biological control of aphids

Aphids are known to be the most damaging group of pests in agriculture. There are several varieties with adult sizes ranging from 1.7 to over 4 mm. Aphids are polyphagous and easily switch to a substitute host when they do not find their preferred prey.

The peach aphid (Hyalopterus pruni) is one of the most polyphagous aphids. Although its primary hosts are trees of the prunus genus, it also likes a large number of secondary hosts, including many vegetables: tomatoes, peppers, potatoes, lettuce, spinach, carrots, courgettes, melons, strawberries, etc. It should be noted that this aphid does not hesitate to invade certain weeds, such as nettles and wild grasses. This aphid is also a vector of a hundred or so viruses against which there is no curative means of control, such as the lettuce mosaic virus and the dreaded plum pox virus.

It is not only market gardeners who suffer from aphid infestation. It is well known that aphids and leafhoppers are fond of soft wheat and barley seedlings. These insects transmit highly damaging viruses through their bites (dwarf yellowing virus affecting barley and wheat in particular for the aphid, foot rot virus for the leafhopper) with sometimes catastrophic yield losses if the farmer does not intervene at the right time with appropriate treatments.

To reduce the nuisance of aphids, some advocates of organic methods recommend water jets, rhubarb decoctions, physical barriers using liquid soap, and others, as the list of remedies of our grandparents who tried everything before the invention of synthetic pesticides is long. In the 48 years that I have been growing a garden and fruit trees, I have tried all these methods alone or in combination, and each time I have had poor results.

Aphid-borne viruses

Cultivation under plastic film delays the invasion of aphids carrying CABYV by a few weeks. For melons, young plants can be protected with insect netting, but this protection must be removed as soon as the first flowers appear. In case of invasion of the gossypii aphid (melon and cotton aphid) and in the absence of the introduction of an efficient predator, the use of pesticides becomes indispensable.

Ecosystem services; the problem of natural aphid control in spring

Some websites claim that where there are aphids, ladybirds will soon arrive and eventually the aphids will disappear. Any outbreak of a pest will eventually encourage the development of its own predators. Thus, a natural regulation takes place, gradually reducing the aphid population. However, this regulation phenomenon occurs at the end of spring and even at the beginning of summer, which can be a serious problem for crops. This regulation is late for the following reason:

The ladybird appears and survives in the wild when the average temperature is 10 to 15° (9 to 12° for the Asian ladybird); this is why ladybird sellers advise introducing these insects when the outside temperature is at least 12°C (1). As for the aphid, it emerges when the average temperature is 4°C (2).

There is therefore a time lag in the appearance of aphids and ladybirds. This time lag also exists for hoverflies, which stop reproducing at a temperature of 15°C or less. Is this time lag a cause for concern? Certainly when this time lag reaches several weeks and when the aphids are virus carriers. The damage to the crops can then be very significant, requiring a preventive treatment against aphids.

Some remarks on the different species of ladybirds

Beneficial insects are more or less specialised in their prey. Some beneficial insects are exclusively monophagous and others polyphagous, often with marked preferences, and this is also the case for ladybirds. For example, the 2 and 7-spot ladybirds prefer aphids, although they may occasionally be satisfied with other prey when the aphid population is low.

Other ladybirds (Chilocorus bipustulatus) have a strong preference for mealy bugs. The Delphastus pusillus ladybird prefers whiteflies and the Stethorus pusillus ladybird prefers mites. The Asian ladybird is known to be very polyphagous, feeding on various aphids, eggs, moths and even the larvae of other ladybirds. This greater or lesser specialisation of ladybirds can be exploited to target pests with a minimum impact on biodiversity. This is why some breeders of beneficial insects have specialised in the production of certain ladybirds to specifically combat aphids or whiteflies.

Ladybirds in their natural environment

In autumn, adult ladybirds look for a place to overwinter. They often gather in groups, huddled together. In a dry shelter (such as attic insulation), ladybirds can resist more easily to the attacks of the dreaded beauveria fungus, also known to parasitise silkworms (ladybird losses can reach 70% in a humid place and/or in case of heavy frosts).

In spring, ladybirds wake up and after mating, the female lays 30 to 50 eggs on the underside of the leaves near the aphid colonies. These elongated eggs with a yellowish-white colour are tiny and therefore difficult to observe. After incubation (about 7 days), larvae are born and are immediately very active. They are difficult to observe during the first week and can even be confused with black aphids that infect some plants. Depending on the amount of food present, ladybird larvae develop within 3 to 4 weeks, undergo 4 moults before turning into pupae. Pupation lasts 8 days to produce adults with a lifespan of approximately 3 months to 1 year.

Any treatment using physical processes to kill aphids such as spraying with Marseille soap or plant extracts (pyrethrin, neem oil) will also kill ladybird larvae.

When aphid colonies are scarce, ladybirds feed on other prey such as thrips, psyllids, mealybugs and whiteflies, the larvae of certain wasps, diptera, butterfly caterpillars and even other species of ladybirds, as they are cannibalistic. Ladybirds can also feed on pollen and nectar from flowers when they run out of prey. In this case, they stop reproducing

Predators of ladybirds

Ladybirds are considered to have few predators. Ladybird larvae can be attacked by polyphagous parasitic wasps (pteromalids) which lay eggs inside their bodies. Pteromalids also parasitise psyllids, mealy bugs, aphids and some species are used in biological control. Spiders and birds also feed on ladybirds. But ladybirds reproduce quickly when their food is abundant. In this case, a female ladybird lays about 50 eggs per day. It is estimated that about 1000 eggs are laid during the lifetime of a ladybird.

