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What do you need to know about parasitic wasps in agriculture

Parasitic wasps in agriculture can play important role in keeping plants protected. In recent years the practical knowledge on leveraging their benefits has been growing rapidly. In this post, we will present information that will help you utilize this practice to improve plant protection in your fields and gardens.

"I cannot persuade myself that a beneficent and omnipotent God would have designedly created parasitic wasps with the express intention of feeding within the living bodies of Caterpillars." Charles Darwin
«I cannot persuade myself that a beneficent and omnipotent God would have designedly created parasitic wasps with the express intention of feeding within the living bodies of Caterpillars.» Charles Darwin

The importance of biocontrols in plant protection programs

The use of biocontrol has been around since the 19th century. Biocontrols have been used to control pests in agriculture and forestry. Biocontrols are a form of pest control that uses natural enemies to combat pests and they are a great way to reduce the amount of pesticides used.

The biggest problem with chemicals is that when you spray, the chemical doesn’t work anymore after a while because the insects build up resistance. Another downside is the toxicity of pesticides to beneficial insects. For example, most chemical sprays that kill whiteflies also kill bumblebees. Biocontrols can be used as an alternative that is friendly to pollinators and other beneficial insects.

Three types of biocontrols: predators, parasitoids, and pathogens

Biocontrols can be divided into three groups: predators, parasitoids, and pathogens. Predators eat the prey, while parasitoids lay their eggs on or in the body of their host. Parasitoids are more specific than predators and have a higher success rate in controlling populations of insects. Pathogens are microbial antagonists used to suppress diseases and eliminate insect pests. Bacillus thuringiensis for example, is a bacteria used as an insecticide that acts against moth caterpillars.

Most parasitic wasps are beneficial insects

After parasitic wasps lay their eggs, the eggs hatch, and the wasps larvae feed on the host’s body fluids. Parasitic wasps are not protected from parasitoidism; some wasps parasitize the parasites and therefore are not considered beneficial.

The number of pesticides available to farmers is running out

Pesticide resistance is a major problem for farmers around the world. Farmers need to find new ways of protecting their crops from pests or suffer the consequences. Due to the developed resistance, farmers must resort to more expensive and toxic pest control methods. The lack of new pesticides on the market is becoming more severe due to regulatory requirements. Even after such pesticides have been invented, it takes years before they can be used in farms because of the length of the approval processes. 

Biology of parasitoid wasps

Parasitoid wasps are fascinating creatures that have been around for millions of years and have evolved to be very successful in their niche. It is estimated that these organisms parasitize over 20% of all insect species. Therefore, understanding their way of action can be very helpful in pest control.

Parasitic wasps can be found worldwide, but they are most prevalent in temperate regions. The insects utilize the parasitoidism mechanism to survive and reproduce. In practice, it is an insect-host relation in which the larvae are dependent on the host while they feed on their tissues. As a result, the host will not be killed until there is a completion of larval development.

How to tell a parasitoid wasp from other insects

Wasps are shaped like a triangle, and their size is partly related to that of the host. They have long, thin waists and large wings. Some wasps are dark brown or black with yellow stripes on their bodies. Some wasps can be orange, red, or yellow with black stripes.

Wasps are distinguished from bees by their lack of pollen-carrying structures on their legs and the fact that they do not have the same kind of hairy body as bees.

How to build a parasitoid wasps population in fields and gardens

Support flowering plants in and around the growing area

Parasitic wasps can feed on nectar from flowers, so it’s not unusual for them to be attracted to the scent of flowers. The wasps can be found on most flowering plants, but they prefer some over others. The female parasitic wasp lays eggs on the host plant and then flies off to find more prey.

Allow at least a low level of pest presence on plants 

Some products allow the introduction of the wasps with the target insect to the growing area to support them even when the plants are not infested. But more generally, the wasps should be released when the host is present, and the infestation is mild.

Use insecticides that are selective and don’t harm parasitic wasps

Organophosphates such as malathion, disulfoton, and acephate are highly toxic to natural enemies. Carbaryl harms bees, natural enemies, and earthworms. Systemic neonicotinoids, such as imidacloprid, can be very toxic to bees and parasitic wasps, especially when applied to flowering plants. Pyrethroids, Afidopyropen, chlorantraniliprole, chlorpyrifos, dimethoate, fenpropathrin, methomyl, and thiamethoxam are toxic to parasitic wasps.

