4. DISCUSS
In this study, the distribution of PWD was predicted based on the MaxEnt
model. Environmental factors and actual geographical distribution spaces
of species were combined when this model was used to study the potential
distribution of species (Yuan et al. , 2020). The actual
geographical distribution of species and the environmental factors of
species have a stable state when the state parameters obtained through
the operation of the system obtain the maximum entropy to determine the
potential geographical distribution of the species (Rong et al. ,
2019). The MaxEnt model is easy to overfit when simulating the potential
distribution of species, resulting in unreliable prediction results.
This can seriously affect its application in global change biology and
other research fields (Liu et al. , 2018). Therefore, the ENMeval
data package was used to adjust the MaxEnt model parameters in the
study. The parameters with the lowest complexity were selected to
predict the potential distribution areas of PWD disease by analyzing
model complexity under various parameter conditions (Muscarella et
al. , 2014). The average value of training AUC reached 0.940,
indicating that the prediction results of the model had high accuracy
and feasibility.
The main climatic factors affecting the potential geographical
distribution of PWD were temperature (max temperature of the warmest
month [bio5], mean temperature of the driest quarter [bio9]),
rainfall (coefficient of the variation of precipitation seasonality
[bio14], and precipitation of the wettest quarter [bio16]).
Kobayashi et al. (1970) studied precipitation as a crucial factor
affecting damage caused by PWD, including the many pine trees killed by
PWD during a summer drought. Naoko et al. (2001) studied pine forests in
the warm temperature zone of Japan, which PWD had seriously damaged. The
maximum temperature in the warmest month affected the diffusion of adultMonochamus alternatus (M.alternatus ), the vector of pine
wood nematodes. The population density of B.xylophilus was
significantly increased by drought (Zhao et al. , 2003).Currently, researchers agree that the climate conditions of high
temperature and drought are conducive to the occurrence of PWD (Kanzaki
& Giblin-Davis, 2018).
Under the next six greenhouse gas emission scenarios, the potential
distribution area of PWD predicted by MaxEnt will shift to the
Northeast, and the potential suitable area will expand significantly
under the 2070s SSP5-8.5 scenario. Climate is the key factor affecting
the distribution area of species. Climate change will have a
far-reaching impact on the distribution of pests, and global warming
will be the trend of climate change in the future (Volney & Fleming,
2000; Walther et al. , 2009). Previous studies have reported that
the suitable distribution area of PWD in China will expand nearly twice
by 2100 with the intensification of climate change, showing a trend of
acceleration of the diffusion rate to the North and West (Cheng et
al. , 2015). The results of the correlation between temperature
and B.xylophilus showed that low temperature may inhibit the
spread of B.xylophilus by affecting the reproduction and activity
range of M.alternatus (Jikumaru et al., 2008). Global warming is
conducive to the activity of the nematode vector B.xylophilus ,
which significantly enhances the damage caused by PWD (Jikumaru &
Togashi, 2000). However, predictions of the potential distribution area
of PWD also need to be combined with host plants, topography, soil, and
human activities. Abiotic factors, biological factors, and species
migration affect the distribution of species during their long-term
evolution, and the distribution range of species is different in
different historical periods (Soberón, 2007).
If effective prevention and control of PWD is not realized as soon as
possible, this major disease may soon spread across a larger area in
China, causing hundreds of millions of pine tree deaths every year. This
will be an ecological disaster (Lee, 2014). The techniques adopted to
control PWD in China mainly include disease quarantine and epidemic
situation monitoring, diseased wood removal, and vector insect control.
Further, there is an active disease prevention measure, namely, trunk
injection. Trunk injection involves injecting effective components into
the tree and exerting a drug effect by distributing the chemicals based
on the transpiration of the tree. It has advantages of accurate
application and high control efficiency, and it is environmentally
friendly (Byrne et al. , 2014; Takai et al. , 2000b)
(Aćimović et al. , 2014; VanWoerkom et al. , 2014).
Emamectin benzoate is a semi-synthetic second-generation
avermectin-derived insecticide found to have the strongest nematicide
activity against B.xylophilus among different chemical substances
(Takai et al. , 2000a). Therefore, it can be considered a strong
candidate for use as a preventive trunk injection against pine wilt
disease.