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Brassicaceae members are economically important crops throughout the world, both as vegetables for human and livestock consumption, and for oilseed for culinary use, industrial use (Wightman, 1999) and biofuels (Booth et al. 2005). In addition, the Brassica family members are frequently cited as playing a role in integrated pest management of their own family, and other plant families, due to their biological and phytochemical profiles; though these roles are not always clearly defined. Some examples include habitat manipulation or physical methods, such as intercropping, companion planting, trap cropping, field margins, mulching and cover cropping; or utilising chemical defences such as alleopathy and biofumigation. (Eng-Chong and Douglas, 2004) In the present paper these methods are investigated.
It is hypothesised that utilising Brassica family members for pest management, and therefore increased yield, either with or instead of traditional methods such as chemical spraying, will reduce pest populations and increase yield. The following literature reviews attempt to demonstrate and support this hypothesis.
Keywords: intercropping, Brassica, trap cropping; glucosinolates
Trap crops have been defined as "plant stands grown to attract insects or other organisms like nematodes to protect target crops from pest attack, preventing the pests from reaching the crop or concentrating them in a certain part of the field where they can be economically destroyed" (Hokkanen, 1991) An example of such is the pest cabbage seedpod weevil which prefers the trap crop Brassica rapa to the main crop Brassica napus. Cárcamo et al.'s 2006 study shows that the seed weevils clearly preferred the trap crop, with a factor of 3-5 times more weevils present in the trap than the main crop. Cabbage seed weevils are well known (Dosdall et al. 2006) for invasion from the outside of fields into the field, then distributing throughout the crop, and a field strip of trap crops on the outside of the field is a successful strategy here. Economically this is a sound strategy also, as the authors explain "Assuming a cost (Canadian dollars) of $10/ha for chemical and $10/ha in labor and ground equipment costs, a 25 ha strip would cost $1000 to spray twice. Aerial blanket-spraying a field of 260 ha would cost approximately $6500 at $25/ha which translates into a $5500 savings." There would be additional savings of time and potentially other pests with the same invasion strategy such as flea beetles would be sprayed off also.
For other pests and Brassica species, evidence for successful trap cropping is based on the trap crop physically taking the role of the host in terms of the pest's life cycle. Cabbage root fly (Delia radicum) is a pest of economic significance for all members of the genus Brassica in Europe, and the most significant pest of Brassica species for Scotland (Evans, 2003). In a research article by Kostal and Finch (1994) one specific hypothesis was looked at, that cabbage root fly females exhibited a reduction in oviposition on their host plants intercropped or undersown with non-Brassica plants. The authors believed that normal female cabbage root fly flight behavior is interrupted by the non-host plants surrounding the hosts, that crucial host-plant stimulus preceding oviposition is lost, thus ovipositioning on hosts is reduced by at least 50%. This experiment was carried out on a mixed Brassica cropping, with bare soil, peas, clover, grass and four non-living materials. The results proved their hypothesis, and showed that visual disruptions are much more important for pest reduction that chemical repulsion, which could mean the cheapest and easiest to grow of local cover crops or mulches could be used for pest reduction, which would be an economic benefit to farmers. A further, simplified version of the study by the authors looking at time and behaviour on both host and non-host plants confirmed that female egg laying was arrested by visual disruption rather than chemical; in line with the 1994 study, the authors concluded that it was this protracted time spent on the non-host plants that reduced ovipositioning, and therefore pest numbers. This is in contrast to the previous study discussed, where weevils were chemically attracted to the trap crops, the cabbage root flies were seen to be confused "leading to a reduced host-plant acceptance."
Biofumigation is a sustainable strategy to manage soil-borne pathogens, nematodes, insects and weeds. Defined by Ciancio and Mukerji (2007) as "the pest suppressive action of decomposing Brassica tissues, and plant residues." Motisi et al.'s 2009's study concerned biofumigation as a method to control soil-borne pathogens, focusing on Rhizoctonia solani root rot of sugar beet, using Brassica juncea in three ways. It was used as a cover crop until it flowered, then pulled; and allowed to flower, then incorporated the biomass into the soil; and without Brassica juncea present as a control. The crop was artificially inoculated with R.solani in two ways, in two trials: by sowing infected seed, and by infesting the crop at six to eight leaf stage; the two ways by how a typical infection occur. This paper elaborates on the mechanisms by which biofumigation through cover cropping of Brassica species can potentially affect soil-borne diseases. This study expands understanding of the methods by which mustard affects pathogens: that '(i) growing mustard can affect the initial inoculum or the spread of the pathogen through primary/allo-infections and (ii) the incorporation of residues has an additional effect on these mechanisms, reducing pathogenic activity of R. solani within the root. R. solani is known to affect all major cereal crops in the UK, and as there is no effective fungicide available on the market at the moment (HGCA, 2008) other methods of control are needed. For minimum or no-till systems the lack of a sterile seed bed created from cultivation can encourage and harbour pathogens, a method by which cover cropping could reduce these pathogens would be of economic benefit which still retaining minimum tillage benefits.
