Avoiding insecticides by mixing vegetables and sowing flowers
Séverin Hatt
While farming, it may be frightening to discover cabbages infested by caterpillars starting
to devour the leaves, or lettuces full of aphids ready to transmit viruses to the plants. Whatcan be done? The temptation could be high to rely on insecticides to eradicate these insect
pests and save the vegetables. Nevertheless, insecticides should not be the solution because
they are poisonous for humans and the whole environment (see Agenda no. 64). Alternativestrategies exist! First, mixing the plants instead of cultivating them separately can reduce
the ability of insect pests to colonize them and spread in fields. Second, sowing flowering
plants can attract beneficial insects that are natural enemies of the pests.
Why mixing plants?
Most of insect pests feed on a limited number of plants. Cultivating a single plant species ina field provides high amount of food for the insects that feed on this single plant.
Consequently, these insects will be attracted, and they will be able to multiply and spread in
the field because all of the available plants will be suitable for them. Conversely, by mixing
plants that are consumed by different insects, the amount of food available for each of themis reduced.
In addition, it would become difficult for the insects to identify the specific plant they aresearching for. Indeed, insects use colors, contrasts of colors, but also odor blends to identify
their host plant. Mixing plants would lead to mixing colors, modify the contrasts between
them as well as odor blends. Some plants also produce repellent odors. In the end, insects
would be disoriented and have difficulties to identify their host plants in the mixture.
Some insect pests would eventually succeed to colonize a plant. However, when different
plants are mixed, they will have difficulties to spread because plants surrounding themwould not be suitable. Indeed, each non-host plant for an insect will act as a barrier andlimit pest spreading.
How to mix plants?
Plants are distinguished by their family, species and variety that partly describe their levelof differences and similarities. Two plants can be: from different families, for example
tomato (Solanaceae family) and cabbage (Brassicaceae family); from two species of thesame family, for example tomato and potato (both of the Solanaceae family); or from twovarieties of the same species, for example two varieties of tomato (which species name is
Solanum lycopersicum). Family differences are generally larger than species differences,being also larger than variety differences.

1Many insects are specialized in a single family of plant. This is the case of several pests of
the Brassicaceae (cabbage, mustard, radish) like the cabbage aphid (Brevicoryne brassicae)or the cabbage butterflies (Pieris rapae and Pieris brassicae). Some insects are evenlimited to few cultivated plant species, or few varieties of a species, for example the lettuce
aphid (Nasonovia ribisnigri). Mixing plants of different families maximizes the chance oflimiting pest colonization. Mixing varieties of a same species or two species of a samefamily can also help to reduce risks from insect pests, compared to planting large patches ofa same plant.
Nevertheless, there are also insects that can feed on plants from different families, forexample the Japanese beetle (Popillia japonica). These insects are not easy to manage andmixing plants from different families may not be enough to ward them off. Carefully
choosing plants that are known not to be the hosts of such generalist pests would be needed.
Experiments have shown examples of plant mixing that proved to efficiently reduce pestcolonization and damages. Mixing carrots with onions can reduce attacks by the carrot fly(Psila rosae) and the onion thrips (Thrips tabaci) (1). Associating cabbages with tomatoes
or green peppers can limit attacks and damages from the diamondback moth (Plutellaxylostella) on cabbages (2). Onions can protect potatoes from attacks from the potato tuber
moth (Phthorimaea operculella), aphids (Myzus persicae, Aphis gossypii), leaf hoppers(Empoasca spp.) and herbaceous ladybird beetles (Henosepilachna sparsa) (3). Growing
zucchinis with buckwheat can lower colonization of aphids (Aphis gossypii) on zucchiniand can delay the transmission of viruses (4). In addition, flowers of buckwheat can attract
beneficial insects that are natural enemies of the pests.
Why sowing flowers?
While mixing plants is a first practice to limit the colonization and spread of pests onvegetables, sowing flowers in fields is a second and complementary technique. Its aim is toencourage a natural control of the pests by their natural enemies. Natural enemies areinsects that are predators or parasites of other insects. When they attack pests of vegetables,
natural enemies can be seen as farmers’ ‘helpers’.

Hoverfly adult feeding on
coriander

Predatory wasp adult feedingon shiso2Lacewing larvae feeding on
aphidTo attract and reward these ‘helpers’, a farmer can sow flowers in their fields. Indeed,
nectar and pollen of flowers represent important food source for many insects. Nutrient
composition of pollen and nectar varies between flower species, but in general, nectar isrich in sugar that provides energy to insects, and pollen contains proteins useful for theirfecundity. Although there are insects that feed only on flowers, like bees, others feed bothon flowers and on other insects.
Some of these natural enemies are very well known and loved by humans. This is the caseof the ladybird beetles (Coccinellidae). Ladybird beetles mostly feed on other insects but
they can also survive by feeding on flower nectar and pollen. Other natural enemies are notas popular among humans. This is the case of the large wasps (Vespidae), of which people
are often afraid of being bitten. However, many large wasps that feed on sugar sources suchas flower nectar as adults can be very helpful because they attack other insects to feed theiryoung.
Most natural enemies are often unknown to non-entomologists but they play great roles inregulating insect populations. For example, hoverflies (Syrphidae) and lacewings(Chrysopidae) only feed on flowers as adults but the larvae of many species are predators,especially of aphids. Also, many small wasps feed on flower nectar as adults and parasitize
other insects to reproduce. Parasitizing means that the adults lay their eggs on―or into―another insect’s body (the host) and their larvae will feed on it until killing it. Such hosts
can be pests of vegetables, like aphids or caterpillars. To wrap up, flowers can attract andsustain adults of natural enemies in a first step, and these adults will search for prey or hosts
for their young in a second step, finally leading to a natural control of the pests.

