It has completely adapted itself to lettuce, hides deep inside the head and multiplies rapidly. We’re talking about Nasonovia ribisnigri, the currant-lettuce aphid feared by all lettuce growers. However, it ignores Rijk Zwaan’s new lettuce varieties with Nr:0,1 resistance. Anneke Kroes, a phytopathologist at Rijk Zwaan, explains how her research contributed to achieving this all-important breakthrough for growers.

“I view the currant-lettuce aphid as an interesting challenge, not only because it’s such a troublesome pest in lettuce crops, but also because it’s important to Rijk Zwaan,” says Anneke. “It was one of the first resistances our company introduced, back in 1996. Unfortunately the resistance was overcome in 2007 because the aphid adapted. That’s why I was keen to play a role in finding a new resistance.” 

Devoted to the aphid 

Anneke, who joined Rijk Zwaan three years ago, has been able to pursue that goal in the company’s phytopathology department, where around a hundred colleagues are focused on pests and diseases. For the past three years, Anneke has enjoyed devoting every workday to the currant-lettuce aphid, which is mainly found in temperate climates worldwide. Her aim? To develop tests as the basis for research into Nasonovia ribisnigri resistance. 

Lettuce specialists 

In order to develop a testing method, she first had to study the currant-lettuce aphid closely. Today, she is an expert on it. “The aphids are usually green, but they can also have a yellow or reddish-brown colour, with characteristic black markings on their legs. They feed on phloem – the living tissue in plants that transports things like sugars and proteins – using an efficient mouthpart called a stylet,” she says. 

Another distinguishing feature of these aphids is that they can only survive on lettuce or endive. “They are real specialists. They have completely adapted themselves to lettuce, making it easy for them to evolve,” explains Anneke. 

Blackcurrants 

However, the aphid actually originated in a different crop: blackcurrants. “That’s how it got the name ribisnigri. The aphids survive the winter as eggs on blackcurrant bushes, where they hatch into nymphs in the spring. After maturing into adults, the aphids then migrate to lettuce crops,” continues the phytopathologist. 

Giving birth to live young 

Notably, in lettuce, the adults not only reproduce asexually to lay eggs, but also give birth to live young. “That contributes to their success. After all, eggs are vulnerable. It’s much more beneficial to give birth to live offspring that can feed immediately. Moreover, young aphids can produce their own offspring after just one week, so they multiply very rapidly.” 

Besides that, adult aphids can develop wings and fly to another plant if their existing plant becomes overcrowded. That’s how the aphids spread through a lettuce field so quickly. 

Difficult to detect and tackle 

Besides rapidly multiplying and spreading, the aphid has found even more ways to ensure its survival. For example, it hides and forms colonies deep inside the lettuce head. This is a clever strategy, according to Anneke: “It not only makes it hard for growers to detect the aphids, but also makes it difficult to tackle them with pesticides or biologicals. And lettuces infested with aphids are unsaleable.” 

Wild lettuce plants 

All of this underlines why a resistance to this extremely troublesome pest was so essential for lettuce growers. Anneke provides a brief summary of the years-long process that led to Rijk Zwaan’s team of specialists achieving that: “We started by setting up large-scale testing based on ‘accessions’, in which our team introduced the aphid onto wild lettuce. Needless to say, plants that occur in the wild suffer from pests too, so there was a chance that they had developed the resistance we were interested in – which would give us a new source,” states the phytopathologist. 

Eureka moment 

So how does it feel when you find a new source? “When you see one plant coping very well among the infested plants and you realise you’re onto something, that’s a Eureka moment for the whole team,” replies Anneke. 

Once an accession had been found that was strong against the aphids, the team performed countless crossings with today’s commercial lettuce varieties to ensure that the resistance could be passed on to the offspring.  

Markers 

Other Rijk Zwaan colleagues assisted in this process by studying the resistance at molecular level. With the knowledge they gained, they developed a genetic marker as a ‘label’ enabling the breeders to specifically monitor the favourable trait in crossings. This accelerated the selection of good offspring with the favourable trait.  

Under the microscope 

Meanwhile, Anneke used her understanding of the aphids to explore precisely how the resistance mechanism worked and why aphids chose to ignore resistant lettuces. “After pricking the plant, an aphid produces saliva while feeding. That saliva contains effector proteins which help suppress the resistance present in the phloem. In the case of a new source, this suppression fails. The effector proteins have no impact on the plant’s resistance, leading to an adverse effect on the aphid’s growth and survival on the lettuce.” 

More insight into the resistance 

Additionally, she investigated whether – and if so, for how long – the aphid could survive on the resistant lettuce plant and whether it produced offspring. Anneke: “Each research activity gave us more insight into the inner workings of the resistance. For example, we discovered that it also has some impact on the potato aphid Macrosiphum euphorbiae. We’re now looking at how effective it is.”  

Very satisfying 

Anneke is extremely pleased that Rijk Zwaan has introduced lettuce varieties with this resistance. “It’s very satisfying. I’m happy and proud – especially because so many people from different departments have worked so hard on this. It’s a fantastic achievement.”  

Even after the market launch, Anneke’s work still revolves around Nasonovia ribisnigri. “We’re continuously looking for new sources of resistance, including with a view to the future. These and other aphids are definitely not rid of me yet. My goal is to help growers by finding resistances against all aphid types,” she says.