Defense mechanisms of polycultures against pests

The lower damage from pests in polycultures may be due to the fact that they are less attractive for them because (Alonso, A., Guzman, G., 2000):
1. The pest cannot find the crop on which it usually feeds. This is due to the fact that the accompanying crop alters the physical (microenvironment, light reflectance pattern, etc.) or chemical conditions (diffusion of attraction, odor masking, repellency, etc.) that normally indicate to the pest that the crop is present which reduces the probability of finding it. Let’s look at some examples.
The insect’s search for the host plant often involves olfactory mechanisms. Plants attractive to pests associated with plants that are not attractive can be an important component in defense against pests, due to the effect of masking crop odors by companion plants. For example:
– Carrot onion.
– Carrot- alfalfa.
– Intercropping of tomato and cabbage-cabbage that results in less presence and laying of eggs of the Plutella xylostella moth in cabbage, because it is repelled by the odors that the tomato gives off.
– This mechanism seems to also act in the case of phytopathogenic nematodes when some companion species, such as carnation or sesame, are present.
– Aromatic herbs (rosemary, sage, etc.) have been indicated to repel pest insects in orchards, where they are usually implanted at the edges of the plots.
Other pests detect the crop through visual mechanisms that are also affected by the presence of a second crop. This is the case of aphids (aphids), which are confused by the lower intensity of the light reflected by the crop growing with the plant cover, compared to monoculture
2.    In other cases, the pest colonizes the polyculture less and, furthermore, the rate of migration of the pest to other sites is much higher than when it encounters a monoculture plot. This seems to be due to the fact that the pest has to invest more energy to move and feed on the polyculture, which is why it is not as “profitable” as the monoculture.
This mechanism occurs, for example, when the cruciferous flea (Phyllotreta cruciferae) reaches a broccoli polyculture covered with vetch. The flea finds serious difficulties, when it falls on a vetch plant, to reach the top of it, and to be able to jump or fly towards another broccoli plant. This is due to the large biomass produced by vetch and its complex architecture of tangled branches. For this reason, they quickly leave the polyculture plot.

3.  The lower attraction or development of the pest in the polyculture is sometimes due to the different “quality” of the host plant, which is therefore less valued by the pest. This can occur because in polyculture there is competition for nutrients between the crops involved, which reduces the extraction carried out by each one of them.

An example of this are aphids or aphids. The fertility of these is usually proportional to the content of soluble nitrogen in the phloem (sap) of the plant, increasing its population the higher the nitrogen consumption by it. Polyculture can reduce nitrogen uptake by participating crops, avoiding luxury consumption and limiting aphid population development. This is the reason for the lower level of aphids in the broad bean crop when it is sown with a cover of barley or oats.
4.    On other occasions, the lower presence of plague on the plants of the main crop is due to the fact that it has preferred to be located on the companion crop, which thus acts as a trap crop.
Some proven examples of these trap crops are:
a) When corn is planted in strips in cotton fields, it attracts the cotton weevil, keeping it away from this crop.
b) In the Jewish / tomato polyculture, the bean acts as a trap crop against the attacks of the donut (Spodoptera sunia) on the tomato, which is hardly attacked.
c) The broccoli crop mixed with another cruciferous host, wild mustard (Brassica kaber) suffered fewer attacks from the insect pest Phyllotreta cruciferae (fleas). This is because these insects concentrated more on the wild mustard than on the broccoli in the mix. This preference is chemically based, since wild mustard produces a greater quantity of a chemical that strongly attracts the flea.
5.    Hypothesis of natural enemies.
When moving from a monoculture to a polyculture, the presence of pest predators and their effectiveness increases. This is due, among other reasons, to the fact that:
a) In polycultures they find other insects that they can feed on when the pest is not present. In this way they survive, and when the pest insect appears they can control it.
b) They find other sources of food (pollen and nectar) that, as before, allow them to survive.
c) They more easily find shelters to spend the winter, to reproduce, etc.
Successful polycultures in pest control

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