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How does organic agriculture affect above and below ground organisms including pests and their natural enemies?

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1. Introduction:

During the ensuing decades to come, biodiversity will be threatened due to farming practises on a macro scale. Figures predict a doubling or trebling of nitrogen and phosphorous inputs, with potentially 1,000,000,000 hectares of natural ecosystems being lost for agriculture purposes by the middle of the 21st century (Tillman et al 2001), producing knock on effects for above and below ground biota (Hole et al 2004).

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Intensive farming is renowned for its consequential loss of soil nutrients and simultaneous denitrification. The net result is a simplified food web, with potential doubling of pests such as aphids in intensive farming practises (Birkhofer et al 2008).

Organic farming practises generally tend to improve soil structure, with the addition of compost and plant material improving fertility, with techniques such as cover crops helping to reduce soil erosion (Abawi et al, 2000). Conversely, soil pathogens such as sclerotium rolfsii sacc, are residents of organic soils, and have been linked to crop infestations such as tomato blight, as well as crown rot being caused by Rhizoctonia Solani (Gurkin et al 1985; Punja 1985). Above ground organisms have also been shown to be vulnerable, with biotic interactions between brassicia oleracea and plant pests such as Plutella xylastella and Brevicoryne brassicea, with data showing that population numbers of both species are heavily dependent on fertiliser regimes. This is an important aspect of the review, as food web dynamics need to be considered, with regard to the effect of fertiliser use on host induced defences (Staley et al, 2010).

This paper highlights not only the effects on above and below ground organisms, but also how parasotoid and pathogen distribution is effected as a result of this, with particular consideration towards the effects on crop biochemistry, and how potential drawbacks of contemporary farming practises are being overcome. Increased weed richness due to organic management will also be discussed (Staley et al 2004).

2. Affects of organic agriculture on soil communities:

Nematodes play a critical role in nutrient cycling and soil community trophic structures (Ferris et al 2004). (Nahar et al 2006) observed a negative correlation between free living and plant parasitic nematodes in organically farmed soils that were used to grow tomatoes, with nematodes feeding on fungi and bacteria producing as much as 27% of the readily available nitrogen in soil. High levels of free living and reduced levels of parasitic nematodes should therefore be a goal for organic farmers. (Briar et al 2011) investigated the abundance of nematodes among different soil plots. The four different plots included tilled fallow (FA), mixed species hay (HA), low intensity open field vegetable production (FV), and intensive vegetable production under high tunnels (HT), each plot measured with and without compost. The study showed a greater number of plant feeding nematodes in HT without compost, with plant yield being significantly higher in HT than in open field plots regardless of treatment, with tomato yield also slightly but not significantly higher in the compost amended plots compared with non amended plots (Briar et al 2011). Tilled fallow produced the lowest tomato yield, which is to be expected, due to the fact that tillage moves soil from deeper layers of the soil profile, which has been shown to be more abundant in pathogens compared to soils which have been exposed to mulching (Liu et al 2007). (Mikanova et al 2009) showed that dehydrogenase enzyme activity was higher in tillage treatments, which is a positive sign of nutrient release. (Dick 1984) reached the conclusion that non tilled soils had higher enzyme activity, such as alkaline phosphatise and invertase. It is generally accepted that conventional tillage supports organic matter distribution, however excessive tillage can pulverise the soil and cause excessive sun exposure, leading to a loss of organic matter , whereas no tillage causes decomposition rates to slow, hence organic matter accumulates, stimulating biological as well as enzymatic activity of microbial organisms(Arshad et al 1990). Hu and co workers analysed the characteristics of nematode communities through measuring the absolute abundance of nematodes per one hundred kilograms, as well as the ratio of bacteriovores, fungivores and plant parasites. Results showed higher nutrient and nitrogen avilability in compost than in fertiliser synthetic treatments; as a result, total nematode density was higher in compost treatments, including higher numbers of bacteriovores. This however could be due to more present bacteria because of compost treatment. There were higher numbers of plant parasitic nematodes in organic plots, with fungal feeders and bacterial feeders high in chemical treatments, hence organic treatment is more efficient at maintaining soil fertility. The drawback is a higher incidence of plant damage (Hu et al, 2010). Mulching has been effective in reducing pathogen outbreak, especially in reducing the incidence of southern blight (Liu et al 2007).

