Revista Mexicana de Ciencias Forestales Vol. 13 (69)

Enero – Febrero (2022)

La temperatura en latitudes septentrionales en el hemisferio norte es el factor más importante que determina la abundancia de los descortezadores. En los bosques de pino del centro-norte de México existen otras variables climáticas que influyen en este suceso. Se consideró como hipótesis que existe una relación entre las variables climáticas con la abundancia de Dendroctonus mexicanus y se planteó como objetivo describir la abundancia de D. mexicanus ocurrida durante los años 2015 y 2016, en relación con las variables climáticas: temperatura, precipitación, índice de aridez e índice de precipitación estandarizado en el bosque de pino de Hidalgo, mediante el monitoreo de la abundancia de los escarabajos descortezadores con trampas tipo Lindgren. Se observó una correlación entre la abundancia de D. mexicanus y las variables climáticas, de ellas destacaron la precipitación (r2=0.59, p=0.0035) y el índice de precipitación estandarizado (r2=0.85, p=0.0082). La mayor abundancia de D. mexicanus se presentó en un ambiente de ligero a moderadamente húmedo. El aumento en la abundancia del insecto descortezador en los bosques de pino del estado de Hidalgo está correlacionado con los regímenes de variables climáticas como la temperatura, el índice de aridez y en especial con la precipitación e índice de precipitación estandarizado.

Palabras clave: Abundacia, escarabajos descortezadores, estaciones del año, índice de precipitación estandarizado, precipitación, temperatura.

The temperature in northern hemisphere latitudes in the north seems to be the most important factor that determines the greater abundance of bark beetles, in the pine forests of north-central Mexico there are other climatic variables that influence this event. There is a relationship between the changes in the climatic variables that influence the changes in the abundance of D. mexicanus, such as temperature, precipitation, aridity index and standardized precipitation index in the pine forest of Hidalgo. This study consisted of monitoring the abundance of bark beetles using Lindgren-type traps. Observing in the results, a relationship is apparent between the abundance of D. mexicanus and climatic variables, among which the precipitation (r2 = 0.59) and the standardized precipitation index (r2 = 0.85) stand out. The increase in the abundance of D. mexicanus in pine forests in Hidalgo is related to changes in the regimes of climatic variables such as temperature, aridity index and especially with precipitation and standardized precipitation index.

Key words: Abundance, bark beetles, seasons, standardized precipitation index, precipitation, temperature.

The survival of bark beetles in more northern latitudes decreases in winter because they are poikilothermic organisms; thus, the increase in environmental temperature regulates the number of individuals (Faccoli, 2002; Safranyik et al., 2010). In addition, the number of generations per year increases because their metabolic rate accelerates and the diapause time decreases (Amat-García et al., 2005; Aukema et al., 2016) ―as in the case of Dendroctonus frontalis Zimmerman and D. ponderosae Hopkins―, and, hence, they become pests (Trần et al., 2007; Safranyik et al., 2010; Six and Bracewell, 2015). For this reason, temperature variation is considered an important factor in increasing the abundance and population dynamics of bark beetles throughout the year (Chapman et al., 2012; Gaylord et al., 2013; Hart et al., 2014).

There are studies that relate precipitation and extreme temperature events to a higher abundance of bark beetles (Chapman et al., 2012; Cervantes-Martínez et al., 2019). In contrast to extreme weather events, such as unseasonal cold snaps, which reduce bark beetle populations during winter (Sambaraju et al., 2012; Weed et al., 2015; Rosenberger et al., 2017). This is because the insects are unable to colonize the host trees in the presence of a cold and humid climate.

Temperature and precipitation are two determining variables in the life cycle of insects (Bentz and Jönsson, 2015; Six and Bracewell, 2015), as has been acknowledged for D. mexicanus Hopkins and D. adjunctus Blandford (Salinas-Moreno et al., 2010; Six and Bracewell, 2015; Soto-Correa et al., 2019). In Mexico, the number of D. frontalis beetles has been observed to increase as precipitation augments; however, such increase has not been observed in relation to the temperature when it tends to average approximately 19 °C (Soto-Correa et al., 2019).

Since 1836, outbreaks of insect pests of the genus Dendroctonus Erichson in the pine forests of Hidalgo, Mexico, have become more frequent and are the cause of high mortality of trees of the genus Pinus (Servicios Forestales de Hidalgo, 2017). In this respect, the death of individuals of the species Pinus patula Schltdl. & Cham (Fonseca-Gonzáles et al., 2014), Pinus teocote Shiede ex Schltdl et Cham and Pinus leiophylla Shiede ex Schltdl et Cham (Sánchez-Martínez, 2004) has been documented in large forested areas.

