Tag Archives: intercropping

Ten Papers that shook my World – Root (1973) – When more means less – crop diversity reduces pest incidence

I can’t remember when I first read this paper but judging by the record card and the state of the actual hard copy of the paper, it was probably when I was doing my PhD in the late 1970s. This paper and its companion, which was published a year earlier* (Tahvanainen & Root, 1972), have had a significant effect on the scientific understanding and development of inter-cropping as a method of crop protection worldwide. Although inter-cropping in some form or another has been around a long time, the idea that it could be used as part of an integrated pest management programme was not proven.  In this landmark study, Root compared pure stands (plots) of collards (spring greens in the UK) (Brassica olercaea) with adjacent rows of collards grown intermingled with other herbaceous plants.  His premise being that it was well documented that pest outbreaks tend to be associated with pure monocultures of crops (Pimentel, 1961; Janzen, 1970) and he wished to test the hypothesis that natural enemies were more abundant and effective in vegetationally diverse areas  than in pure monocultures, the so-called ‘enemies hypothesis’.  This idea had been around a surprisingly long time e.g. Ullyett (1947) who remarked  “where weeds occur around headlands and in hedges, they should be left for the purpose of supporting parasites and predators important in the natural control of the diamond-back moth (Plutclla maculipennis Curt)”.  A decade later, Elton (1958,) refers to this statement, explaining that “these hedge rows form a reservoir for enemies and parasites of insects and mite pests of crops”.  I am not sure what it indicates but note that many groups around the world, including mine, are still working on this both at the local (field-scale) level (e.g. Ramsden et al., 2014) and landscape level (e.g. Rusch et al., 2013; Raymond et al., 2015).

Root explained the premise of the ‘enemies hypothesis’ as follows.  Predators and parasites are more effective at controlling herbivore populations in diverse habitats or plant communities because, diverse plant communities support a diversity of herbivores with a variety of phenologies, providing a steady supply of prey for the predators.  In addition, complex environments provide prey refugia, thus allowing the prey not to be completely eradicated.  Diverse plant communities also provide a broad range of additional resources for adult natural enemies e.g. pollen and nectar.

Root ran his experiment for three years and did indeed find a significant difference in herbivore load between the pure plots and the weedy rows, the former having a greater abundance of pests (mainly aphids and flea beetles) than the latter.

Fig 1

From Root (1973)

To his disappointment (I assume), he did not find any difference in the numbers of natural enemies between the two treatments. He thus had to come up with another idea to explain his results. His ingenious explanation is encapsulated in what he termed the Resource concentration hypothesis which states that herbivores are more likely to find and stay on hosts growing in dense or nearly pure stands and that the most specialised species often reach higher relative densities in simple environments.

Fig 2

Typical modern monocultures, beans, cabbages and wheat

He hypothesised that specialist herbivores were ‘trapped’ on the crop and accumulated whilst more generalist herbivores were able to and likely to move away from the crops to other host plants.  Root added that the ‘trapping effect’ of host patches depends on several factors such as stand size and purity.

In 1968, presumably as a result of what Root was discovering, Jorma Tahvanainen (one of the many great Finnish entomologists who appeared on the scene in the 1970s -, he retired in 2004) came to Cornell to do his PhD with Richard Root. Working on the same system and in the same meadow, Tahbanainen developed two new hypotheses to explain why more diverse cropping systems have fewer pest problems than monocultures. His experiments as he too found little evidence of natural enemies having an effect. He developed two new hypotheses, one he termed Associational resistance which I reproduce below exactly as published:

A natural community, such as a meadow, can be treated as a compound system composed of smaller, component communities (Root, 1973). The arthropods associated with different plant species represent important components in terrestrial systems. The available information indicates that the biotic, structural and microclimatic complexity of natural vegetation greatly ameliorates the herbivore pressure on these individual components, and consequently, on the system as a whole. Thus, it can be said that in a compound community there exists an “associational resistance” to herbivores in addition to the resistance of individual plant species. If the complex pattern of natural vegetation is broken down by growing plants in monocultures, most of this associational resistance is lost. As a result, specialized herbivores which are adapted to overcome the resistance of a particular plant species, and against which the associational resistance is most effective, can easily exploit the simplified system. Population outbreaks of such herbivores are thus more likely to occur in monocultures where their essential resources are highly concentrated

The other, is the Chemical Interference Hypothesis, in which he postulated that reduced herbivory in diverse communities due to chemical stimuli produced by non-host plants interfering with host finding or feeding behaviour of specialist herbivores.  His experimental set-up was very simple, but very effective.

Fig 3

How to send mixed signals to specialist herbivores – reproduced from Tahvanainen & Root (1972)

In simple terms, a monoculture sends out a very strong signal, it could be olfactory, e.g. a strong bouquet of crucifer volatiles, or for other herbivores it could be visual, or a combination of the two.

