When farmers consider cover crops in vineyards, they likely look at plants, such as clover or rye, growing between rows of vines. These, and other common cover crops, are known to provide farmers with many benefits, including reducing soil erosion, nutrient cycling and weed inhibition. It is, however, possible to grow cover crops that are attractive to beneficial insects right in the rows of vines. In doing so, farmers may encourage beneficial organisms, such as parasitoids (i.e., parasitic wasps) to target the pests feeding on the grapevines.
Here at Brock University in Niagara, Ontario, we are currently testing cover crops that could be planted in vine rows. One of these is sweet alyssum (Lobularia maritima). This plant may attract more parasitoids to the vine rows than common cover crops. Studies in the United States and New Zealand suggest that alyssum may increase parasitism of pest insects when planted under the vines and, thereby, reduce the number of pests. Understanding how individual species of cover crops interact with the surrounding agroecosystem is a small, yet integral, part of our project: “Unique Cover Crops, Rootstocks, and Irrigation Techniques for Canadian Vineyards.” Parasitoids are insects that lay eggs into the bodies of other insects, such as leafhoppers.
Parasitoids mainly belong to two insect orders: Hymenoptera (including wasps and bees) and Diptera (true flies). Hymenoptera boasts the largest assemblage of parasitoids with thousands of species in more than 40 families. Most parasitic wasps are smaller than a centimetre and some as tiny as a fraction of a millimetre long. Mymaridae, the family of parasitic wasps also known as fairyflies, includes the smallest insects known to science. They are important to agriculture because members of this family parasitize pests, particularly leafhoppers. Trichogrammatidae is a family of particular importance to viticulture, as some of these wasps parasitize the grape berry moth (Paralobesia viteana), a common pest of grape vines. Parasitic wasps can significantly affect pest insect populations and this is why they need to be studied. Note that even though parasitic wasps can ravage insect pest populations, they don’t bite or sting humans.
When we think of a parasite, we may think of something that harms but does not kill the host. This is not the case with parasitic wasps: they are parasitoids rather than true parasites. They nearly always end up killing the host during their lifecycle. The females will lay their eggs inside a host, where the larvae will feed before it pupates and becomes an adult. Different wasps target different life stages of the pest insect, and can lay their eggs into an egg, larva or adult. The wasp larvae usually start by eating the nonessential tissue and slowly move on to consume vital organs of the pest. This way, the wasp larvae are sustained throughout their growing period. Then they pupate either inside the host or in a cocoon attached on or near the host, before emerging as adults. It is easy to take these wasps for granted. They occur naturally throughout the landscape but are so small that we are more likely to see the parasitized pests than the wasps themselves. Some farmers choose to buy adult wasps and spread them in their fields. The introduction of beneficial insects is referred to as classical biological control. This can be an important part of an effective and sustainable integrated pest management program. Conservation biological control, on the other hand, involves creating the right conditions in the landscape to attract naturally occurring beneficial insects. Whether the population is natural or released, farmers can support and encourage the beneficials by providing them with nectar and pollen, which are food sources for the adults.
This is where sweet alyssum comes to play. This popular horticultural plant provides a floral display, as well as sustenance for beneficial insects. The flowers on alyssum are wide and shallow enough that parasitic wasps, which often have small mouthparts, can feed on them effectively. Many flowering plants may be good food sources for larger beneficial insects, such as pollinators, but might not be a suitable choice for parasitic wasps if the pollen and nectar are out of reach, for example, in a deep flower.
Another common plant used to attract beneficial insects in agriculture is buckwheat. A study done in China suggests that while both alyssum and buckwheat may attract beneficial insects, buckwheat, unfortunately, is likely to attract the pest insects as well. Depending on the pest species you want to target, it is important to consider how selectively the cover crop attracts insects (i.e., does it attract just beneficials or beneficials and pests). Other studies have shown that alyssum may be as effective as certain native plants in attracting beneficial insects selectively. It is important for us to understand what types of insects alyssum attracts and in what numbers before we can truly understand if it is a useful plant for vineyard managers.
The flowers of alyssum attract parasitoids and therefore the plant mainly serves its purpose while in bloom. Since it grows as a hardy annual in Ontario, it usually needs to be seeded every year and does not necessarily provide the kind of winter habitat that is important for many beneficial insects. We continue to work with our vineyard growers to better understand their seasonal needs and how alyssum may fit into the broader management scheme. Our next steps will be to identify and quantify the numbers of parasitoids attracted to the vineyard alyssum.
Alternatives to pesticides based on conservation biological control are gaining momentum and expanding, and are especially important for organic vineyards like ours. Research is needed to learn which plants can better attract beneficial insects, such as parasitic wasps, and to help farmers make more informed decisions about their approach to pest control. Farm management and climate change, especially extreme weather events such as flooding and droughts, may affect the performance of alyssum and other cover crops. This may indirectly affect populations of parasitic wasps, which are sensitive little creatures. It is important to understand how all these various factors may affect performance of plants and parasitoids. This is all part of our research work here at Brock University.
(OSC3) Originally published in Organic Science Canada magazine, Issue #3, Spring 2021
