and brush weeding, combined with the previously mentioned false seedbed and row hoeing. However, weed control will generally be less effective than in transplanted crops, and supplementary hand weeding will be necessary if herbicides are to be avoided completely. Hand weeding is extremely timeconsuming and requires additional manpower, which might be difficult to provide.

Other potentially beneficial methods of weed control are currently being developed.

The twoyear crop management system entails a fixed crop rotation with, for example, a cereal crop preceding a vegetable crop. During the firstyear cereal crop, mechanical weed control throughout the growth period aims to reduce the number of viable weed seeds within bands. The secondyear vegetable crop is then grown in these bands. It must be pointed out that there is a need for continued development of cultivation methods for the vegetables produced in the second year of the system.

In seeded vegetable crops, with poor competitive capability, heat treatment of bands of soil immediately before sowing is considered to have potential in the control of weeds. There is, however, a need for further development of the method, and for guidelines and recommendations for its use in practice.

In the future, it can be expected that mechanical implements for weeding will be attached to hitech sensors or will use vision technology. Such methods will make it possible to distinguish crop plants from weed plants in crops that have been transplanted early in the season and that have welldefined and precise plant spacing.

Other crop management practices also have a certain potential for the prevention or elimination of weeds. Crop rotation systems where crops with strong or poor competitive capabilities are alternated may help reduce specific weed problems created by standard crop rotations. In the same way, crop manipulation - by altering plant density and/or transplanting instead of seeding - may strengthen crop competitiveness and reduce the need for weed control.

Mulching entire fields or rows with synthetic or natural materials may have potential in a number of vegetables. The method may be expensive, and the technology and methodology involved require further development. Integrated pest management systems for the prevention and control of weeds can generally reduce the use of herbicides in vegetable production. Herbicide application in narrow bands only covering crop plant rows may be combined with sowing the crop or row hoeing of the seeded or transplanted crop. Band application is highly effective in the sprayed strips, but should be combined with betweenrow mechanical weed control to provide overall effectiveness. Band application can reduce herbicide use by 50-80%, but special spraying equipment is required.

The general optimisation of herbicide dosage, spraying technique and treatment timing may also reduce pesticide use. This, however, requires up-to-date and accessible knowledge on the subjects in decision support systems like "PC Plant Protection" and "Decision Support". An overview of strategies for the use of alternative methods in weed control, and their effects, is shown in Table 10.

Alternative methods for the prevention and control of diseases

Promising results with biological control of both root and leaf pathogens have been achieved in numerous research projects around the world. However, only few investigations have shown positive effects of biological methods under field conditions. Methods based on the use of specific antagonists are primarily suitable for closed systems like greenhouses.

Although several products are available in other countries, there are presently no biological control products available in Denmark for disease prevention in fieldgrown vegetables. In time, it can be expected that microbiological products will also be available in Denmark. It is, however, doubtful whether microbiological products alone will be able to replace chemical pesticides in disease control in fieldgrown vegetable crops. Biological control should be considered more as one of several factors in a comprehensive strategy for disease prevention in horticulture in general.

The development and marketing of biological control products is still hampered by uncertainty about approval requirements, test methodologies, residue concentrations in the products and the possible production of toxins. It should be mentioned that most microbiological products are difficult to patent.

Crop rotation or other cultivation techniques must be used to prevent soilborne diseases in fieldgrown vegetable crops as no approved chemical pesticides exist. However, there is only limited documentation of the effectiveness of these methods on the occurrence and epidemic development of leaf diseases.

It has been shown that attack by downy mildew (Peronospora destructor) in onion develops more rapidly in highdensity crops than in more open crops. In agricultural crops, experience shows that plants that have been highly fertilised with nitrogen are more prone to attack by fungus diseases than plants that have been more moderately fertilised. Whether the same applies to vegetables in general is not presently known. However, there are examples of vegetable crops that have been lightly fertilised with nitrogen developing more diseases than heavily fertilised crops. This is the case with neck rot (Botrytis allii) of onion during storage, for example.

A number of pest prediction models are available in other countries. The models are designed to limit pesticide use and make prevention and control more targetspecific. They are generally estimated to forecast with 85% certainty. It is estimated that the use of forecasting systems can reduce pesticide use by up to 30-50%. The extent to which these models are used is not known, but they are not systematically used in Danish vegetable production.Several attempts have been made to implement and test systems for onion and lettuce downy mildew. However, as the models have been devised and developed for different climates, the systems need adjustment and adaptation to Danish conditions and have therefore not become widespread. It is estimated that a number of foreign systems for forecasting fungus disease can be introduced for testing in the short run (1-2 years). Other systems will require more time. Forecast systems are useful primarily when relevant pesticides are commercially available.

The effectiveness of plant extracts and nonsynthetic natural substances under field conditions is very sparsely documented in the literature, and knowledge of such substances is largely based on practical experience. Plant extracts and nonsynthetic natural substances for use in the prevention and control of disease, are subject to the same regulations and approval systems as chemical pesticides. This means that they must be approved by the Danish Environmental Protection Agency before they are marketed in Denmark, and in principle the same amount of documentation is required for their approval as is required for pesticides. Because of limited experience with these substances it is difficult to quantitatively estimate their potential in disease control. Clarification of their potential requires further investigation of application techniques and efficiency under controlled conditions.

Growing diseaseresistant varieties can reduce the need for pesticides. Differences in disease susceptibility between varieties have often been found in ongoing variety trials, but no commercially available vegetable varieties are completely resistant to specific diseases. It is also well known that any resistance will be rapidly degraded if it is based on a single gene. However, varieties with lower disease susceptibility as a result of multigene resistance are becoming more common. In current research projects, work is underway to breed varieties that are resistant to various diseases.