Fig 4

Conventional intensive agricultural landscape sending out strong ‘signals’ to specialist herbivores

Inter-cropping increases crop diversity and changes the crop ‘signal’ to one that now ‘confuses’ specialists. Note that I am not necessarily advocating a combined crop of wheat, beans and cabbages, as harvesting would be a nightmare 😉

Fig 5

 

The intercrop melange effect

These two papers have had a huge influence on the theory and practice of inter-cropping and agricultural diversification, although Root (1973) has had many more citations (1393 according to Web of Science on 11th December 2015) than Tahvanainen & Root (1972) which has only had a meagre 429 citation to date.  The message coming out from the many studies that have now investigated the effect of intercropping crop diversification on pest abundance, is, that in general, polyculture is beneficial in terms of promoting biological control and that incorporating legumes into the system gives the best yield outcomes (Iverson et al,  2014).

Another take on intercropping that overcomes the potential problems of harvesting different crops from the same field, is the concept of planting different genotypes of the same species. Resistant plants tend to have fewer generalists present, although their individual yield may be reduced.  By planting a mixture of susceptible and resistant genotypes it is however, possible to have your cake and eat it, especially if it is not essential to have a single genotype crop.  This approach has been used to good effect in the production of short rotation willow coppice, where planting diverse genotypes of the same species reduces both pest and disease levels (Peacock et al., 2000, 2001).

Who would have that two simple field experiments conducted in an abandoned hay meadow outside Ithaca, New York almost fifty years ago would have such a far-reaching influence?

 

References

Elton, C. S. (1958) The Ecology of Invasions by Animals and Plants. London: Methuen & Co., Ltd. 159 pp.

Iverson, A. L., Makin, L. E., Ennis, K. K., Gonthier, D. J., Connor-Barrie, B. T., Remfret, J. L., Cardinale, B. J. &Perfecto, I. (2014). Do polycultures promote win-win or trade-offs in agricultural ecosystem services? A meta-analysis. Journal of Applied Ecology. 51, 1593-1602.

Peacock, L. & Herrick, S. (2000) Responses of the willow beetle Phratora vulgatissima to genetically and spatially diverse Salix spp. plantations. Journal of Applied Ecology, 37, 821-831.

Peacock, L., Hunter, T., Turner, H., & Brain, P. (2001) Does host genotype diversity affect the distribution of insect and disease in willow cropping systems? Journal of Applied Ecology, 38, 1070-1081

Janzen, D.H. (1970) The unexploited tropics.  Bulletin of the Ecological Society of America, 51, 4-7

Pimentel, D. (1961). Species diversity and insect population outbreaks. Annals of the Entomological Society of America, 54, 76-86.

Ramsden, M. W., Menéndez, R., Leather, S. R. & Wackers, F. (2014). Optimizing field margins for biocontrol services: the relative roles of aphid abundance, annual floral resource, and overwinter habitat in enhancing aphid natural enemies. Agriculture Ecosystems and Environment, 199, 94-104.

Raymond, L., Ortiz-Martinez, S. A. &Lavandero, B. (2015). Temporal variability of aphid biological control in contrasting landscape contexts. Biological Control , 90, 148-156.

Root, R. B. (1973). Organization of a plant-arthropod association in simple and diverse habitats: the fauna of collards. Ecological Monographs, 43, 95-124.  1393 citations

Rusch, A., Bommarco, R., Jonsson, M., Smith, H. G. &Ekbom, B. (2013). Flow and stability of natural pest control services depend on complexity and crop rotation at the landscape scale. Journal of Applied Ecology, 50, 345-354.

Tahvanainen, J. & Root, R. B. (1972). The influence of vegetational diversity on the population ecology of a specialized herbivore Phyllotreta cruciferae (Coleoptera: Chrysomelidae). Oecologia, 10, 321-346. 429 citations

Ullyett, G. C. (1947) Mortality factors in populations of Plutella maculipennis Curtis (Tineidae: Lep.) and their relation to the problem of control. Union of South Africa, Department of Agriculture and Forestry, Entomology Memoirs, 2, 77-202.

Post script

*I suspect, judging by how the two papers cite each other, that the Root (1973) paper was actually submitted first but that the vagaries of the publication system ,  meant that follow-up paper, Tahvanainen & Root (1972) appeared first.

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Pest Managers are ecologists too

Over the course of the last decade I have come to the conclusion that British undergraduates have no idea of what pest management actually is.  For the last twenty years or so, I have run a suite of MSc courses, initially at the Silwood Park campus of Imperial College and since 2012, at Harper Adams University, the leading land-based university in the UK.  The four MSc courses I run are Conservation and Forest Protection, Ecological Applications, Entomology and Integrated Pest and Disease Management.  These are, as you can see, pretty much specialist vocational courses.  The students who do these courses are highly motivated and come from a range of backgrounds.  Many had established careers in other areas and found those careers lacking.  Others have had a burning desire to embark on advanced training in entomology since they were at school and been deeply frustrated that no undergraduate degrees exist in the UK in that subject. [The last BSc in Entomology in the UK was awarded in 1993 by Imperial College to Andy Salisbury (now Dr Andrew Salisbury and Senior Entomologist at the Royal Horticulture Society’s research offices at Wisley].  Yet others have developed an interest in one of the areas covered by my MSc courses as undergraduates and decided to make a career in those areas, but found that their BSc degrees had not prepared them to the required level in their chosen subject and that they needed the extra training provided by a postgraduate course.  The interesting thing to me is how the absolute interest in integrated pest management has remained at pretty much the same level year on year, at just under 5 with a high of 9 students in 1989 and in 1998 no students at all.  Over the same period the average number of entomologists on the course was twelve. As a proportion of the student body the pest management contingent has ranged from a very disappointing zero to the memorable year, 2004, when there were twice as many pest managers as entomologists, albeit out of total student body of only nine.

