Increasing Beneficial
Insect Populations by Increasing Biodiversity in Organic Agricultural Systems
An ant, CAN, move
a rubber tree plant
If you’ve ever been to a fruit
orchard, eaten fresh fruit or vegetables from a local stand or farmers market
it is likely that you have seen the effects of herbivory (the act of feeding on
plants) on market produce. This may present in the form of minor cosmetic
damage or as a wiggly bit of a worm staring at you from the apple you just bit
into. While these conditions are problematic they do not demonstrate the full
effect of the problem. Pest damage, including herbivory, is the second highest
cause of infield decrease in yield for market produce (fruits and vegetables)
and the leading cause of loss of yield for cereal grains, as well as being the
cause of the majority of “marketable” defects in fresh market produce(fruits
and vegetables meant to be eaten fresh) (1).
In conventional agricultural
systems pesticides have been the primary tool producers have used to control
pests. However, these pesticides, along with synthetic fertilizers are agricultural
inputs which are causing massive damage to the natural ecosystem and in the
case of pesticides can even be dangerous to human health (2). Among ecologists,
agricultural researchers and other interested parties (read: all of humanity) this
problem of sustainably controlling pests from an ecological perspective while
still achieving sustainable economic productivity should be of paramount
concern (2). Simplified, how do we feed the world without destroying the ecosystem
that keeps us all alive in the process? A possible solution comes in the form
sustainable organic agricultural systems.
The USDA National Organic Program,
the regulatory body for products labeled “Organic”, has a fairly straight forward
approach to the problem, no synthetic pesticides of any kind are permitted to be
used in any field or even livestock feed grains for any product which is to be
labeled “Organic”, at anytime during production or in the three years prior to
planting (3). This, however, leaves the organic
producer with a difficult task, how do they maintain economic levels of
production and loss without using synthetic pesticides? Natural pesticides such
as oil from the Neem tree are permitted but are not as entirely effective or economically
viable as repeated treatments of traditional pesticide.
The current solution is creating a
robust production system along side an integrated pest management system(IPM);
a system that encourages the crop to grow most vigorously while also employing
a pest management system that seeks to holistically control pests by
controlling all facets of their ability to thrive, not just kill them outright.
Various strategies are used, many of them ecological, such as: controlling the
pests environment, interrupting the pest’s life cycles, removing plants which
are already infested, using traps or even “trap crops”(crops which have no
market value but serve to distract pests), or, most interesting to me, using
beneficial predatory insects to control pest populations. To the outsider this
seems illogical, weren’t bugs the problem? But, by promoting a more natural
beneficial arthropod (insect) population, pest insects can be controlled to
economic levels.
The pollinators
are great, but save the Ladybugs!
The importance of natural
pollinators and the threat to their continued survival has not been understated
recently, at least in the ecological and agricultural communities, I cant speak
for the whole of America, but I know the “Save the Bees” campaign was quite
visible in this area. The effects of agricultural systems on the natural
ecosystem as a whole has been less visible to the layperson. The line between pollinator
and “we need food” is direct and obvious to most. I would say that the
importance of promoting a more naturally balanced arthropod community in an
effort to reduce explosions of pest populations is less intuitive. The next
campaign should be called “Save the ladybugs”, they’re cute like bees and are
also voracious predators (crazy right?).
Conventional agricultural systems are fairly
sterile from an ecological standpoint, there is the cash crop and the soil,
nothing else belongs there and is to be removed. Producers remove any biota
that may effect the crop (weeds and pests) and attempt to grow as much produce
as possible while using herbicides and pesticides to control any problems that
appear; environmentally conscious producers will employee minimal pesticides
but this is not always the rule. Conventional
producers fill a field with tasty treats for that crop’s pests and then control
any pest “Booms” with pesticides. The problems with this strategy are two
sided. On one hand, the pesticides are almost necessary at this point for
traditional producers to have economic returns, without them pests would devastate
most crops and pesticides are generally effective at stopping this,
disregarding acquired pesticide resistance. On the other side of the coin, these
pesticides are causing significant non-point pollution problems; they are
killing insects they aren’t intended too (4), are doing so outside of the area
where they are applied (4) and the entirety of these effects is not at all thoroughly
understood (4), like many topics ecological in nature.
When these pesticides are removed
from the equation in organic agricultural systems the first problem persists, pests
will devastate a crop where they are not controlled. How can they be
controlled? One of the most effective strategies is to promote beneficial arthropods
that will consume pests, by providing bio-diverse habitats that attract them
and promote their competitiveness (5).
But does it work?