Dendroctonus mexicanus is a generalist species, as it colonizes 21 of the 47 Pinus taxa present in Mexico (Salinas-Moreno et al., 2004); therefore, it is of great importance to understand the population dynamics of bark beetles in the pine forests of the state of Hidalgo in order to propose management strategies to prevent and control the incidence of these pests (del Val and Sáenz-Romero, 2017).

The present paper describes the relationship between climatic variables and the abundance of D. mexicanus in the pine forest of the state of Hidalgo during 2015 and 2016. The importance of this study lies in the lack of studies involving continuous collections in the spatiotemporal distribution of D. mexicanus. The result will be the guideline for understanding the dynamics of the abundance of bark beetles under scenarios of future changes in temperature, precipitation, and aridity.

The study was carried out in pine forests in the state of Hidalgo, located between 21°24'22'' and 19°38'3'' N, and 99°53'43'' and 97°59'8'' W ―an area that is part of the Mexican Transversal Volcanic Belt and the Sierra Madre Oriental, comprising the municipalities of Jacala, Pacula, Metztitlán, and Zimapán. The forest occurs within an altitudinal range of 1 800 to 2 600 m (Conabio, 2004). The climate is predominantly temperate, with an average annual temperature of 16 °C; an average minimum temperature of 4 °C in January (the coldest month), and an average maximum of 27 °C in April and May. Rainfall occurs in the summer, from June to September, with an average rainfall of 800 mm, with fluctuations of 600 to 1 500 mm, and a dry season between October and May (Conabio, 1998; Conabio, 2004).

The sampling consisted of four transects, one per locality, within the pine-oak forest; the transects consisted of four to six sampling sites with an altitudinal separation interval of 100 m (Table 1). The sampling was carried out using paired traps. Two Lindgren-type traps consisting of eight funnels were set up at each sampling site ―a trap with an artificial bait (Dendroctonus attractant Alpha-pinene + Frontalin + Endobrevicomine; Synergy Semiochemicals Corporation™) and an unbaited control trap―, separated by a distance of 50 m. In each trap, a (BioQuip™) collecting jar with a mixture of equal parts of (Prestone af ex™) antifreeze and 70 % alcohol was installed for the purpose of slaughtering and preserving the captured insects (Macías-Sámano et al., 2004).

Table 1. Dendroctonus mexicanus Hopkins sampling sites in the pine forest in Hidalgo, abundances per site in both years and modeled climatic data (1961-1990).

MAT = Mean annual temperature; MAP = Mean annual precipitation; DD5 = Degrees day; AAI=Annual aridity index; ---= No data available.

Individuals of the genus Dendroctonus were collected on a fortnightly basis from February 2015 to December 2016. Subsequently, they were taken to the Faculty of Natural Sciences of the Autonomous University of Querétaro (Universidad Autónoma de Querétaro) and identified according to the codes proposed by Cibrián-Tovar et al. (1995).

Data from nine meteorological stations of the Mexican Institute of Water Technology (IMTA, 2019) near the sampling sites were used for the purpose of learning about climate behavior in 2015 and 2016, and the averages per day were analyzed. Since 1945, historical temperature and precipitation data were drawn from three stations (Huichapan, Ixmiquilpan, and Zacualtipán); since 1955, from five stations (Actopan, El Salto, Metztitlán, Mixquiahuala, and San Cristóbal), and since 1975, from one station (Presa de Endho). Daily and monthly average point data were used every ten years up to 2016; standardized precipitation index (SPI) data were also used (OMM, 2012) for the years 2015 and 2016 for the abovementioned stations.

For the modeling of the specific climatic variables of each sampling site, a thin plate spline climate model developed for Mexico was applied (Hutchinson, 2004; Crookston, 2010). The estimated variables were mean monthly temperature (MMT), mean monthly precipitation (MMP), monthly aridity index (MAI = (DD50.5)/MMP; DD5 = degrees day >5 °C). The value of the standardized precipitation index (SPI) was estimated for Hidalgo, based on data from the National Weather Service (Servicio Meteorológico Nacional (SMN, 2019). The SPI was estimated based on the values of 24 months and these were grouped according to the seasons of the year in Mexico: winter (December, January, February), spring (March, April, May), summer (June, July, August), autumn (September, October, November). SPI is a drought indicator that represents parameters such as soil moisture and precipitation anomalies, among others (McKee et al., 1993; OMM, 2012).

Data analysis was performed with the SAS® statistical package version 9.3 (SAS, 2004). A Pearson analysis of the correlation between the abundance of D. mexicanus and climatic variables (temperature, precipitation, aridity index, SPI) was performed, and a regression between the number of D. mexicanus, precipitation and the standardized precipitation index (SPI) was carried out.