An overview of application strategies for alternative methods of disease prevention and control, and their effects, is shown in Table 12.

Alternative methods for the prevention and control of insect pests

The biological control of insect pests is widely used in greenhouse vegetable production as a broad range of massproduced natural predators of insect pests are available and these can be applied according to requirements in individual crops. The situation is quite different in fieldgrown vegetables where biological control is only used to a very limited extent. A few farmers have experimented with biological control products based on virus or bacteria.

A number of new biological control methods are currently being developed. Entomopathogenic fungi are being tested in the biological control of beetles and aphids, for example, but further development is required before practical implementation is possible. In some cases, the biological effect of the methods on various pests and their natural antagonists is not fully clarified and optimisation of application methods is required. In other cases, control efficiency requires further documentation.

A number of crop management strategies, including crop rotation and cultivation techniques, can be used as alternatives to pesticides. Such methods are welldocumented in the literature and welltried in practice. Some of the methods are already common in practice (e.g. irrigation to prevent cutworms (Agrotis segetum) and crop rotation to prevent nematodes) while others are still under development and/or lack documentation for their effectiveness in pest control. Greater use of crop rotation as an alternative means of insect pest control and the optimisation of its effectiveness are hampered by current crop management systems, choice of crops and marketing conditions.

Covering crops with nets to prevent flying insects is estimated to have some potential in valuable vegetable crops. Covering crops with bonded fibre fabrics or insect nets can prevent flying insects in cabbage and carrot provided that optimal covering is supplied during insect swarming. However, crop covering is not problemfree, and there are a number of sideeffects from covering fields for long periods - including direct effects on crop plants and indirect effects on microclimate, weed growth, disease incidence, etc. Other barriers to a more widespread use of the method are higher labour requirements for covering and uncovering and higher costs in general. Ongoing research with net covering is attempting to solve some of the abovementioned problems and overcome barriers to implementation in practice.

Models which relate insect pest population dynamics to climate factors (pest models) and models which also incorporate plant development parameters (pestcrop models) can be very effective decisionmaking tools in vegetable crop production. Pestcrop models are extremely complicated and require individual measurement of selected climate parameters, a fact that can limit their practical application. Simpler models are normally based on the relationship between insect development and temperature, calculated as degree days. Three such temperaturebased models are currently available for vegetable growers in Denmark. One describes the swarming of the cabbage root fly (Delia radicum), another describes the development of the turnip moth (Agrotis segetum) and the third calculates the critical harvest time to avoid severe damage from carrot fly attacks (Psilae rosae). A more complex model that simulates cabbage root fly development in cauliflower has been designed but requires more input data before it can be used.

Experience gained from developing the cabbage root fly model could beneficially be used to develop similar carrot fly models. As in the model for the critical harvest time of carrots, data for the number of insects caught on yellow sticky traps could be used to develop a temperaturebased model for carrot fly swarming. A Dutch model for gall midge (Contarinia nasturii) in cauliflower could possibly be quickly adapted to Danish conditions. In the same way, the applicability of other existing models under Danish conditions could be investigated.

Pest resistance is found in only a few vegetable crop species, and is normally limited to a single insect pest. General resistance to several pests, both insects and diseases, has not been described or documented. A variety that is resistant - or less susceptible - to a specific pest (e.g. aphids) is not guaranteed widespread use, as it may also be highly susceptible to other important insect pests or diseases. For less susceptible varieties to become widely used, they must necessarily meet requirements for product quality and stability in cultivation. Leading varieties in individual crop species are often characterised by a high degree of adaptability to growing conditions (climate, soil, pests, etc.) and therefore generally have less need for pest control.

An overview of strategies and methods of alternative insect pest control, and their effects, is shown in Table 14.

Environmental evaluation of alternative methods and their consequences

The most realistic means of reducing or eliminating herbicide use is often an increased use of mechanical weed control or heat treatment (steam or flame weeding). Naturally, the associated environmental effect will be that the risk of herbicide percolation and runoff is reduced or eliminated. However, alternative weed control methods like harrowing or flame weeding also have environmental costs. Fuel consumption per unit field area is greatly increased. Comparison of energy use in sprayed and unsprayed fields also requires calculation of the energy used in herbicide production (including chemical works) and the manufacture of tractors and implements. No detailed calculations of energy consumption or CO2 emission have been made in this report. No suitable tools are available for comparing for example ground water pollution with CO2 emission.

Mechanical weed control will have effects on flora and fauna similar to chemical control provided the methods are as effective as herbicides. However, this is seldom the case. Hoeing and brush weeding can also affect the fauna directly, for example by injuring large arthropods and damaging skylark nests. More traffic in the field increases soil compaction. Soil treatment can increase the risk of nutrient leaching and runoff by affecting soil structure.

The effect of heat treatment on fauna is not well documented, but even brief heating can affect arthropods. It is to be expected that insects living on the weed plants will be eradicated.

Experience from organic cultivation of fieldgrown vegetables

A number of alternative strategies and methods are used in organic vegetable production to control weeds and insect pests in particular. The methods have been developed simultaneously with the increase in the area used for organic vegetable production (to the current 10% of total area) that has occurred during recent years. The methods and strategies have been developed by a combination of practical solutions at farm level and ongoing efforts in research and development. However, solutions have not been found to all cultivationrelated problems. So far, efforts have concentrated on the vegetable species with the fewest problems as regards diseases and insect pests.

| Forside | | Indhold | | Forrige | | Næste | | Top |