Parasitoids are essential for the biological control of aphids in greenhouse crops. However, their efficacy can be strongly reduced by secondary parasitoids, so-called hyperparasitoids, which parasitize the primary parasitoids. An inventory by Wageningen UR Greenhouse Horticulture among 10 organic pepper growers has made clear that hyperparasitoids are a common phenomenon and can occur as early as the start of a new cropping cycle in January. A better understanding of hyperparasitoids is necessary to develop measures that minimize the risks of hyperparasitoids.
Sweet pepper is very susceptible to different aphid species, such as the peach aphid, foxglove aphid, potato aphid and, increasingly, cotton aphid. Parasitoids are very important in biological programmes for aphid control. They can respond quickly to outbreaks of aphids and do an excellent job in detecting small aphid colonies. The rapidly developing Aphidius species are the most popular: Aphidius ervi against the larger foxglove and potato aphids, Aphidius matricariae against peach aphids and Aphidius colemani against peach aphid and cotton aphid. Additionally, some slower developing Aphelinus species are used and the wasp Praon volucre often occurs spontaneously.
One dominant species
In January this year, Wageningen UR Greenhouse Horticulture has started to draw up an inventory among 10 organic pepper growers which parasitoids and hyperparasitoids occur. The first half of the year there was clearly one dominant species; the generalist Dendrocerus aphidum. Laboratory tests showed that all of the above mentioned parasitoid species were susceptible to this hyperparasitoid. This is in contrast to what was believed that Aphelinus and Praon species would not be susceptible. Even more striking was the fact that this hyperparasitoid occurred very early in the growing season at the start of a new cropping cycle. In early February, the species already infected 5 of the 10 companies. In April, all 10 companies were infected with this hyperparasitoid. Organic growers often use banker plants of winter wheat with the grain aphid Sitobion avenae as an alternative host for parasitoids in order to support early establishment of released parasitoids. However, our survey showed that hyperparasitoids were often present on these banker plants even before crops were infested with aphids. Hence, banker plants can facilitate early hyperparasitism, which is disruptive for biological control of aphids with parasitoids.
From May another hyperparasitoid occurred on half of all companies, namely Asaphes suspensus. A better understanding of the biology and ecology of these species would help to develop programmes for controlling multiple species of aphids with parasitoids while minimizing risks of hyperparasitism.