Dendroctonus mexicanus exhibited the highest abundance in 2015 in the months of February and October, with about 3 500 individuals per month, while the lowest abundance occurred in 2016, during the period from June to October, with abundances of less than 500 individuals; during the other months, abundance remained above 1 000 individuals (Figure 1). Regarding the annual accumulation of all traps, a higher abundance was observed in 2015, with 15 949 individuals occurring at an average annual temperature of 16 °C and 752 mm of precipitation. In 2016, the abundance was lower, as only 9 263 individuals of D. mexicanus were captured, with an average annual temperature of 16.4 °C and 598 mm of precipitation.

$G:\Memo 2\REV Ciencias forestales inifap\Figura 3 Mensual 2015 2016.xlsx_archivos\Figura 1 Abundancia por mes feb15 dic16_21382_image001.png$

Figure 1. Total abundance of Dendroctonus mexicanus Hopkins by month during 2015 and 2016 in the pine forest of Hidalgo, Mexico.

The monthly mean temperature, monthly mean precipitation and monthly mean aridity index are characterized by a pattern throughout the year. Temperatures are high from March to September; precipitation occurs from June to September, and aridity, from November to May (Figure 2).

$G:\Memo 2\REV Ciencias forestales inifap\FIGU2 DATOS CLIMATICO_archivos\Figura 2 Pre y Temp Current Clim 2015 2016_30211_image004.png$

Figure 2. Monthly temperature (A); Monthly precipitation (B); Monthly aridity index (C) in 2015, 2016 and contemporary data (1963-1993) in the pine forest of the state of Hidalgo.

The correlation results exhibited a low association of D. mexicanus abundance with the mean minimum temperature in 2015 (r = -0.51), and with the mean minimum temperature (r = -0.63), the mean minimum precipitation (r = -0.77), the aridity index (r = 0.52) and the SPI (r = 0.61 in 2016 (Table 2). On the other hand, there is a functional relationship between the monthly abundance and the mean monthly precipitation for the year 2016 (R2 = 0.59); while, for 2015, no relationship was observed (Figure 3A, Figure 3B; Table 2).

Table 2. Pearson's correlation analysis (r) and a regression analysis (R2) between data from climate stations near the sampling sites and modeled climate data regarding the abundance of Dendroctonus mexicanus Hopkins during 2015 and 2016 in the pine forest of Hidalgo.

$G:\Memo 2\REV Ciencias forestales inifap\reggresion_archivos\reggresion_22195_image010.png$

Figure 3. Regression analysis of monthly abundance of Dendroctonus mexicanus Hopkins with station precipitation in 2015 (A) and in 2016 (B) in the pine forest in Hidalgo, Mexico.

There was also a relationship between the abundance of D. mexicanus and the SPI organized by season, in the winter, spring, summer and fall of both years (r2 = 0.854). The abundance of D. mexicanus was observed to be high in the four seasons of the year 2015; likewise, SPI presented higher values. There was a higher abundance during the winter and spring of 2016, and a lower abundance in the summer and fall (Figure 4).

$C:\Users\Horticultura\Downloads\Figura 4 SPI por temporada (1)_4651_image003.png$

Figure 4. Relationship between the abundance of Dendroctonus mexicanus Hopkins and the standardized precipitation indices during 2015 and 2016 in the pine forest of Hidalgo, Mexico, and SPI intervals indicating moisture.

The pattern of climatic variables, such as MMT, MMP and MAI partially explain the occurrence of a higher or lower abundance of D. mexicanus throughout 2015 and 2016. For example, the temperature range in the study area shows a pattern in the MMT throughout the year, in which it is observed to decrease from November to January by 14 °C (-/+1 °C); then, the temperature increases gradually from February onwards, reaching 20 °C (-/+2 °C) in April and May; from June to September, the average temperature decreases to 19 °C (-/+2 °C), and the rainy season begins; the temperature decreases from September onwards, reaching 14 °C (-/+1) in December (Figure 2A).

During 2015, the MMT was higher than the historical levels in the months of February, March and April. This change may have led to an increase in the number of bark beetles during this period (Figure 2A). In other studies, the change in the regimes of climatic variables caused an increase in the abundance and changes in the population dynamics of bark beetles (Chapman et al., 2012; Gaylord et al., 2013; Hart et al., 2014; Bentz and Jönsson, 2015; Six and Bracewell, 2015; Soto-Correa et al., 2019).