In an agricultural setting, food
for pest insects is abundant and generally problems arise when there is no
balance in the natural system and these pest insects are not adequately
controlled by their natural predators. This problem could arise from a variety
of reasons. The predatory insects life cycle may be interrupted by agricultural
practices, the predatory insects may be separated spatially or temporally from
their prey, or the predatory insects are not adapted to the crop plant and do
not have suitable habitat. In recent years it has emerged that these non-crop
habitats are very important to the life cycle and competitiveness of these
beneficial “natural enemy” species (6). It has been shown that many types of
native “weeds” and selected species can be used at field margins, within fields
and even in the surrounding landscape to promote natural enemy species (5).
I have selected three studies which
demonstrate the effectiveness of increasing biodiversity on reducing pest
populations. The first is a study on intercropping polyculture fields of
vegetables with sunflower, that is, planting sunflowers adjacent to multiple vegetable
species in the same area. Jones Et al. found that sunflower plants were shown
to attract parasitoid insects almost immediately after establishment and
pollinators were attracted after flowering (7). The second trial in the study
found that sunflowers attracted these parasitoid insects and also had showed a
similar increase in the area (1m) around the sunflowers as well as higher levels
of pests on the sunflowers suggesting that they could function as both a
habitat for beneficial insects and as a trap for pests. The study also showed
that a huge variety of beneficial insects occurred on or near the sunflowers,
as shown in Table 1.
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| Table 1: Occurance of beneficial insects in and around sunflower plots |
The second study I found related to
this topic examined the effects of managed hedgerows on field margins and
compared them to sites with weedy edges. Moradin et al. found that parasitoids
were more abundant in sites with hedgerows, that pest control was enhanced in
sites with hedgerows and that the benefits of these hedgerows extended up to
200m into the field. Most importantly to this study it was found that fewer of
the fields adjacent to hedgerows reached threshold levels of pests would require
pesticides.
The third study examined the use of
relay cropping cereal grains and other species (wheat, canola, sorghum) into cotton fields. This study by Parajulee et al. was interesting
to me because while it is not a market vegetable like I mentioned in the
beginning, cotton is notoriously plagued by pests and before Bt Cotton (pest resistant,
modified genetics) required extensive pesticide use. This is likely to become a
problem again in the future as pests acquire resistance to this new poisonous
cotton. As shown in figures one and two cotton aphid populations were drastically decreased during the critical growing and harvest season, the occurance of aphids in the relay cropping systems decreased over-all and the occurance of predatory insects increased over-all.
These studies are not rare or isolated. Increasing biodiversity has been shown to have a significant beneficial effect on many different crop types over many years with several net positive benefits such as increased yields, decreased inputs and improved ecological health.
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| Figure 1: Abundance of Cotton aphids in isolated fields and fields with selected relay crops |
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| Figure 2: Abundance of aphids and predators in isolated and relay cropping systems |
Works Cited:
1: Crop losses to pests. Oerke, E. (2006). The Journal
of Agricultural Science, 144(1), 31-43. doi:10.1017/S0021859605005708
Polyxeni Nicolopoulou-Stamati,
Sotirios Maipas, Chrysanthi Kotampasi, Panagiotis Stamatis, Luc Hens. Front
Public Health. 2016; 4: 148. Published online 2016 Jul 18. doi: 10.3389/fpubh.2016.00148
3: United States Department of Agriculture; Guidelines for
Organic Crop Certification, https://www.ams.usda.gov/sites/default/files/media/Crop%20-%20Guidelines.pdf
4: The Effects of Microbial Pesticides on Non-target, Beneficial
Arthropods. J. Flexner, B. Lighthart, B.A. Croft. Agriculture, Ecosystems &
Environment, 16, 203-254, July 1986. https://doi.org/10.1016/0167-8809(86)90005-8
5: Landis, D., Menalled, F., Costamagna, A., &
Wilkinson, T. (2005). Manipulating plant resources to enhance beneficial
arthropods in agricultural landscapes. Weed Science, 53(6), 902-908.
doi:10.1614/WS-04-050R1.1
Douglas A. Landis Stephen D.
Wratten and Geoff M. Gurr. Annual Review of Entomology 2000 45:1, 175-201
7: Gregory A. Jones and Jennifer L. Gillett "INTERCROPPING WITH SUNFLOWERS
TO ATTRACT BENEFICIAL INSECTS IN ORGANIC AGRICULTURE," Florida
Entomologist 88(1), (1 March 2005).https://doi.org/10.1653/0015-4040(2005)088[0091:IWSTAB]2.0.CO;2
8: Morandin, L. Long, R. Kremen, C.(2014) Hedgerows
enhance beneficial insects on adjacent tomato fields in an intensive
agricultural landscape. Agriculture,
Ecosystems & Environment, 189, 154-170, May 2014. https://www.sciencedirect.com/science/article/pii/S0167880914001662
9: M. N. Parajulee , R. Montandon & J. E. Slosser
(1997) Relay intercropping to
enhance abundance of insect
predators of cotton aphid (Aphis gossypii Glover) in Texas cotton,
International Journal of Pest
Management, 43:3, 227-232, DOI:
10.1080/096708797228726
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