3. Affects of organic and intensive agriculture on
above ground organisms

3.1 Crops

Zoophytophagy is a form of omnivory where facultative predatory insects feed on both plants and prey at the same time period throughout their development (Albajes et al 2006). Zoophytophagous predators can negatively affect growing plant tissues due to mechanical actions of stylets, which puncture and penetrate the plant host, leading to malformations that impair development (Hori, 2000). N.Tenius for example extracts continuous assimilates from plants under low prey availability, which over prolonged time periods reduces crop yields (Arno et al 2010). This highlights the importance of trophic structures in understanding and manipulating the level of damage inflicted on crops. Plants have evolved a range of defence mechanisms when resisting attack from pests, with their main arsenal consisting of lectins, which bind reversibly to specific carbohydrates present on cell membranes of invading erythrocytes (Van Damme et al 2008). These include glyconjugates which are toxic towards phytophagous insects (Vandenborre et al 2011). (Staley et al 2010) showed that Aphids B.brassicae and P.Xylostella responses are largely affected by fertiliser type, with P.Xylostella pupae being significantly heavier on plants fertilised with ammonium nitrate than unfertilised plants. Aliphatic glucosilonates were shown to be higher in ammonium nitrate treatments (Staley et al 2010), which are vital in plant defences against pests (Beekwilder et al 2008). Such research leaves organic farming advocates with a dilemma, as artificial fertiliser not only improves quantity of yield, but increases the armoury of plant host defences against pathogen invasions (Insert citation).

3.2 Aphids

Sap feeding insects such as Aphids show a response in terms of fecundity and longevity in relation to soluble nitrogen levels of their host plant (Van Emden 1996). Grain Aphids are a constant pest and problem to wheat crop management worldwide (carter et al 1989). Aqueel and co conducted a study into the effect of nitrogen fertiliser and selected wheat cultivars on the population dynamics of rhopalosiphum and Sitobion avenae. Experiments were conducted in light and temperature controlled chambers, in order to replicate normal aphid conditions. Results showed that increased fertiliser inputs had a positive effect on the growth and fecundity of S.Avenae and R.Padi. S.Avenae lifespan was shortest (34.4.days) on cvDeben at the lowest fertiliser treatment and longest (36.3 days) on cv solstice variety at higher treatment levels (Aqueel et al 2011). The conclusion was that nitrogen levels directly increased aphid performance and fecundity.

Ai and co investigated the impact of fertilisation on cotton aphid population in BT cotton production systems. Their study looked at the combined effect of potassium and nitrogen levels on aphids. Using an experimental field test in China, they studied the effects of three different potassium fertiliser levels without nitrogen (60, 120 and 180 kg/ha), and a combination of four nitrogen levels (36, 72, 108 and 144 kg/ha) with three potassium levels (60, 120 and 180 kg/ha) on Aphid abundance. Results showed that when 120 kilograms of potassium fertiliser was applied, the aphid densities in the treatments N0, N1, N3 and N4 with applied nitrogen were significantly lower than the aphid densities of N2 in 2005(Ai et al 2011).(Baskaran et al 1985) reported that high levels of potassium negatively influenced insect populations. Ai and co’s study indicates that without nitrogen fertiliser, the population density of cotton aphids in treatment of high concentration was significantly lower than those in low concentration of potassium fertilisers. This suggests that potassium affects hormonal and metabolic processes in plant tissues, as potassium ions regulate transmembrane conductance which in turn leads to downstream events, making crops less desirable for herbivorous pests (Amtmann et al 2008).