An alternative procedure for using Brassica extracts is for that of nematode suppression. Nematodes are significant pests for a number of plant species, both for vectors of viruses and for plant root attack. For example, yield loss of cotton in the USA attributed to nematodes was 4.39% in 2000.(NCCA, 2001) Brassicas produce glucosinolates as secondary metabolites then isothiocyanates (ITCs) are derived from these compounds during hydrolysis. These ITCs have been extensively studied for pest suppression properties.
Table 1: Food sources of selected isothiocyanates and their glucosinolate precursors
Allyl Isothiocyanate (AITC)
Broccoli, Brussels sprouts, cabbage, horseradish, mustard, radish
Benzyl Isothiocyanate (BITC)
Cabbage, garden cress, Indian cress
Broccoli, Brussels sprouts, cabbage
Source: Linus Pauling Institute at Oregon State University. (2009)
In Zasada and Ferris's 2004 study they explored the using Brassica soil amendments for nematode suppression. In a previous study in 2003 the authors had determined the sensitivity of two common nematode species, Meloidogyne javanica and Tylenchulus semipenetrans, to isothiocyanates using laboratory assays. The authors state that "Glucosinolate profiles differ among plant species and their (ITC) derivatives differ in toxicity to nematodes. "Brassica hirta suppressed Meloidogyne javanica and Tylenchulus semipenetrans, while M. javanica was suppressed by Brassica juncea, using the typical benchmarks of LC50 and LC90. The authors found "that brassicaceous amendments can be applied to achieve consistent and repeatable nematode suppression when based upon the chemistry of the incorporated material." and "allows the selection and application of species of Brassicaceae containing glucosinolate precursors of the ITCs most toxic to target nematodes." This study is important as it validates the lab based studies in a field situation, in a targeted application.
Cover and inter-cropping:
A cover crop is broadly defined as any plant grown to improve any number of conditions associated with sustainable agriculture, generally for weed and pest suppression through interspecific competition, for soil improvements, water management and increased diversity.
Broad et al.'s 2009 study deals with interspecific competition of a cover crop for weed suppression, namely a cover crop of rye for a main commercial crop of broccoli. It was shown that when grown with a rye cover crop, broccoli had less leaf and stem biomass, and fewer, larger leaves; typical shading avoidance strategies. This meant a yield lowering of 7%, and a week harvest delay. However, broccoli marketability in terms of branching was improved, the covered crop required no weeding, but the standard crop required it three times during the growing season. There are very few selective herbicides for use in broccoli, and the cover crop provides an excellent barrier for weeds, providing uniform and dense cover. Economically this particular crop was not successful, but if the increased marketability resulted in a high profit margin, factoring in reduction of labour costs and herbicide usage, using a rye cover crop with broccoli may be a successful strategy in the future.
Intercropping, the practice of growing two or more crops in the same space at the same time has seen positive results for the Brassica family. Yildirim and Guvenc's 2005 study used cauliflower as an intercrop. The efficiency of this intercrop was determined using the land equivalent ratio (LER) of the as an index of intercropping advantage and of economic net income. Cos lettuce, bean, leaf lettuce, radish or onion was planted between rows of cauliflower simultaneously in separate plots, and in pure strands. This study was successful: 'the values of land equivalent ratio appear to be greater than 1 under intercropping system, this usually indicates the efficiency of this system over the sole cropping system', all intercrops yielded a value over 1, the greatest being 1.36. These values were analysed for economic benefits, and all intercrops, with the exception of radishes will give a higher monetary value to the farmer for the same area. However, these economic values are indicated to only work with short-season crops alongside a longer season crop, though the positive effect of intercropping on net income in this study was evident, "the highest LER values did not always reflect highest monetary return to the farmer." (Muoneke and Asiegbu,1997) also that, "the practical significance of LER can only be fully assessed when related to the actual economic yield." (Willey, 1979) If these LER values and the market values are assessed for each crop prior to planting , then intercropping cauliflower is indicated as a "economically satisfactory intercropping system" when intercropped with cos lettuce, bean, leaf lettuce or onion.
Figure 1: LER comparison from intercropping over three years.
Adapted from: Yildirim and Guvenc (2005)
As the above studies show, integrated pest management and novel intercropping using Brassica species reaps benefits for the farmer both in terms of economic benefits and in pest reduction. In the near future these methods, and others based on the same principles will become standard as our use of pesticides are reduced for financial, health and environmental reasons, but also due to new legislation in the EU due to come into effect in 2011 further reducing the list of allowed agricultural chemicals (European Parliament Press Release, 2009). The guidelines for this legislation specifically mentions integrated pest management strategies to be used whenever possible instead of pesticides. Sustainability is the key factor, these methods are unlikely to cause any build up of pest resistance, specific trap crops will not interfere with beneficals, or honey bees, and the reduction of chemicals will reduce financial outlays throughout the cropping seasons as well as reducing negative human and environmental effects.