Mechanisms related to mixing plants and sowing flowers to limit pest abundance on vegetables

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Which flowers to sow?
Many flower species can be sown in fields and experiments have been carried out toevaluate which ones are useful to attract and support the natural enemies. Some of theseflowering plants can also be consumed by people as food and medicine, thus sowing themwould not mean losing field space. Coriander (Coriandrum sativum), dill (Anethumgraveolens) or fennel (Foeniculum vulgare) are examples of aromatic flower plants that areboth consumed by people and appreciated by insect natural enemies (see table for moreexamples). Also, current research at Kyoto University is evaluating the attractiveness andbenefits of shiso (Perilla frutescens) for natural enemies. Experiments done in laboratory
show that ladybird beetles can survive several days when feeding on shiso flowers only,
and a mixed diet of shiso with prey food can improve their early reproduction (5). In fields,a large diversity of insects has been observed visiting shiso flowers, among them hoverflies
and wasps.
Table Ten flowering plant species that can be sown to attract and support natural enemies against
insect pests
Plant family
Apiaceae

Plant scientific nameAnethum graveolens

Plant common name
Dill

Natural enemies
Ladybird beetles,
hoverflies, lacewings,
parasitic wasps

Apiaceae

Coriandrum sativum

Coriander

Ladybird beetles,
hoverflies, lacewings,
parasitic wasps

Apiaceae

Foeniculum vulgare

FennelLadybird beetles,
hoverflies, lacewings,
parasitic wasps

Boraginaceae
Lamiaceae
Lamiaceae

Borago officinalis
Agastache rugosa
Mentha spp.

BorageKorean mint
Mint

Lamiaceae

Lavendula stoechas

LavenderLamiaceae
Lamiaceae

Ocimum basilicum
Perilla frutescens var.
crispaBasilShiso

Hoverflies, lacewings
Hoverflies
Hoverflies, parasitic
wasps
Hoverflies, parasitic
wasps
Lacewings
Ladybird beetles,
hoverflies, predatory
wasps

Polygonaceae

Fagopyrum esculentum

Buckwheat4

Hoverflies, parasitic
waspsWhen choosing flowering plants to sow, attention should be paid on their blooming period.
While some tend to bloom in spring, like coriander, others bloom in autumn, like shiso.
Sowing different flower species that bloom at various periods is important to provide foodsources to natural enemies from spring to autumn. In this way, natural enemies will besustained all along the cropping season and will be ready to regulate pests in case ofoutbreak.
Re-imagining old ideas
Mixing plants and favoring natural enemies are not new ideas and have been used byfamers before the industrialization of agriculture. In her book Silent Spring (6) published in
1962, Rachel Carson already recalled that ‘an insect that lives on wheat can build up its
population to much higher levels on a farm devoted to wheat than on one in which wheat isintermingled with other crops to which the insect is not adapted.’ The great role of natural
enemies in the self-regulation of insects was also obvious to her, when she cited theentomologist Robert Metcalf highlighting that ‘the greatest single factor in preventing
insects from overwhelming the rest of the world is the internecine warfare which they carryout among themselves.’
More than 50 years have passed since the publication of Silent Spring, and agriculture hasgreatly evolved, but insecticides are still widely used. Therefore, remembering themechanisms leading to the natural regulation of insect pests on plants remains useful.
Imagining how to apply them in today’s agricultural fields is the challenge that must beovercome. While farmers can try out mixing plants, sowing flowers and observing naturalenemies in fields, scientists must work at understanding how ecosystems function.
Consumers can support farmers by buying organic food at farmers’ markets and retailshops, through teikei groups or co-ops. Finally, a general change of the whole food system
will be possible if policy makers assume their responsibility to implement adequate
environmental and agricultural policies, which could start by simultaneously banning theuse of pesticides, and supporting farmers’ transition to a pesticide-free agriculture. As
citizens, our responsibility is to elect representatives that consider such a transition as apriority.

References(1) Uvah I. I. I., Coaker T. H. (1984). Effect of mixed cropping on some insect pests ofcarrots and onions. Entomologia experimentalis et applicata, 36(2), 159-167.
(2) Asare-Bediako E., Addo-Quaye A. A., Mohammed A. (2010). Control of diamondback
moth (Plutella xylostella) on cabbage (Brassica oleracea var capitata) using intercroppingwith non-host crops. American Journal of Food Technology 5(4), 269–274.

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(3) Potts M. J. (1990). Influence of intercropping in warm climates on pests and diseases ofpotato, with special reference to their control. Field Crops Research, 25(1-2), 133-144.
(4) Hooks C. R., Valenzuela H. R., Defrank J. (1998). Incidence of pests and arthropod
natural enemies in zucchini grown with living mulches. Agriculture, ecosystems andenvironment, 69(3), 217-231.
(5) Hatt S., Osawa N. (2019). The role of Perilla frutescens flowers on fitness traits of theladybird beetle Harmonia axyridis. BioControl. DOI: 10.1007/s10526-019-09937-1(6) Rachel Carson (1962). Silent Spring. Houghton Mifflin, USA.

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