Since its first observation in the south of The Netherlands in 1991, the geographical range of the oak processionary caterpillar has increased steadily over the years, moving in north-eastern direction. It shows that it now occurs in the whole southern part of The Netherland

Moraal, L.G.

Oudenhoven, A.P.E., van

Vliet, A.J.H., van

English

Wageningen Yield 

Pagina 236 Gewasbescherming  jaargang 39, nummer 6, november 2008
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(eds). Contribution of Working Group II to the Fourth Assess-
ment Report of the Intergovernmental Panel on Climate Change. 
Cambridge University Press, Cambridge, UK, pp. 33–67.
IPCC, 2007. Climate Change 2007: Synthesis Report. Contribution of 
Working Groups I, II and III to the Fourth Assessment Report of 
the Intergovernmental Panel on Climate Change. [Core Writing 
Team, Pachauri, R.K. and Reisinger, A. (eds)]. IPCC, Geneva, 
Switzerland, 10 pp.
Kriticos, D.J., Yonow, T. and McFadyen, R.E., 2005. The potential 
distribution of Chromolaena odorata (Siam weed) in relation to 
climate. Weed Research 5, 26–25.
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3: User’s Guide. Available from www.hearne.com.au. Hearner 
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Climate change: opportunity 
or difficulty for farmers?
Yvonne Gooijer and Peter Leendertse
CLM (Centre for Agriculture and Environment), P.O. Box 62, 
4100 AB, The Netherlands; e-mail: ygooijer@clm.nl
The climate is changing. Your business too?
This question was asked in three groups of 
arable farmers and three groups of dairy far-
mers during workshops in different parts of 
The Netherlands. We asked them if they already 
experience effects of climate change on their 
farms. We also discussed which climate-rela-
ted difficulties they face now and expect in the 
near future. Besides difficulties, farmers also 
see opportunities. As an arable farmer from 
the province of Groningen stated: “For Global 
Warming we farm at the Gold Coast: fertile soil, 
enough water available, warmer weather and 
–in Groningen- above sea level.” 
Farmers differ in their opinion on the rela-
tion between climate change, pests and crop 
protection. Some dairy farmers relate the 
increase in flies and insects causing diseases 
as Bluetongue and Q fever to climate change. 
Furthermore, they expect more weeds in their 
crops when temperature rises. Arable farmers 
expect an increase in fungal diseases and state 
that they already have more troubles with 
insects. The consequences for their crops and 
cattle are uncertain. They expect more pests, 
but also new natural enemies. Some farmers 
think pests and weeds will become a bigger 
challenge, while others think the situation will 
not change that much. “As farmers, we are used 
to changes.” Overall, most farmers think their 
business eventually will benefit from climate 
change due to improved climatic conditions 
for crop growth.
Climate change exacerbates 
the oak processionary 
caterpillar problem in the 
netherlands
Alexander P.E. van Oudenhoven1, Arnold J.H. van 
Vliet1 and Leen G. Moraal
 
