Imports of beneficials against aphids.
⇒ Against 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.
Yellow aphids on young oleander stems ♦
It is not always wise to wait for native ladybirds to intervene in a garden to control aphids. The stings of aphids can have catastrophic consequences on some plants. The wounds caused by their bites are open doors for all kinds of pathogens (fungi, bacteria and viruses). By taking part of the sap, aphids weaken the plant's natural defences, which are already not very strong at the end of the winter. Infections that originate in aphid wounds continue to develop after the aphids have disappeared.
In arboriculture, aphids, bugs and leafhoppers are known to spread fire blight. There is no curative treatment for this dreaded disease. Once the damage has been done, it is necessary to use the most effective means to prevent the infection from spreading: pruning the diseased organs and curative treatments with pesticides when pruning is insufficient.
Before reaching such extremes, it is easy to understand that it is preferable to intervene quickly against these pests as soon as the first signs of colonisation appear by importing their predators, which can be ordered in certain garden shops or on Internet sites.
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.
Black aphids on green beans ♦
Aphids transmit viruses to parasitized plants in a persistent or non-persistent mode. Viruses are incurable. The green pea aphid transmits virus Y to potato or cucumber in a non-persistent mode, i.e. the aphid transmits the infection by biting an already infected host and then transmitting it to another plant. Eventually, the aphid loses the virus. In the persistent mode, the virus cycles through the aphid's body before being transmitted when it is found in the salivary glands. The aphid may then transmit the virus to many plants. Some aphids retain viruses in their bodies throughout their lives, such as the strawberry SCV.
Aphids allow some pathogens to migrate from one vegetable variety to another. For example, wild strawberry is the host plant par excellence of the viruses SMoV, SMYEV, SVBV, SCV transmitted by the aphid Chaetosiphon fragariafolii to domestic strawberry plants.
Aphids transmit a particularly devastating virus disease in cucurbits. To date, there are no curative control methods for this virus infection in the field. A plant infected with the aphid-borne cucurbit yellows virus (CABYV) will remain so for the rest of its life. The first step is to weed the area around the crop to eliminate the sources of the virus that thrive on weeds. 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.
Nice courgette affected by potyvirus transmitted by aphids ♦
Several aphid species frequently develop on tomatoes and are responsible for the transmission of several viruses, including the "Tomato chlorosis virus" originating from the United States, ToCV (tomato chlorosis virus), which is present mainly in the south of France.
In case of high temperatures, losses of ladybird larvae during transport can be significant. It is preferable to import adults especially to protect crops under cover. On the other hand, importing ladybird larvae that cannot migrate to other plants is more practical for protecting outdoor crops while the larvae adapt to their new environment. In addition, in the spring, it often takes several days to get ladybirds to breed at the breeder's premises (some producers indicate on their website whether their ladybirds are available and are then delivered immediately).
You can also try to collect ladybirds in the wild knowing that some ladybirds prefer mealybugs, mites, whiteflies and others aphids, or even only one species of aphid. Under cover, it is easier to force some adult ladybirds to switch to another form of aphid.
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?
If you decide to import ladybirds into your garden, you should order them when the aphids are already present on the crop, otherwise the ladybirds will starve. However, you should not wait until a crop has been invaded when it shows signs of aphid-borne disease. Ladybirds do not act as quickly as a pesticide and you have to wait for them to reproduce to see a significant reduction in the invasion.
PSbMV virus (still called mosaic) imported by aphids, on green bean leaves ♦
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.
Woolly aphids removed from a carrot root
Woolly aphids can cause significant damage to crops. In 2002-2003, a species of woolly aphid was the main sanitary problem encountered in northern France on chicory crops. Most ladybirds do not attack them unless they are kept in a closed environment and there are no other aphids. They will then attack the aphids that emerge from the roots to infect other plants. The comma beetle (Exochomus quadripustulatus) is known to attack many woolly-bodied prey such as the woolly apple aphid (for sale by clicking here). It also feeds on other aphids and is sometimes found in gardens. I have noticed that a root aphid prone vegetable plant such as celery grown under an insect veil can be protected by the import of a few ladybirds. Once the root aphids have disappeared, a potted plant covered with aphids should be brought in from time to time to maintain the ladybird population.
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.
keeping ladybirds in winter.
To help the ladybirds get through the winter, it is possible to put them in aerated jars in an unheated garage if the temperature does not fall below -5°. This allows you to have a stock of ladybirds immediately available when aphids appear in early spring and which swarm on certain crops (such as fruit trees when the first leaves appear).
The jars should contain moistened cotton wool. The ladybirds need a little moisture to survive by depositing a few drops of non-chlorinated water on the cotton wool, which must always remain moist. The opening of each jar is protected by an anti-insect veil fixed by an elastic band to let the air through. Like humans, ladybirds breathe and produce carbon dioxide, a heavy gas that tends to concentrate at the bottom of the jar. Each jar should therefore be positioned on its side to allow the carbon dioxide to escape more easily.
As soon as the first cold weather appears in autumn, the ladybirds go into diapause. They appear to be dead, but start to move in early spring when the temperature inside the garage rises. During the hibernation period, it is important to check the moisture content of the container from time to time. Beware; the lack of humidity during the diapause period causes many losses. When the temperature drops, ladybirds tend to gather in piles, which can be encouraged by introducing a few crumpled sheets of paper into the container to form shelters.
A farm can suffer severe and sudden losses probably caused by microbial diseases. It is preferable to use several jars with about 50 ladybirds in them, far apart from each other, in order to limit the losses caused by infections.
In order to survive the winter, ladybirds build up reserves of lipids and carbohydrates in their tissues which will be consumed quickly if the temperature of a shelter is too high. In order to strengthen their energy reserves, ladybirds are fed with honey placed in a very thin layer on a sheet of paper as shown in the picture on the left. This feeding should be repeated in the spring about every 3 days as long as the ladybirds are not placed on aphid infested plants. In this way, ladybirds can be kept until the end of May.
A stock of ladybirds can be kept in the same way in midsummer to cope with a possible sudden attack of aphids on open-air crops.
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1)http://fr.jardins-animes.com/larves-coccinelle-europeenne-anti-pucerons-p-347.html#detail_produit
2) INRA -Université de Rennes 1 – Les pucerons, indicateurs de changement climatique ?