(Pareja et al 2008) looked into the effects of crop patch size on aphid density. This is important knowledge for farmers, due to modern day shift from traditional organic to large scale intensive agriculture. Trophic structures varying on spatial scales can lead to herbivores in larger plant patches being of greater risk than herbivores in smaller patches, with theoretical models putting top herbivores at greater risk than their prey (Esch et al, 2005). Pareja and co survey involved 27 by 27 metre squared blocks were used to search for the presence of u.jaceae (aphids). Artificial colonies were used to infest c. Nigra plants, with a plant from each colony size placed in a patch of one, three or six plants (infected plant alone, two healthy plants, and five healthy plants respectively). For the first three observational dates, only A.Funebris was detected in each field. Over the five observed dates, there was a significantly greater occurrence of parasitism by A Funebris with larger aphid colonies (1.45 times greater upon doubling colony size). Effect of plant patch size on the presence of parasitism was not statistically significant, thus there was a positive effect on aphid density throughout the sampling period. This is important information for herbivore mortality caused by natural enemies, as larger colonies of u.jaceae suffered higher parasitism rates (Pareja et al 2008). (Macfayden et al 2009) showed that aphid parasitism was greater in conventional farming than organic, through comparing ten organic and ten conventional farms in order to assess pest outbreaks. Results of the study showed aphid abundance was low, never reaching one aphid per metre in cereal aphids. Organic fields had a greater abundance of aphids per metre (0.29 for organic, 0.13 for conventional). There was however greater parasitism of aphids in conventional farms (thirty percent of aphids being parasitized compared to twenty in organic).

3.3 Carabid beetles

Carabid beetles and ground beetles can be exploited as control agents against invertebrate pests (Symondson et al 2002). Changing management practises in organic systems to increase natural enemy effectiveness via top down enhancements of natural enemies (Gurr et al 2003), with Carabid species being shown to be actively higher in areas of weed strips in cereals (Lys et al 1994). Research into ground beetles has shown them to be predators of cabbage root fly eggs, Finch and co’s results highlighting that for every millimetre increase in beetle length above 2.7mm, an additional 18 eggs were eaten per beetle per day (Finch, 1996). Eyre and co studied the effects of carabids in pest control in three organic and one conventional field. Synthetic fertiliser was applied in conventional field whilst organic fields used mechanical weeding.

Sampling for ground beetles was carried out using pitfall traps (8.5 cm diameter by 10 cm deep), with twenty four sampling points in each field. Two organic fields were planted with cauliflower, and the same fields were planted with leeks one year later. In the other organic field, one third was planted with cabbage and two thirds with broccoli in the middle of May 2006, whilst in the last week of April 2006 Calabrese was planted in the conventional field. Their results showed that Bembidion lampros and pterostichus melanarius were the most abundant in cauliflower plots (Eyre et al 2009). Weed cover was greatest in cauliflower crops, which was badly affected by cabbage root fly. The three Bembidion species were significantly negatively correlated to weed cover, whilst B. Lampros, B.Obtusum and B.Quadrimaculatum significantly preferred weeds in crops. This differed from the conventional managed field, which had fewer carabid species and less activity.

Results indicate that crop type, weeds and vegetation cover affect both species activity and assemblage distribution in the vegetable fields surveyed (Eyre et al 2009). This has large implications for increasing predator activity and abundance for pest control (Bianchi et al 2006). Ekroos and co conducted a similar study in southern boreal vegetation zone in southern Finland. Six patches of farmland were selected at study sites, situated ten to thirty kilometres apart. The farm types compared in this study were conventional cereal, conventional mixed and organic (Ekroos et al 2010).Carabid beetles were collected using pit full traps in three two week sampling methods during the first half of June, July and august respectively. A total of 10,888 carabids were counted. There was higher activity density in organic cereal crops than conventional cereal (188.7 compared to 163.4), with a higher alpha diversity of large and intermediate carabid species on organic compared to conventional cereal farms, of marginal significance (p<0.082). Land scape diversity had a negative impact on large carabid activity density (Ekroos et al 2010),research that contrast with that of (Vesely et al 2008) who found organic farming to promote the survival of larger carabids, via field experiments over a five year crop rotation period.

