mites
Prey diets greatly improve biological control by predatory mites
Predatory mites are the most important natural enemies used for biological pest control in greenhouse horticulture.
Many of these predatory mites are mass-reared on prey mites that feed on fungi growing on bran. In collaboration with companies in the Netherlands and Spain, we investigated whether predatory mites could also be reared directly with artificial diets based on insect proteins. High-quality proteins from sources such as black soldier flies and mealworms were found to be suitable food sources.
However, formulating these semi-liquid diets wasn’t easy as the predatory mites were unable to consume diets offered in microcapsules. The next step, therefore, was to focus on diets for prey mites. Several species of prey mites were reared on diets with different nutritional values. When administered on plants, the prey mites reared on high-protein diets produced predatory mite populations five times the size of those feeding on bran-reared prey.
These nutritious prey mites can therefore enhance pest control by quickly boosting populations of predatory mites.
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‘Before the problem starts, build up a vigorous predator population’
Supplementary feeding of predatory mites with a pollen preparation has outgrown the trial phase. A large number of growers is successfully using this method. While pest numbers are low, predators that also eat pollen have the chance to build up a vigorous population. Dutch growers Wesley Klauwi from sweet pepper nursery Zuidgeest Growers and cucumber grower Bart de Groot explain how they do it.
“Snack peppers are different from standard pepper varieties, and you have to learn to respond to that.” Wesley Klauwi is responsible for crop protection at Zuidgeest Growers, a nursery famous for its Vitapep orange snack pepper. “Caterpillars and aphids are the biggest problem at this nursery. We have the other pests well under control, even though we have had a lot of pressure from thrips here over the last couple of years”, he explains.
The sweet pepper nursery gained experience this year with supplementary feeding of predatory mites with Nutrimite, a preparation based on cattail pollen. It is highly nutritious for predatory mites and unattractive to pests. What’s more, this pollen doesn’t produce allergic reactions.
Spreading pollen
In the first week of January, Amblyseius cucumeris was used to control thrips on the four hectare site in Maasdijk (south-west Netherlands). A bag was hung on every twelfth plant, and Amblyseius degenerans was added two weeks later. The rows in the greenhouse are 50 metres long. Klauwi opened the tubes halfway down the rows. He then provided three to four batches of supplementary pollen at 14-day intervals.
He disperses the product with a small leaf blower, first around the places where he has distributed the A. degenerans and then around the whole greenhouse. “We want to see plenty of predatory mites round about March, and we have achieved that at all our sites,” he says.
Good results
Supplementary feeding of natural predators is becoming more and more popular. Marcel Verbeek of Biobest says that about half of his customers now feed pollen. The major advantage of this method is that it allows natural predators to develop well at a time when there are not yet enough pests to sustain them. Predatory mites in a sweet pepper crop can often survive simply on the pollen from the flowers, but a cucumber crop produces no pollen at all. This makes it much harder to build up a good population of natural predators.
Several predatory mites eat the pollen preparation, namely A. degenerans, A. swirskii and Eurseius gallicus. A. cucumeris also responds to the preparation but less vigorously than swirskii and degenerans. “We are seeing good results. Proponents are finding that it makes the predatory mites more active and more vital. Others believe that the mites can get lazy or that they would prefer to eat pollen than catch pests.” Verbeek doesn’t agree.
Sometimes it is hard to make a sound choice. The advice is to disperse a total of 0.5 kg per ha each time. Supplementary feeding is not cheap but it delivers good returns because of the increased numbers of predatory mites in the crop.
Resilient population
“We are certainly seeing plenty of Amblyseius degenerans running around on the flowers,” says Klauwi. “Thrips are under control and this predatory mite also tackles spider mite. We hope that Orius will help with pest control in around week 25.” They prefer the combination of A. cucumeris, A. degenerans and supplementary feeding with pollen. “As far as we’re concerned, supplementary feeding is the future.”
Verbeek: “Our advice is to build a resilient population of natural predators before problems start arising in the crop. Sometimes this even starts with the breeder. Some growers are already releasing predatory mites at the propagation stage as well as providing supplementary feeding with the preparation.”
Gradual start keeps down costs
“Supplementary feeding doesn’t come cheap,” says Bart de Groot, Aad and Ruud Zwinkels’ partner from Kwintsheul in the Westland area. They have been using the pollen preparation at the 28,000 m2 cucumber nursery for a couple of years now.
The cucumber growers plant three times a year. To begin with they release A. swirskii at a rate of one bag per four plants to control thrips and whitefly. When the first spider mite appears, they add Phytoseiulus. Although the preparation is very effective for the predatory mite population, they decided last year to take a more gradual approach to keep costs down. In April 2015 the pressure from whitefly increased and there were not enough swirskii present to tackle the problem, so they decided to start supplementary feeding again.
This year, the course of events is the same as in 2014. De Groot is using 0.25 kg pollen per hectare per week on this fast growing crop. “But we still notice that this is saving us a round of predatory mites,” he explains. “Incidentally, we don’t have a lot of trouble from thrips. It’s mostly whitefly that we need to keep under control.”
Fan is faster
The leaf blower they used for dispersing the preparation in the first two years has now been replaced by a fan on the pipe rail trolley as dispersing the pollen with the leaf blower took too long. The grower can now get the job done in 45 minutes. He does it early in the morning before the vents are opened. He positions the trolley so that the fan is about half a metre above the wire. Then he rides down eight rows in the 300 metre-long greenhouse. The following week he changes the pattern and takes the eight rows in between.
The advantage of this method of dispersal is that the grower gets a good picture of the condition of his crop in a short space of time. “It means I can see straight away whether the plants are growing well and if there are patches of whitefly, spider mite or other pests remaining.”
Combining bio and pollen
This year, biological control is once again going according to plan. When the third crop starts, De Groot will be using a high dose of standard bags of swirskii, one for every two plants. He wants to keep this population vital until the end of the crop with supplementary feeding.
“We are noticing that supplementary feeding with the pollen preparation is adding a new dimension to our biological control. We’ve seen for ourselves how predatory mites can build up a good population when they get a varied diet, in other words a combination of pests and pollen. With more and more chemical pesticides being banned, I see this as a good strategy for the future,” Verbeek adds.
Summary
The number of sweet pepper and cucumber growers feeding predatory mites pollen preparations is on the rise. The varied diet of pests and pollen creates a good, resilient population of natural predators. Although this strategy means the money has to be spent before the benefits are felt, this trend looks set to continue.
Text and images: Pieternel van Velden.
Controlling Echinothrips americanus with predatory mites and bugs
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.
Switching methods
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.
Predatory bugs
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.
Summary
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).