Plant bugs like the European tarnished plant bug and the common nettle bug are a serious problem in crops such as aubergine, cucumber and chrysanthemum. Even in small numbers they can do considerable damage: abortion of the flower in aubergines, stem and fruit damage in cucumbers and splits in chrysanthemums. As soon as growers spot bugs or bug damage, they feel they need to intervene fast with products that are harmful to the biological predators they are using for other infestations, marking the beginning of the end of their biological pest control.
Bugs usually enter the greenhouse from outside. They can arrive early in the season but most sightings of bugs, particularly the most harmful species, the European tarnished plant bug, are reported in the summer months. A good method of spotting and monitoring the presence of bugs can help growers decide when to use pest control products. It would be even better if bugs could be effectively eliminated from the plants with traps.
Pheromones and plant aromatics
A trap with a pheromone attractant for the European tarnished plant bug (Lygus rugulipennis) was originally only available for outdoor use, mainly in strawberry crops. Producers of biological pest control Entocare biocontrol C.V. and Wageningen University & Research have been working with a number of growers in the Netherlands to study and optimise the use of the trap and pheromone in the greenhouse. This trap is now available for detecting the presence of the European tarnished plant bug in various crops (aubergine, cucumber).
The traps were tested in a season-long trial and show peaks in the occurrence of bugs (Figure 1). The relationship between the numbers of bugs caught and the damage they cause is currently being investigated more closely. The level of the peaks and the increase or decrease in the numbers captured in weekly counts help the grower decide which crop protection measures to take.
Trapping bugs certainly helps control them but as yet it is unclear what proportion of the bugs already present can be eliminated with traps. The distribution of bugs across the greenhouse is very irregular: catches in traps show no evidence of bug hotspots.
During the trial it was discovered that the luring effect of the pheromone works best in the presence of plant or crop aromatics. A simple pheromone trap catches fewer bugs than a trap with the smell of the crop in the background. But it is mainly the males that are attracted by the smell of the pheromone, so the researchers have set about finding attractive alternatives to lure females as well. Their main focus is on plant aromatics.
Several plant substances that seem to attract the females have already been identified. Lab trials indicate that the concentration at which the aromatic is offered is critical, however (Figures 2 and 3). Too little fails to attract them while too much scares them off. Some substances are attractive to both males and females (substance B) while others only attract males (substance D) or only females (substance A). The research is now focusing on finding the right substances, or combination of substances, and on finding a formulation that will go on producing the right intensity of aromatics over a long period of time in practice.
Trap colour and shape
The funnel trap with pheromone which researchers are currently using needs to be further optimised for catching bugs. Video recordings of males landing on these traps showed that less than five per cent of landings on the trap actually resulted in capture. The colour and the shape of the trap will be studied in more depth in future research, along with ways of further optimising the trap. If a combination of pheromone, plant aromatics and better traps proves successful in trapping both males and females, this will open up new opportunities for tackling the bug problem.
Besides observations, the aim is also to improve biological control with an effective combination of attractants and biological agents. To begin with, the researchers are looking at a biological agent based on an insecticidal (entomopathogenic) fungus. They are investigating whether it is possible to use attractants to target fungal spores better in the crop in order to increase their effectiveness in controlling bugs. With a modified formulation of the fungal spores and an effective method of transferring them to the bugs, it has been shown that it is possible to get at least three to four times more spores onto a bug.
The next question is whether this combination of methods (luring, infecting and transferring) actually helps combat the infestation. Further research on this will be taking place during the coming year.
For some years now, scientists and growers have been working together to come up with a better monitoring and control plan for the European tarnished plant bug. A pheromone trap that captures males has already been successfully tested. Research into attractants for females has yielded some new substances that are effective, offering new options for controlling this bug.
Text and image: Rob van Tol (Wageningen UR), Maedeli Hennekam and Daowei Yang (Entocare biocontrol).