In the forests of Hidalgo, the rainy season and the dry season are very different (Conabio, 1998), and the changes in climatic variables are responsible for the increase in the abundance of bark beetles. For example, throughout 2015 there were changes in the temperature and precipitation patterns; the average minimum temperature in January was 4 °C, and, therefore, the populations of bark beetles did not decrease (Sambaraju et al., 2012; Weed et al., 2015; Rosenberger et al., 2017). Temperatures close to 0 °C are a limiting factor for the abundance of this species (Hicke et al., 2006; Aukema et al., 2016), while the ideal average temperature for the presence of bark beetles ranges between 5 and 11 °C (Hicke et al., 2006).

Moreover, an increase of in the minimum winter temperature by 3.3 °C results in epidemic outbreaks (Trần et al., 2007; Bentz et al., 2016). Increased ambient temperature reduces insect mortality in the winter (Faccoli, 2002; Safranyik et al., 2010). In addition, there was an oscillatory pattern of precipitation in 2015, when precipitation was out of phase with respect to the historical precipitation. In that year, there were three important peaks: the first, in March, with 90 mm (an atypical event), and the second, in June, with 90 mm; in July, it decreased to 50 mm, while the third peak occurred in August and September.

In 2016, the precipitation was more similar to the historical average (Figure 2B). The presence of rainfall in February led to a lower aridity index during that season, compared to the historical level. In February 2016, climatic conditions (temperature, precipitation, and aridity) were similar to the historical conditions, and there was a lower abundance of D. mexicanus. Precipitation occurred usually during the months of June-September, with no fluctuations throughout the year. The lack of rainfall in February of that year may have led to a higher aridity index, resulting in insect mortality due to lack of moisture (Amat-García et al., 2005).

The average temperature suitable for a higher abundance of D. mexicanus has been estimated to be 18 °C (between 13 and 25 °C) (Méndez-Encina et al., 2020); likewise, another study determined an optimum average of 16.6 to 18 °C (Morales-Rangel et al., 2018). However, a high average temperature also affects abundance; such was the case of D. frontalis, which registered a lower abundance with an MMT of 19 °C (Soto-Correa et al., 2019). In this study, the mean annual temperature was 19 °C (with a range of 14 to 21 °C). Throughout the year, the average temperatures were adequate for the existence of a higher abundance of bark beetles; however, it was observed that average maximum temperatures can act as a limiting factor of abundance, as was estimated for D. frontalis, which exhibited a relationship between maximum temperatures above 30 °C and a lower abundance (Soto-Correa et al., 2019). This can be attributed to the fact that certain growth states in the life cycle of the beetles are very sensitive to desiccation (Amat-García et al., 2005).

In north-central Mexico, D. mexicanus lives in forests where there is an association between the anticipated increase in temperature at the end of the winter and an explosion in the population of bark beetles (Hernández-Muñoz et al., 2017). Changes in the rainfall pattern are another factor that promotes a greater abundance of D. mexicanus in places where most of the year the temperature is appropriate for the development of this species, as a result of the presence of humidity in the environment due to precipitation (López-Gómez et al., 2017; Cervantes-Martínez et al., 2019). This relationship has been observed in 15 Mexican states (Cervantes-Martínez et al., 2019).

The life cycles of the beetles are in accordance and in harmony with the seasonality of the annual cycle of the region where they live (Amat-García et al., 2005). Climate change modifies annual seasonality and leads to favorable conditions for bark beetles (del-Val and Sáenz-Romero, 2017). In a study conducted in Hidalgo, outbreaks of Dendroctonus coinciding with lower rainfall in the rainy season were registered in 1940, 2011, and 2013 (Cervantes-Martínez et al., 2019).

Another characteristic to consider is the average minimum temperatures below 5 °C, or average maximum temperatures above 32 °C, which limit the abundance of bark beetles. However, if rainfall occurs, the right conditions are generated for a greater abundance, as is the case for various types of beetles in tropical areas with the temperature required to fulfill vital functions and the necessary humidity to prevent dessication (Amat-García et al., 2005).

On the other hand, the relationship between the abundance of D. mexicanus and the standardized precipitation index (SPI) reinforces the importance of the presence of humidity in the environment in increasing the abundance; it also coincides with higher SPI values within the analyzed interval, while lower abundance corresponds to lower SPI values. The SPI must be slightly to moderately moist in order for a higher abundance to exist (OMM, 2012).

The present study is a contribution that provides information on the abundance of D. mexicanus associated with climatic variables. However, knowledge of temperature- and precipitation-dependent physiological processes is still limited for most of the bark beetle species both in the southwestern United States and in Mexico (Bentz et al., 2016).

