How active are insect predators in orchards?

Gordon Brown reviews global research on the use of sentinel prey technology in orchards to assess management practices for their impact on predator activity. 

One of the aims of both organic and integrated pest management is to minimise insecticide usage. A tool that is commonly used is to encourage insect predators and parasitoids by only spraying when pheromone traps indicate the pest is present above threshold levels. The problem is that both organic and predator friendly insecticides are only tested against a handful of beneficial species.  

In the natural environment, the community of beneficial species in an orchard is very diverse, including bats, birds, spiders and many different insect species. Unfortunately, the impacts of insecticides against all these beneficial species is poorly researched or understood.  

In Washington State a survey of apple orchards over two seasons identified 80 species of spiders, ladybeetles, lacewings and predatory bugs in unsprayed orchards.¹ To add to the complex nature of this ecosystem, they also identified that different regions had different predator compositions which changed as the season progressed. Hence the decision whether to spray or not should account for the population density of predators and parasitoids as well as pest insects in the orchard. The question is how do we simply measure this activity. 

This is also a problem for scientists wishing to determine how a particular practice or treatment impacts the entire predator population. A method has been developed for these studies which involves the use of sentinel prey. The method involves placing potential predator food in an environment, then coming back some time later to see if a predator has taken any of the food. For insect predators, the food will be insects in one of their growth phases. In science the use of this method is rapidly increasing to study the impacts of agricultural management practices, pollutants and other environmental conditions on predator activity (Figure 1). 

Figure 1: The number of scientific articles with the wording ‘sentinel prey’ in the title or keywords.⁶

There are a few relevant research studies in orchards. In 2012 French research identified that predation on codling moth eggs was higher in August than in June and that the level of predator activity was related to the toxicity of the insecticides used in the orchard and the surrounding area.²

In 2022 research in Belgium found greater predation of codling moth eggs in the natural landscape compared to both IPM and organic orchards.³ There was slightly more predation in the organic orchards which may have been due to differences in weed populations or pesticides used. Unfortunately, damaged apples were greater in the organic orchard due to increased pest pressure, demonstrating the need to monitor both predator and pest populations. 

While it is easy to assume that predation will be greater in organic orchards, this is not necessarily the case. In Austrian vineyards it was found that the rate of predation of grape berry moth eggs and larvae (sentinel species) was greater in the IPM vineyards than the organic vineyards.⁴ This was thought to be due to the different fungicides impacting predator populations, demonstrating the potential of all management practices, including organic ones, to affect predator activity. 

Most recent study 

The most recent study of sentinel prey in apple orchards occurred in France over a period of nine years.⁵ The project aimed at refining the method of determining the predator activity in 14 apple orchards, initially with four under organic production, which became eight by the end of the trial due to test orchards converting to organic production. This project studied the efficiency of three prey species, three live aphids, eight to 12 codling moth eggs or a small number of Mediterranean flour moth eggs glued to small sandpaper cards prior to freezing. Fine grit sandpaper cards were used to provide predators with grip to consume the glued-down prey. About a month after freezing, cards were stapled to the underside of leaves at eye level in 10 trees spread out in the centre of the orchard. Each tree had a card of each prey species. This was performed at the end of April and again at the end of May. 

Aphid cards were collected after a day in the field and the egg cards left for four days prior to removal for assessment. They were then studied for the number of remnant prey and each card recorded as predated or not. 

Results 

While there was a difference due to organic orchard production (Figure 2), all different sentinel prey had similar levels of predation, as did the two timings of assessment. This data shows that IPM production had 30 per cent less predation by naturally occurring predators compared to the organic orchards. Unfortunately the high pest pressure in the organic orchards (which may have attracted the predators) meant that the level of fruit and shoot damage due to codling moth and apple aphids was higher in the organic orchards. This indicates that IPM pest control strategies were not completely replaced by organic pesticides plus predator activity. 

Figure 2: The impact of organic versus IPM production on the percentage of sentinel species cards that had predated prey on them.

If sentinel prey methods are to be used in conjunction with pest monitoring information to decide if pesticides are needed, then the sentinel prey system needs to be sensitive to yearly variations in predator activity. The appendices to the “Sentinel prey settings” paper provide graphs, without commentary, on the yearly predator activity levels.⁶ Figure 3 combines the prey species data for the median number of predated cards over the nine seasons. Here, card predation varied from just over 20 per cent in 2017 (a low predator activity season) to close to 60 per cent in 2020 (a high predator activity season). While no pest population densities or fruit damage data is provided, this data does highlight the potential of the sentinel prey system to detect small changes in predator activity with each season. 

Figure 3: The average median of predation in the orchards over the nine-year period.⁶

It will be many years before research is completed on the use of sentinel prey technology in orchards for making pesticide use decisions. However, its use for assessing management practices for their impact on predator activity should become more common in the immediate future. This will allow for improved environmentally friendly practices to become widely used in orchards. 

Take home messages 

Sentinel prey systems: 

• involve sticking predator prey to sandpaper cards and placing them in trees to detect the level of predator activity in the orchard 
• can determine predator activity within four days of card placement within the orchard 
• are sensitive to and can detect small changes in predator activity due to management practices such as organic production methods 
• can be used to detect the impacts of existing and future management practices on predator activity 
• could be used, in the future, in conjunction with pest trapping to determine if pesticide sprays should be applied 

Further reading 

1 “Diversity and phenology of the generalist predator community in apple orchards of Central Washington State (Insecta, Araneae)”, https://doi.org/10.4039/tce.2012.72 

2 “Predation of codling moth eggs is affected by pest management practices at orchard and landscape levels”, www.sciencedirect.com/science/article/abs/pii/S0167880911003471 

3 “Direct and indirect effects of management and landscape on biological pest control and crop pest infestation in apple orchards”, https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2664.14319 

4 “Organic farming and cover-crop management reduce pest predation in Austrian vineyards”, www.mdpi.com/2075-4450/12/3/220 

5 “Sentinel prey settings differ in their ability to distinguish organic from conventional apple orchards and in their sensitivity to weather conditions during field exposure”, https://doi.org/10.1016/j.biocontrol.2025.105875 

6 Appendices to “Sentinel prey settings” paper, https://ars.els-cdn.com/content/image/1-s2.0-S1049964425001859-mmc1.docx 

This article was first published in the Autumn 2026 edition of AFG.