Controlling thrips in cut flowers biologically may be difficult but it’s not impossible. Supplementary feeding with pollen can help to build up and maintain a vigorous predatory mite population. But this strategy isn’t without risk, since pollen is also a food source for thrips. This was the starting point for a research project on roses which also looked at how other predatory mites react to supplementary feeding.
Supplementary feeding of biological predators with pollen preparations is a strategy that is becoming increasingly popular. In vegetable production, particularly in cucumbers and sweet peppers, a lot of growers are using this method to build up a healthy population of predatory mites. But is it also suitable for ornamentals, an area in which pest tolerance is low?
Entomologists Gerben Messelink and Ada Leman of Wageningen University & Research in the Netherlands are studying the development of Californian thrips in roses and their control with a range of predatory mites. Part of this research involves supplementary feeding with pollen, a method that has raised questions in the past.
Taking no chances
“Biological control in ornamental production is no easy option,” Messelink says. “Pest tolerance is so low that it is difficult to build up a good biological balance. A few thrips on a glue trap are enough to trigger panic.” In other words, growers don’t want to take any chances if there is a risk of infestation, especially with Californian thrips. But there are not enough chemicals available to control or correct an infestation.
The entomologists’ research is focused primarily on getting biological agents, in this case predatory mites, better established in the greenhouse and the crop. They call this the “standing army principle” – an army that is on standby ready for deployment.
If biological agents are difficult to establish in a crop, banker plants can sometimes help. These are plants on which useful predators establish readily and then spread throughout the crop. But this method doesn’t work for predatory mites because they don’t fly and only move around slowly.
Pollen a firm favourite
Amblyseius swirskii has so far proved one of the most successful predatory mites for controlling thrips in ornamentals. But to do its job, this creature must be in tip-top condition. As thrips pressure is kept as low as possible, there isn’t very much food around. Fortunately these mites are omnivores and are happy to tuck into a good meal of pollen. So providing pollen to help build up the predatory mite population would seem an obvious choice.
And yet there’s a danger lurking in this method. Thrips also like to feed on the same pollen. So the question is whether providing pollen actually does more harm than good. There is also the risk that the predatory mites could become lazy and leave the thrips in peace.
So the researchers first wanted to get a clear idea of how both thrips and predatory mites would react to supplementary feeding. They studied a thrips population fed with cattail pollen, maize pollen, Ephestia (Mediterranean flour moth) eggs and cysts of Artemia (brine shrimp). The thrips population remained lowest in the control treatment without supplementary feeding and increased most in the treatments with pollen and Ephestia. “We noticed that the number of eggs laid by the thrips tripled on average when pollen was given,” says Messelink. This confirms that thrips love pollen and thrive on it.
The predatory mite A. swirskii was studied separately and was given the same food sources. This study also showed that pollen and Ephestia had a considerable impact on egg laying.
Supplementary feeding needed
With these results in mind, the researchers devised a strategy for the rose, a crop in which a predatory mite population can be difficult to maintain. Using pollen, they quickly built up a vigorous population, and stopped supplementary feeding when the thrips started flying in. But that didn’t work: the number of predatory mites dropped to such an extent that the thrips gained the upper hand.
Messelink: “We have changed our strategy now. Supplementary feeding is still needed once the thrips have arrived. A vigorous predatory mite population can easily tackle a nascent thrips infestation despite the presence of pollen.”
The question then arises as to how much pollen is needed to maintain the population. It’s difficult to say, and it also depends on the conditions on site. The equivalent of 5 kg of pollen per hectare per week was administered in the study, but in practice it has been found that good results can also be achieved with smaller quantities.
Research in 2015 with a combination of A. swirskii supplemented with the predatory mite Macrocheles robustulus also delivered quite promising results. A. swirskii attacks the adult thrips and M. robustulus goes for the pupae that fall on the ground. The two predatory mites make a good team.
Other good natural predators
In many tests, Amblyseius swirskii has proven to be one of the better predatory mites for controlling thrips, but this has always been tested without the use of pollen. It may well be that other species are even more effective when given a supplementary food source. So in greenhouse trials with roses, A. swirskii was compared with Iphiseius degenerans, Euseius gallicus and Euseius ovalis. They were all given pollen.
The test confirmed that both I. degenerans and E. ovalis responded very well to supplementary pollen feeding – even better than A. swirskii, in fact, proving that these predatory mites are an interesting option for use in roses. Many predatory mites don’t do well in a short period of heat with low humidity, but I. degenerans appears to be reasonably resistant to that, making it a useful addition to the overall package.
If Echinothrips also occurs alongside Californian thrips, E. ovalis in combination with pollen is a good species to use. In short, various species of predatory mite are suitable for controlling thrips in roses in conjunction with pollen.
Messelink predicts that good thrips control will consist of a total approach involving a range of biological agents at different times. Close monitoring and providing supplementary pollen feeding at the right moment are also part of this mix.
In their follow-up research, the entomologists are not only looking at control with predators but also at the “push-pull” method, which is based around repelling and attracting pests. For example, the crop can be made unattractive to insects with certain smells, or the pests can be lured away with plants or smells that are more attractive to them. Increasing plant resistance also plays a role in this strategy. Research is currently being carried out in wind tunnels, in particular using smells that should scare off thrips.
Controlling thrips in rose cultivation biologically is difficult: due to the low pest tolerance, the predatory mite population has trouble establishing properly and staying vigorous. Supplementary feeding with pollen goes a long way towards solving this problem. This strategy also puts other predatory mites back in the picture. A total approach with a range of biological agents is likely to be the most effective one.
Text and image: Pieternel van Velden and Wageningen University & Research.