Knowledge of the relationship between the greater or lesser abundance of D. mexicanus in pine forests in Hidalgo and the patterns of temperature, precipitation, aridity index, and, especially, the standardized precipitation index is very important because it allows us to have an approximation of where the right conditions for the existence of an outbreak of D. mexicanus are present. This study generates evidence of the increase in the abundance of D. mexicanus in the forests of Hidalgo and its relationship with the humidity of the environment, which should be characterized as slightly (SPI = 0.51-0.79) to moderately humid (SPI = 0.8-1.29). Having data for only two years may not be sufficient to know the dynamics of the abundance of D. mexicanus, as it will depend on the particular conditions of each site.

The authors are grateful to the Conafor-Conacyt fund C01-234547 for the support provided to carry out the project.

The authors of this article had no conflict of interest in the conduct of the study, the drafting of the manuscript, or the evaluation of the article.

José Carmen Soto-Correa: participated in data analysis, experiment planning and writing of the manuscript; Guillermo Hernández-Muñoz: identification of individuals, data analysis, and drafting of the manuscript; Víctor Hugo Cambrón-Sandoval: conduct of the experiment and drafting of the paper.

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Municipality	N Latitude	W Longitude		Altitude (m)	Abundance		MAT	MAP	DD5	AAI
Municipality	N Latitude	W Longitude		Altitude (m)	2015	2016	(ºC)	(mm)	(°C)
Metztitlán	20º42’40.8”	98º46’52.9”	2 222		802	---	16.1	846	4016	0.21
Metztitlán	20º40’20.3”	98º46’43.7”	2 216		969	---	16.0	844	4010	0.21
Metztitlán	20º42’36.4”	98º46’48.1”	2 163		978	---	16.1	855	4019	0.21
Metztitlán	20º42’53.3”	98º47’40.6”	2 046		1 290	---	16.0	903	4007	0.23
Zimapán	20º55’49.9”	99º13’38.4”	2 098		143	363	17.1	521	4406	0.12
Zimapán	20º56’03.7”	99º13’29.5”	1 998		436	818	17.4	502	4499	0.11
Zimapán	20º56’13.1”	99º13’19.5”	1 880		838	625	17.7	476	4602	0.10
Zimapán	20º56’14.8”	99º13’15.5”	1 812		436	318	17.9	456	4662	0.10
Zimapán	20º56’15.4”	99º13’09.8”	1 722		1 493	1386	18.0	439	4725	0.09
Zimapán	20º56’17.0”	99º13’03.8”	1 654		717	1190	18.1	432	4755	0.09
Pacula	20º55’46.8”	99º14’14.4”	2 117		1 535	2063	17.1	523	4379	0.12
Pacula	20º56’24.7”	99º13’37.2’’	2 014		311	83	17.3	506	4469	0.11
Pacula	20º56’45.7’’	99º14’42.8’’	1 938		246	183	17.6	489	4553	0.11
Pacula	20º56’46’’	99º14’20.3’’	1 880		1 993	1675	17.7	476	4602	0.10
Jacala	20º54’05.2”	99º09’21.0”	1 611		1 421	281	18.3	469	4814	0.10
Jacala	20º54’14.8”	99º09’10.6”	1 532		570	210	18.6	455	4927	0.09
Jacala	20º54’21.5”	99º09’12.3”	1 440		998	15	18.7	447	4974	0.09
Jacala	20º54’25.8”	99º09’15.0”	1 342		773	53	18.7	447	4992	0.09

Climate variable	Abundance of D. mexicanus
	Pearson’s coefficient (r)		R2 coefficient
	Year	Year	Year	Year
	2015	2016	2015	2016
Mean temperature (Stations)	-0.47	-0.44	0.23	0.20
Mean temperature (Modeled)	-0.37	-0.41	0.14	0.17
Maximum mean temperature (Stations)	-0.34	-0.02	0.11	0.00
Maximum mean temperature (Modeled)	-0.19	0.00	0.04	0.00
Minimum mean temperature (Stations)	-0.51	-0.63	0.26	0.40
Minimum mean temperature (Modeled)	-0.44	-0.68	0.20	0.46
Mean precipitation (Stations)	-0.16	-0.77	0.03	0.59
Mean precipitation (Modeled)	-0.42	-0.86	0.18	0.74
Aridity index (Stations)	-0.11	0.52	0.01	0.27
Aridity index (Modeled)	0.37	0.77	0.13	0.60
Standardized precipitation index	-0.29	0.61	0.09	0.37
Maximum extreme temperature (Stations)	-0.24	0.42	0.06	0.18
Minimum extreme temperature (Modeled)	-0.62	-0.67	0.38	0.45