Less toxic insecticides are Potassium salts of fatty acids, Insecticidal oils, Microbial insecticides such as Bacillus thuringiensis and spinosad, and Botanical insecticides such as Pyrethrins and Azadirachtin.

Insect growth regulators such as pyriproxyfen and buprofezin are safe for parasitic wasps (but very toxic to beetles such as the vedalia beetle that can help in soft-scale insect control).

The persistence of the insecticides should be considered as well. While there are some whose effects will disappear after a few days, others will persist for months. For example, Carbaryl will have a bad effect on parasitic wasps for up to five months after the pesticide application.

Moth caterpillar killed by a braconid wasp parasitoid
Moth caterpillar killed by a wasp parasitoid

What are the disadvantages of the parasitoid wasps approach?

The negatives to parasitoids are the cost of purchasing them and the maintenance costs associated with keeping them alive in your garden or farm. Parasitoids do not harm your plants or crops, so you can use them without worrying about causing damage to the plants.

How to time the release of wasps?

The average lifespan for adults is around 1-2 weeks. It might be necessary to release more than once, depending on the size of the infestation.

Groups of parasitoid wasps

Ichneumonid wasps

This group of parasitoid wasps preys mainly on caterpillars of butterflies and moths

Ichneumonid wasps
Ichneumonid wasp

Braconid wasps

This group of parasitoid wasps attack caterpillars and a wide range of other insects, including greenflies and aphids. 

Braconid wasps
Braconid wasps

Chalcidoid wasps

This group of parasitoid wasps parasitizes eggs and larvae of greenfly, whitefly, cabbage caterpillars, and scale insects.

Examples of parasitoid wasps that are important to agriculture

Aphid wasp parasitoid

Aphid wasp parasitoid is an insect that feeds on aphids. It has a long, thin body with a black and yellow pattern. The female aphid wasp parasitoid lays her eggs inside the aphids. The eggs hatch inside the aphids, and the larvae feed on them until they are fully grown before emerging from their hosts as adult wasps.

The life cycle of an aphid wasp parasitoid starts with the egg stage, where females lay their eggs inside the bodies of aphids. This can happen in two ways: either through oviposition or ovipositor. Oviposition is when females lay eggs directly into the body of an aphid, whereas ovipositor is when they use a long tube-like organ to insert the eggs.

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Encarsia formosa

Encarsia formosa is a species of chalcidoid wasp. It is a commercially available parasitoid of greenhouse whitefly,

Scoliid wasp

Scoliid wasps are a family that feeds on beetles. The female finds the beetle grubs in the soil, and lays an egg on it. The young wasps feed on the beetle grub.

Anagyrus lopezi

Acts against cassava mealybugs. These insects can reduce crop yields by 60-80%. A successful preventative program was conducted in Thailand in which parasitic wasps were being reared by the millions and mass-released. A similar experiment was conducted in West Africa in the early 1980s. The result was a suppression of the pest population levels from more than 100 individuals on each cassava tip to fewer than 10-20.

Hyperparasitism wasps

Hyperparasites usually appear at the end of summer, when conditions are favorable for them and their host. Hyperparasites can disrupt the effectiveness of a biological control program. It is difficult to eliminate them, but you can minimize their population. Make sure to remove the parasitic wasps in late summer to remove the host of the hyperparasites.

Dendrocerus spp. and Alloxysta spp. attack Aphidius spp. You can know they are present by examining the exit hole in the aphid mummy.

Summary

In this article, we summarized the benefits of using parasitic wasps in agriculture. In practice, no biocontrol is perfect. Instead, a balance should be found between conventional insecticide applications and biocontrol. Greenhouses have been the domain of biocontrol for decades, even when chemical pesticide usage was on the rise. Partly thanks to the fact that the value of the crop is higher. In addition, greenhouses have a significant advantage as they function as a closed environment. Beneficial insects are going to stay inside. Recently biocontrols have become more prevalent in sectors such as floriculture, viticulture, and outdoor fruits. Track the number of wasps in the growing area and see what works best for you. When the control of different insects is needed, more wasps should be released, and the costs can be higher than conventional treatment. 

We are still in the early days, and robust protocols do not always exist. There is a need to start practicing it to gain knowledge. Join our community where we discuss such practices in a case-by-case manner, we are happy to support your experimentations and help you learn and improve.