3.4 Tomato Blight

More sustainable forms of agricultural production have increased in recent years in many regions of the U.S. (Liu et al 2007). (Fraser et al 1994) reported a noticeable increase in microbial biomass under organic management, of at least ten percent. Numerous species of free living soil fungi from the genus trichoderma, commonly abundant in root ecosystems (Harman et al 2004) have long been recognised as agents for the control of plant pathogenic fungi, and have the ability of promoting plant growth, as well being universally present in soils (Samuels 2006). Liu and co conducted a study into the relationship between propagule numbers and genetic diversity of trichoderma species and southern blight on tomato crops, a symptom commonly caused by the soil borne pathogen rolfsii sacc (Punja 1985). In the study, soil from ten farms in North Carolina with a history of long term organic, sustainable and conventional crop production. Soils were infested with mycelia of s. Rolfsii that were grown for two weeks in sterilised oat grains. Six colonised oat grains were buried at depth of 0.6 cm below the surface. Four week old seedlings (tomato) were transplanted two weeks after infestation of soil. Plants were watered from below. Disease incidence was then measured weekly for five weeks, and rated from 0 to 4(0 = healthy, 4 equals dead). Results showed that incidence of southern blight in the greenhouse bioassays was significantly greater in soils from conventional farms than soils from organic (Liu et al 2008).Canonical correspondence analysis showed that total soil microbial communities were generally grouped based on organic and conventional management systems, with such grouping explaining the difference in disease incidence between farming schemes (Liu et al 2008). Trichoderma species have been reported to be lower in conventionally managed soils (Bulluck et al 2002).

(Curtis et al 2010) conducted a two year assessment of tomatoes in southern Italy, showing two pathogens consistent with tomato production. Emerging strategies for plant disease management involve biological and integrated control by applying antagonistic micro organisms, which is becoming more frequent due to increasing value of crops (Paulitz et al 2001). In this study, 23 day old tomato seedlings were used. Plants were obtained from seeds sown in 220 hole plastic pots and incubated in a germination chamber at 20 degrees. Both pathogens were used for artificial inoculations. Growth chamber experiments used tomato plants grown in the plastic pots under eight treatments (1, bio control bacteria Burkolderia cepacia, 2, bio control bacteria Pseudomonas species, 3, Bio fungicide based on bacillus subtilis, 4, based on trichoderma asperullum, 5, chemical fungicide tolclofosmethyl,6, azoxystrobin,7,fosetyl-Al,8, fosetyl-Al +propamocarb.) The results showed that treatment one significantly inhibited damping off caused by rolfsii, reducing the disease index by 81% compared to the untreated control (pathogen alone). This is an important breakthrough for organic farmers(Liu et al 2008), although more traditional methods such as mulching and grafting have had positive effects on tomato growth, noticeably through Beaufort rootstock grafting, which Hasna and co showed to great effect in reducing the incidence of corky root disease, compared to tomato plots that were ungrafted. Mulching has been viewed as a good form of management, due to promotion of root development, and inhibition of weed growth (Hasna et al 2008).