Ants that feed on the honeydew secreted by aphids are known to defend aphids against their enemies. A nest of ants in an insect shelter quickly results in a reduction of ladybirds introduced by the grower to control aphids. It is likely that the ladybird larvae are destroyed by the ants. It is therefore recommended to eliminate the ants by treating their nests with pyrethrins (or pyrethroids) or nematodes, which can be purchased here or here. Glue barriers around tree trunks when they are frequented by ants are also effective.

When should ladybirds be imported?

The case of woolly aphids

Some aphids lodge in the roots of vegetables; they are called “root aphids” or “woolly aphids”. These aphids produce a waxy secretion consisting of cottony filaments covering the insect. These aphid colonies can also be found on the roots of many weeds surrounded by nests of ants that feed on their sugary ejecta. In nature, these aphids are very common, but because they grow on the roots, they are not easy to detect.

Root aphids are polyphagous pests of fruit, vegetable and ornamental crops. Some varieties of woolly aphids (Eriosoma lanigerum) also have an aerial life. Large colonies can form on the branches of the apple tree, which these aphids love to the point of forming fungus-infected cankers. Some aphids are associated with poplars (Phloeomyzus passerinii) and need two hosts to complete their reproductive cycle; the primary host is the black poplar and the secondary hosts are often artichokes, ball celery, curly lettuce, endives, carrots. In late summer, winged females migrate to poplars to form galls and lay their eggs

A fly feeds on woolly aphids. This is the so-called “chloropid dipteran”, which is the only known predator of this root pest in the INRA library. It is a small fly that lays eggs near infested roots. The 1 to 7 mm long, whitish maggots feed on aphids.

Maintenance of a colony of 7-spotted ladybirds

The 7-spotted ladybird (Coccinella. septempunctata L.) is between 5 and 8 mm long in adulthood and is very common throughout France. It is often reproduced on farms because of its extensive polyphagia and because it reproduces very easily in a closed environment.

This ladybird has a certain preference for the green peach aphid and the black bean aphid, but will feed on other aphids when it cannot find its favourite prey. In nature, this ladybird is mainly found on low and herbaceous plants when these are overrun with aphids. In autumn, the adults gather on high ground to spend the winter. Oviposition occurs 7 to 15 days after mating and the long yellow eggs, 1.3 mm long, are laid near aphid colonies. When food is plentiful, a female lays about 50 eggs per day, giving a total of 500-1000 eggs during her lifetime.

The larval stage lasts about 18 days to give birth to a nymph which will produce an adult 8 days later. The 7-spotted ladybird is known for its migratory behaviour and easily moves to another territory when its prey is scarce. It should therefore be reared in a closed shelter.

The 7-spotted ladybird can be reared in this way :

• In cages of 40 to 50 cm on each side made of wire mesh covered with a veil preventing insects from migrating (muslin, anti-insect net, etc.). The entrance at the top of the cage is not screened and the protective veil is closed with an elastic band. This makes it possible to introduce pots containing sprouted beans or radish seeds into a substrate of moistened wood chips or potting soil. The pots are introduced into the cages when the plants are sufficiently covered with aphids. As soon as the first eggs appear, they are isolated and placed in other boxes so as to obtain about 50 adults per box.
• In the open air, but in shelters made of hoops and covered with anti-insect netting. Beforehand, you should grow plants that easily take the aphid, such as radish or turnip. Aphids that are found in abundance everywhere in nature in early spring (nasturtiums, carnations, hibiscus, chives, bush beans, etc.) are imported into the shelter. Nitrogen application on the plants favours the development of aphids.

The introduction of aphids into the rearing systems is done when the aphid colony is sufficiently abundant. Otherwise, ladybird larvae become cannibalistic and adult production will be derisory.

Natural treatment methods of low concern for aphids

In organic methods, fern purin, tansy decoction and nettle purin are among the recipes belonging to the “low concern preparations” that are supposed to replace synthetic pesticides. They are especially of little concern for root aphids because of their ineffectiveness. As long as there is no effective biocontrol method for outdoor infections, deltramethrin sprays at the base of the roots will remain the most appropriate treatment method to get rid of this pest when it becomes aggressive.

However, it is especially at the time of the first flight of adult insects from poplars that treatment is needed. Plant decoctions and purines are of no use when the plants are grown under an insect shelter. Simply introducing ladybirds as soon as possible will get rid of this problem. Because ladybirds under cover always end up devouring everything they find when their favourite prey disappears.

It should be noted that preparations based on rapeseed oil (registered in the list of biocontrol plant protection products) seem to be effective against the woolly adelgid, provided that the necks of the plants, which are often covered with soil brought in by ants, are cleared and that several sprays are carried out at intervals of one or two weeks

Ladybird shelters

To preserve the local establishment of regional or imported ladybirds, insect shelters to protect them in winter are sold in garden shops. Websites and books explain how to build these protective shelters. However, in the Mediterranean region, you do not have to build these shelters if your garden is located near a modern building.

In the autumn, ladybirds do indeed look for natural shelters to overwinter in. They can easily find shelter in insulating materials in attics and roofs through the tiny holes between the tiles. When tiles are laid on top of the sloping roof, there are often large openings that give direct access to the insulation materials in the attic where ladybirds will take refuge to spend the winter comfortably.

The presence of ladybirds the following spring is guaranteed if you have kept a ladybird farm all summer and above all, if you have not used any pesticides against aphids, including a solution based on Marseille soap which also kills ladybird larvae.