2
1 Environmental Systems Analysis Group, Wageningen 
University, PO Box 47, 6700 AA, Wageningen, The 
Netherlands; e-mail: alexander.vanoudenhoven@wur.nl;  
arnold.vanvliet@wur.nl; website: www.natuurkalender.nl
2 Alterra Research Institute, Wageningen UR
Since its first observation in the south of The 
Netherlands in 1991, the geographical range of 
the Oak processionary caterpillar (Thaumeto-
poea processionea, “OPC” in the following text) 
has increased steadily over the years, moving in 
north-eastern direction. Figure 1 shows that it 
now occurs in the whole southern part of The 
Netherlands. During its expansion, the observed 
numbers clearly peaked in 1996 and in 200. 
Both peaks were followed by severe decreases 
in 1997 as well as in 2005. The spreading of the 
caterpillar causes considerable health problems 
as each caterpillar has over 1.8 million urticating 
hairs. 
The causes of the observed change in occurrence 
as well as potential future changes are unknown. 
The OPC is an egg-overwintering insect, a group 
of insect species that as a whole has become in-
creasingly successful over the years. As the OPC 
is mainly found on solitary oaks and then mainly 
on the southern side of the trees we hypothesise 
that the OPC prefers warm conditions. There-
fore, it is likely that the observed 1°C increase 
in temperature and the corresponding increase 
in growing season length in the recent decades 
have stimulated the spreading. 
The first objective of our study was to deter-
mine which climate variables could explain the 
observed changes in the spatial distribution and 
changing population dynamics of the OPC. Our 
second objective was to determine the potential 
future changes in its distribution under different 
climate scenarios.
We created annual distribution maps of the 
OPC based on observations gathered by Alterra 
Wageningen UR, the Dutch Butterfly Conser-
vation and the Dutch phenological network 
Nature’s Calendar. The period covered was 1991 
to 2007. We correlated the distribution maps of 
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the OPC with weather data that were based on 
data from thirty to fourty MeteoConsult weather 
stations, comparing the region where the OPC 
occurred to regions without OPC observations. 
The large number of weather stations gave us a 
unique insight in regional climate differences. By 
combining the found relation between climate 
and occurrence with the four climate change 
scenarios of the KNMI, we assessed the potential 
future distribution of the OPC.
We conclude that the temperatures in May, June, 
July, September and October are significantly 
higher in the OPC region. May, June and July 
are the months in which the eggs hatch and the 
caterpillars grow, September and October is the 
period of flight and reproduction of the OPC. 
The summer temperature in the OPC region cur-
rently averages 17.6°C. In the north of the coun-
try this is 16.7°C. Our analysis also showed that 
September on average was significantly drier in 
the OPC region. This suggests that much rain 
during the flight period has a negative impact on 
reproduction.
Our scenario analysis showed that the average 
summer temperature in the north of The Neth-
erlands will be between 16.6 and 17.6°C in 2020 
and between 17.0 and 19,0 in 2050 (depending 
on the scenario). Therefore, we conclude that the 
OPC distribution area is likely to expand further; 
in 2020 the entire country could become an 
“OPC region”. 
The need for more detailed information about 
the OPC and the locations has become clear. By 
improving the level of detail in the information 
as well as the cooperation in the management 
the nationwide risk posed by the OPC could be 
contained or prevented more efficiently. Moreo-
ver, attention must be paid to the distribution 
of potentially infected oak trees throughout The 
Netherlands. Important for the management 
of the OPC (locations) is the coupling between 
reported observations on the one hand and in-
formation about the confirmation and manage-
ment practices on the other hand.
additional references 
Van den Hurk, B., Klein Tank, A., Lenderink, G., Van Ulden, A., Van Ol-
denborgh, G. J., Katsman, C., Van den Brink, H., Keller, F., Bessem-
binder, J., Burgers, G., Komen, G., Hazeleger, W. & Drijfhout, S., 
2006. KNMI Climate Change Scenarios 2006 for the Netherlands. 
KNMI Scientific Report WR 2006-01. De Bilt: KNMI (Royal Dutch 
Meteorological Institute).
Website ‘De Natuurkalender’: www.natuurkalender.nl 
Climate change and 
bioinvasiveness of plant 
pathogens: comparing 
pathogens from wild and 
cultivated hosts in the past 
and the present
Aad J. Termorshuizen
Blgg, Nieuwe Kanaal 7F, 6709 PA Wageningen, The 
Netherlands; e-mail: aad.termorshuizen@blgg.nl
Shifts in the distribution of organisms occur 
permanently and worldwide, involving orga-
nisms from all taxonomic groups. Shifts towards 
previously uncolonized areas are now commonly 
referred to as biological invasions. Invasions 
by pests and pathogens have a huge impact on 
agriculture. The successfulness of these invaders 
has been ascribed to absence of their natural 
enemies, but the successfulness of many in-
vaders is also comfortably explained by lack of 
sufficient resistance in many crop/pathogen 
combinations, the area cropped in monoculture, 
and narrow rotations. 
Many plant pathogens of agricultural crops 
made a great shift in their geographical distri-
bution due to simple enlargement of the area 
cropped to hosts, mostly in combination with 
the oblivious transportation of these pathogens 
in plant material. However, it is also interesting 
to consider cases where invasion has not yet oc-
curred. This can be due to inability of the patho-
gen to bridge large distances or to unfavourable 
climatic conditions. For example, although their 
Figure 1: Oak processionary caterpillar (OPC) 
distribution maps of the periods 1991/1993 and 
2006/2007 in The Netherlands. The darker dots 
represent the more recent years. Since the OPC’s 
first appearance in 1991 its distribution area 
has moved in north-eastern direction, expanded 
rapidly as well.


Climate change exacerbates the oak processionary caterpillar problem in The Netherlands

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