3.5 Weeds (Strigalactones)

The significant yield reductions caused by striga in staple cereals like Sorghum drives African poverty. The parasite has a complex life cycle, intimately linked to that of its host’s plant. Upon warm moist conditions, the seed germinates (Worsham 1987). A germ tube grows, and upon contact, develops a haustorium that attaches and penetrates the plant xylem vessel, stealing water and nutrients at the detriment of the plant host (Rich et al 2007). During a 1999 survey in Northern Cameroon, 120 farmers named at least their biggest constraints on crop production, with weed infestation, such as striga, being the highest, followed closely by soil infertility (Ayongwa et al 2010). The biological control of striga hermonthica has been boosted in recent years due to extensive studying of the soil borne fungus Fusarium Oxysporum as a mycoherbicide (Ciotola et al 1995). Elzein and co found that Foxy 2 treated plants (plants treated with the mycoherbicide in question) developed stronger roots than control plants. Efficacy of Foxy2 in controlling S.Hermonthica was evaluated based on the diseased/dead striga seedlings in the treated chambers compared to control. The Foxy2 caused severe disease and death of striga seedlings compared to the control treatments, where striga seedlings were healthy and vigorous (Elzein et al 2010). Crop rotations between legumes and cereals have also been used to reduce striga seed banks and increase maize yield by up to 90%, with application of super phosphate fertilisers on areas of high soybean density not only increasing root length, but reducing striga emergence on maize crops (Carsky et al 2000). Trials were implemented in farmers’ fields in Siaya and Vihga districts of western Kenya between 2003 and 2005. Frequent striga infestation made this area an ideal candidate (Hassan et al 1995) as well poor soil fertility status, which is considered one of the major limiting factors of small hold farming (Tittonell et al 2005). Soils were sampled for physiochemical properties, and to estimate striga seed bank density (Vanlauwe et al 2008). Results showed that the push pull intercrop system outperformed all others for controlling striga emergence. Desmodium plants were seen to be effective at not only contributing to the soil biomass, but also at producing the necessary allelochemicals to inhibit striga well before complete soil cover. (Vanlauwe et al 2008).

4. Affect of organic agriculture on soil pests and pathogens

4. 1 Potato blight

Brassica rotations and green manures could reduce soil borne diseases of potato through the production of volatile sulphur compounds, and via alterations to microbial communities (Larkin et al 2007). Through greenhouse trials and soil microbial assessments, Larkin and co showed that AMF (arbiscular mycorrhizae) biological amendments, recduced stem canker incidence, with only slight to substantial lesions present (Larkin 2008). Potato is the fourth most important food crop in the world after paddy, wheat and corn. The potato late blight caused by phytophora infestans de Bary is the most serious disease in potato production (Yang et al 2011). Yang and co studied the effects of three strains of P.Infestans, ZY15, LSX18, and XH05-5-4. The toxin was extracted and filtered, and its effects on three different potato varieties showed a greater grade of potato blight by LSX18 than ZY15 and XH05-5-4 in PB06(potato species), however ‘XH05-5-4 refined’ had an acute reaction of maximum grade potato blight in Mira potato species(Yang et al 2011). This is important information for developing antagonistic strains of mycofungicides against the more stressful forms of potato blight. Other studies have shown artificial fungicides to be more effective. (Kromann et al 2008) looked into effects of fungicide application strategies on potato blight.

In Ecuador, poor fungicide knowledge and management is hurting crop yield, due to lack of technology and inability to monitor pathogens (Nelson et al 2001) , with 2001 surveys in Kenya showing only 79% of farmers being able to identify late blight(Nyankanga et al 2004). The study compared the efficacy of two fungicide strategies, the first strategy alternating between compounds, with the second being sequential. Measurements taken 113 days after planting showed a 70% disease severity with no fungicide, with only 3 to 5 % incidence with alternation fertiliser. This was decreased further with the application of sequential fertiliser, and improved equipment via the use of a constant flow valve (Kromann et al 2008). Despite disease incidence being reduced compared to organic strategies, many fungicides are non specific, and this will therefore reduce beneficial symbiotic fungi such as mycorrhizae, which will inevitably reduce crop yield. Such research also highlights how equipment efficiency is vital in farming practises, and this is holding back farming management advances in developing parts of the world.

