September 13, 2021

In despear: The impact of climate change on pears, pests and natural enemies

Written by: Laura Reeves

I am a first year PhD student on the Waitrose CTP programme at Reading University. My research currently focuses on how tri-trophic interactions between pears, pests and natural enemies are impacted by climate change.

Pear flowering times:

Pears are an economically important crop within the UK, contributing to 4.0% of total fruit production; with a planted area of over 1500 hectares and an economic value of £22.8 million in 2019 [1]. There is concern that climatic warming may be impacting flowering times and budburst in fruit trees, including pear [2, 3]. Many crops go into a dormancy phase over winter, this is a period of restricted growth to protect them from cold temperatures and frost damage. A minimum amount of chilling time (number of hours below a particular temperature), followed by heating/forcing time (a certain number of hours above a particular temperature) is then required in order to stimulate vegetative growth [4, 5]. Reducing chilling time may delay flowering, whilst increasing forcing time may advance flowering [2, 3]. There is evidence to suggest that flowering date and budburst is advancing in the UK with respect to climate change; one study by Atkinson et al., (2004) predicted that blossom dates of fruit could be 2 weeks earlier by 2080, under the UKCIP02 high emissions scenario, with earliest flowering times in the South-East of England. Furthermore, a study by Drepper et al., (2020) showed that median pear flowering dates were 11.5 days earlier after warm dormancy periods compared to cold ones.

Potential impacts on pest species and their natural enemies:

The dominant pest in UK pear production is the pear psyllid Cacopsylla pyri (Figure.1), which is estimated to cost the pear industry £5 million per annum, due to crop damage and control costs [6]. Pear psyllid nymphs can damage fruits and leaves by secreting a sticky substance called honeydew, which encourages the growth of black sooty mould (Figure.1A). Climatic warming is a concern for pear psyllids, as warmer temperatures may speed up their development rate [7], this could lead to a mismatch between pear psyllids and the emergence of their natural enemies. Natural enemies of pear psyllids include anthocorids, earwigs, ladybirds and spiders [8]. Although ladybirds and spiders are present in orchards for most of the year, anthocorids often migrate into orchards during the spring, whilst earwigs are not seen in trees until mid-June. Therefore, if earlier pear psyllid development is not matched by migration of natural enemies to pear orchards, then pear psyllid population density could potentially increase.

It is also important to consider the interaction between pear trees and pear psyllid. Earlier budburst in pear could potentially lead to earlier oviposition; female pear psyllids often wait until shoots develop before laying eggs [9], therefore if shoots develop earlier in the year, pear pyllids may respond to this with earlier egg laying. Earlier flowering and foliage development in pear could also provide shelter for pear psyllid nymphs from agrochemical sprays and natural enemies, earlier in the year; as nymphs are often more protected inside buds or rolled leaves [10].

Overwintering of pear psyllid is a concern with respect to climate change. Previous studies on pear psyllid in the UK have found that only adults overwinter, going into reproductive diapause during the winter and ceasing to lay eggs until around late January to February [11]. However, after talking to growers and looking at pear psyllid data, there is concern that psyllid nymphs could be overwintering as well as adults. Overwintering of nymphs has been previously recorded in countries with milder winters such as Israel and Sicily [12, 13]. Therefore, warmer winters within the UK could be mild enough for nymphs to survive, although further analysis is required, leading to my current research.

Current research:

My current research is working with multiple datasets as well as collecting data in the field. I have records of pear flowering times in Kent from 1960-2020 and data from 2010-2021 for pear psyllid (eggs, nymphs and adults) and their natural enemies. Alongside this I have been collecting samples of pear psyllids and monitoring natural enemies at 7 orchards throughout the year. My exploratory data analysis so far has indicated that pear flowering is on average 11.98 days earlier in 1990-2019 compared to 1960-1989. Furthermore, psyllid nymphs have been observed in late December 2020 and early January 2021, suggesting that they could be overwintering. Although, my current fieldwork monitoring pear psyllid this winter may give more support to this hypothesis.


  1. DEFRA, Horticulture statistics – dataset 2020, [cited 21/08/2021].
  2. Atkinson, C., et al., Winter chill in fruit. UK Department of Food, Environment and Rural Affairs Report NoCTC026, 2004.
  3. Drepper, B., et al., Comparing Apple and Pear Phenology and Model Performance: What Seven Decades of Observations Reveal. Agronomy, 2020. 10(1): p. 73.
  4. Atkinson, C., R. Brennan, and H. Jones, Declining chilling and its impact on temperate perennial crops. Environmental and Experimental Botany, 2013. 91: p. 48-62.
  5. Sunley, R., C. Atkinson, and H. Jones, Chill unit models and recent changes in the occurrence of winter chill and spring frost in the United Kingdom. The Journal of Horticultural Science and Biotechnology, 2006. 81(6): p. 949-958.
  6. AHDB, Grower Summary-Exploiting semiochemicals, conservation biocontrol and selective physical controls in integrated management of pear sucker. 2011.
  7. McMullen, R. and C. Jong, Effect of temperature on developmental rate and fecundity of the pear psylla, Psylla pyricola (Homoptera: Psyllidae). The Canadian Entomologist, 1977. 109(2): p. 165-169.
  8. Fountain, M., et al., Importance of naturally occurring predators for pear sucker control. Importance of naturally occurring predators for pear sucker control., 2013. 91: p. 117-125.
  9. Horton, D.R., Oviposition by overwintering morph of pear psylla (Homoptera: Psyllidae) with information on conditioning. Environmental entomology, 1990. 19(2): p. 357-361.
  10. Solomon, M., et al., Control of the pear psyllid, Cacopsylla pyricola, in south east England by predators and pesticides. Crop Protection, 1989. 8(3): p. 197-205.
  11. Hodgson, C. and T. Mustafa, The dispersal and flight activity of Psylla pyricola Foerster in southern England. Bulletin SROP, 1984. 7(5): p. 97-124.
  12. Swirski, E., The bionomics of the pear psylla Psylla pyricola Foerst. Israel. Ktavim, 1954. 4(4): p. 61-68.
  13. Nin, S., et al., Pear resistance to Psilla (Cacopsylla pyri L.). A review. Advances in Horticultural Science, 2012: p. 59-74.