As every grower knows, thrips are a huge problem in ornamentals grown under glass. The usual suspect is the Californian thrips, a species with a strong preference for flowers. But in recent years another polyphagous thrips has been increasingly raising its head: Echinothrips americanus. Without timely intervention, this typical leaf-dwelling thrips can cause considerable damage to ornamentals such as gerbera and rose. Scientists have been taking another close look at how to control this thrips with a range of species of predatory mites and bugs.
In this study, striking differences were found between the four species of predatory mite used and the controlling effect was boosted in some cases by providing pollen. Predatory bugs in the Miridae family have been found to be very effective predators of Echinothrips but the options for using them very much depend on the type of crop.
Unlike the Californian thrips, which is a typical flower thrips, Echinothrips americanus prefers leaves. It is easily identified by its black body with two distinct white spots on the wings. But because it often hides away low down in the crop, its presence in the greenhouse can be something of a surprise. Although the species has been present in the Dutch greenhouse horticulture sector for about 20 years (it was imported from North America, as its name suggests), we still know very little about this creature.
In recent years there has been a clear increase in this species at nurseries switching to integrated pest control. The reduction in the use of broad-spectrum insecticides has given this thrips more chance to establish itself in crops. To avoid disturbing biological control of other pests, it makes sense to also tackle this insect with natural predators. Predatory mites and bugs are good candidates for this.
Control with predatory mites
In the lab, the researchers investigated how susceptible the various stages are to predatory mites and how many individuals of each stage are eaten per day. This was tested with the predatory mites Amblyseius swirskii, Amblydromalus limonicus, Euseius ovalis and Euseius gallicus. The latter two Euseius species establish readily in rose and are therefore interesting candidates for this crop.
Striking differences were found between the species. A. limonicus ate the most Echinothrips larvae, followed by E. ovalis and A. swirskii (see figure). A. limonicus also laid the largest number of eggs out of these mites. Surprisingly, the thrips were more or less left untouched by E. gallicus. The exact reason for this is unclear.
Interestingly, the pupae were also susceptible to the mites. This immobile stage, which is also found on the leaf, cannot defend itself and is therefore suitable prey.
With the exception of E. gallicus, all species of predatory mite ate the pupal stage. However, as the pupae are much larger than the larvae, the number of individuals eaten per day was a lot lower than in the larval stage. None of the predatory mites ate adult Echinothrips. In addition to the number of thrips being eaten by each individual predator, an important indicator in pest control in a crop is how well a predatory mite establishes itself and in what densities: after all, a high density can easily make up for lower predation rates per individual.
Results in the crop
The predatory mites A. swirskii and A. limonicus were compared in a greenhouse gerbera crop, with the results matching those in the lab quite closely. The thrips were controlled better with A. limonicus than with A. swirskii. This difference has also been observed in other studies with roses and sweet peppers.
Curiously, though, Echinothrips often doesn’t disappear completely: it is somehow able to survive the pressure from the predatory mite. So in a subsequent study the scientists looked into whether control could be improved by offering pollen as an alternative food source. Pollen can massively boost the density of predatory mites, which could lead to better pest control. The downside is that the Californian thrips also feeds on pollen, potentially causing scenarios that are detrimental to thrips control.
Effects of pollen
An important question, therefore, is whether this also affects Echinothrips. With bulrush pollen, the answer was a resounding no. Unlike with the Californian thrips, there was no impact on development time, egg laying or population development whatsoever. This is helpful in that it means supplementary food can be provided for predatory mites selectively, as long as there are no Californian thrips present.
The effects of pollen on thrips control were tested on non-flowering pepper plants using the predatory mites A. swirskii, E. gallicus and E. ovalis. Providing pollen increased the densities of all these mites. Control of Echinothrips using this method was significantly better with A. swirskii but despite the higher densities there was no control effect with E. gallicus. E. ovalis responded extremely well to pollen but the control effect was just as good on plants without pollen.
It has been known for a long time that the bug Orius majusculus is an effective predator of Echinothrips on sweet peppers. Orius is used to control thrips in this crop with great success. But the bug doesn’t establish in many ornamental crops. In recent years, scientists have been looking at the effect of omnivorous predatory bugs in the Miridae family on gerbera. In addition to Macrolophus pygmaeus, various Dicyphus species such as D. maroccanus, D. tamaninii and D. errans have also been trialled. All these species controlled thrips extremely well. Plants were made completely clean and remained so.
The bugs seem to be excellent candidates for preventive use against a range of pests and they can maintain themselves well because they eat a variety of prey and plant sap. This can also be a disadvantage, however, because feeding on plants can cause flower damage. Further research will be carried out in the future to see whether and when that happens and whether it can be avoided. The use of these bugs in gerbera could be a breakthrough in biological control of Echinothrips as well as other pests. Future research in other crops, such as roses and pot plants, is planned to ascertain whether the use of predators with banker plants can be supported.
Echinothrips is appearing more and more frequently in ornamentals grown under glass. In trials with a number of species of predatory mite, A. limonicus provided the best control while E. gallicus had little if any effect. Control with A. swirskii was improved by offering pollen. Besides predatory mites, predatory bugs of the Miridae family were found to be excellent predators but further research is needed to prevent potential flower damage and to improve establishment in different crops.
Text and image: Gerben Messelink, Somayyeh Gasemzadeh and Ada Leman (Wageningen University & Research).