4.2 Cabbage root fly

Distribution of plants can be highly heterogenous at several spatial scales, creating patches of various species composition (Hamback et al 2008). The probability of a plant being attacked depends on the quality of surrounding vegetation (Hjalten et al 1993)Earlier studies suggest that egg laying mistakes between the plants distribution and herbivore search behaviour typically increase the attack rates on low quality plants, when growing in proximity to high quality plants in the same combination, and it is believed that many insects such as the goldenrod lead beetle use odours to track down host plants (Hamback et al 2003; Morrow et al 1989). Plant secondary compounds have shown to be vital in herbivore tracking of plant hosts (Moyes et al 2001), whilst certain crop species gain protection via plant refuges (Pfister et al 1988). Hamback and co investigated attack rates by cabbage root flies. They noticed that in low contrast habitats, Delia laid more eggs on early cabbages, and the conclusion was for cabbage farming in mixed cropping systems, with multiple genotypes of the same crop species, insects may be pulled away (Hamback et al 2008). Biological control of an herbivorous pest such as this has been of considerable research on Canada, where there are high levels of cabbage root maggot infestation, a serious pest of canola taproots (Soroka et al 2004). The most important control species found in European areas that is yet to be sampled in Canada is Aleochara bipustulata L., potentially making it a candidate for canola crops (Hemachandra et al 2007). Adult female A.bipustulata lays eggs in soil, where the newly hatched first instar larva locates a puparium, chews a hole in the puparial cuticle, and enters (Find citation).

Andreassen and co showed Delia Racidium emergence is reduced under A.bipustulata larvae presence, regardless of other non target species present. An extreme case was Farnia scalaris, replicated 67 times, where D.Racidium had a 78 percent emergence under non parasitic conditions, and a nine percent emergence under parasitic conditions (Andreassen et al 2008). There is a high potential for bio control based on these results, however (Dosdall et al 2010) showed that the effectiveness of control agents such as A.bilineata on root fly infections, depends largely on crop type and production scheme. In the study, experimental plots had higher parasitism rates by A.bilineata; however intercropping of canola with wheat did not have an effect on the frequency of super-parasitism. From a bio control perspective, neither super parasitism nor multi parasitism may be desirable, as they can significantly reduce survival of parasatoids, this has been noted in A.bilineata, which were unable to differentiate between cabbage root flies parasitized and un parasitized states containing T.rapae (reader et al 1990) reducing the longevity/survival rate of the desired parasatoids (Fournet et al 1999).

5. Conclusions

There is much debate over the benefits and draw backs of organic farming. Genetic diversity is undoubtedly an advantage over current conventional methods; however crop yields are highly susceptible to pest and pathogen invasions. Mycofungicides are an attractive replacement for synthetic fungicides, especially as they help to preserve beneficial fungi such as mycorrhizae, which have a positive effect on plant yield, whilst at the same time reducing disease incidence in potato crops (Larkin et al 2008). Many fungi however have proved parasitic such as striga, in which haustoria steal water and nutrients from plant hosts. Comparitive studies between organic and conventional farming have highlighted how different nutrient inputs can have synergistic and antagonistic affects (Li et al, 2005), hence more research needs to be carried out into crop yields in response to nitrogen and potassium levels, in order to find an optimum level of nutrient input. Sap feeding pests such as aphids have shown a strong response to nitrogen levels in plants, as well as potassium levels. Aphid numbers have been shown to be greater in organic fields, however greater parasitism on aphids in conventional fields. This means that there is a fine balancing act between potentially eradicating pests, and potentially eradicating crops. The majority of current studies fail to take into account the affects of temperature and abiotic factors on pest life cycle. Future studies need to be carried out into testing how abiotic factors affect lifecycles, especially as climate change is expected to accelerate the life cycle of numerous pests, such as the green spruce aphid, which has already been seen to migrate earlier(Westgarth et al 2007). Studies also need to investigate how arable land availability will change in the future. As already discussed, less arable land such as farming plots in Ecuador, which struggle with reduced levels of clay composition (look up citation), have lead to rapid infestations of striga. Studies in this area are vital if we are to predict the percentage of less fertile land in the future. Organic farming highlights beneficial practises such as mulching and intercropping, however Conventional farming has been shown to induce greater host defense mechanisms in crops through the addition of ammonium nitrate, as well as the induction of beneficial impacts of parasatoid population numbers.

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