MSc Entomology and IPM students

MSc Entomology and IPM students

Pest managers are in great demand from industry –biological control and agrochemical companies have more positions than there are suitably qualified graduates; yet undergraduates seem reluctant to train in this area.  I think that there are two root causes for this problem; a lack of exposure to the subject as undergraduates and a misunderstanding of exactly what pest managers are and what they actually do.  Mention pest management to most people, not just students, and their first reaction is rats, cockroaches and Rentokil operatives spraying.

Cockroaches

Cockroaches

The mythical spray man

The mythical spray man

A pest!

A pest!

Yes pest control does involve poisoning vermin and spray operations against domestic insect pests, but that is only one very minor, albeit important, aspect of pest management.  Pest management, or as it is more formally known, Integrated Pest Management, is the intelligent selection and use of pest control measures to ensure  favourable economic, ecological  and sociological consequences.  Yes this does include the use of pesticides but only as part of an integrated programme that could include biological control, the use of resistant plant varieties, and the diversification of the farm landscape through conservation headlands, and cultural methods such as crop rotation or inter-cropping.   Even scare-crows, traditional or modern, can be part of an IPM programme. The list goes on and all this is backed up by a detailed knowledge and understanding of the biology and ecology of the pest species, their natural enemies and the habitats that they live in.

Conservation headland

Conservation headland

Traditional scarecrow

Traditional scarecrow

Intercropping

Intercropping

Biological control

Biological control

The idea of pest management is not an entirely modern one ; Benjamin Walsh a British born pioneer applied entomologist  working in the USA, said in 1866, and I quote,  “Let a man profess to have discovered some new patent powder pimperlimplimp, a single pinch of which being thrown into each corner of a field will kill every bug throughout its whole extent, and people will listen to him with attention and respect.  But tell them of any simple common-sense plan, based upon correct scientific principles, to check and keep within reasonable bounds the insect foes of the farmer, and they will laugh you to scorn”.  I consider this to be the first modern reference to integrated pest management.

               

Pest managers do not just work in domestic and urban situations.  Most pest managers, as opposed to pest control operatives work in agriculture, forestry and horticulture, safeguarding our crops and ensuring global food security.  Pest managers, because of the complexity of the problems facing sustainable crop production in the modern world, have to have a much greater depth and breadth of knowledge than pure ecologists.   Although many pest managers have entomological backgrounds, they also have a more than nodding acquaintance with plant pathology, nematology and pesticide application and chemistry.  They also need to have a good grasp of the economics of both pest control and the farming/cropping system that they are working in.  They must fully appreciate what is feasible and appropriate in the context of the farmer’s/forester’s year, budgets and targets in terms of yields and profits.  There is no point in coming up with the ideal ecological or conservation solution that cannot be implemented because of the constraints of the real world.  Pest managers, even those working in academia, interact closely with their end-users, to ensure a reduction in pest numbers and abundance, not necessarily eradication, which is acceptable to growers, society and the natural world.   As Lorna Cole said on Twitter on September 8th 2013….

Lorna Cole

So when you hear the word pest management in future, don’t just think spray, think conservation headlands, beetle banks, biological control, crop rotation, resistant varieties, chemical ecology, forecasting, monitoring , cultural control, holistic farming, multi-purpose forestry, sociology, economics and sustainability.  These are the elements of the armoury most commonly used by pest managers, not pesticides as so many people mistakenly think.

So for prospective students, don’t be put off by the word pest, if you want an exciting, satisfying and possibly international career, embrace the application of ecology and make a difference to the world.   Become a pest manager.  Without integrated pest managers food production will never be sustainable or as ecologically friendly as it now is.

Post Script

Sadly, even to this day (it was much worse when I started my career), there is a perception among some academics that being applied is second best; on one memorable occasion I was introduced to some visitors by one of my colleagues as, “this is Simon Leather, he’s an ecologist, ALBEIT, an applied one”

 

Post post script

 

For those interested in joining the MSc course in Integrated Pest Management based at Harper Adams University here is the appropriate link.  I should also add that unfortunately we are NOT able to offer three annual Scholarships funded by the Horticultural Development Council, HDC, of £5000 each to particularly deserving students as despite ALL our graduates finding employment in the industry they felt that the scheme was not working!

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