Introduction

The bark beetles of the Dendroctonus genus are distributed in the pine forests of North America (^{Islas-Salas, 1980}), and extend to Nicaragua. Its presence in the state of Chihuahua, in central Mexico and, in particular, in the states of Michoacán and the State of Mexico (^{Islas-Salas 1980}) is outstanding. It has also been reported abroad, such as in Honduras and Belize (^{Payne, 1980}; ^{Wood, 1982}; ^{Monser et al., 2005}; ^{Clarke and Nowak, 2009}; ^{Armendáriz-Toledano et al., 2014}).

There is an estimation of two to five generations of Dendroctonus mexicanus Hopkins, 1905 per year as this species is dependent on temperature; in this regard, it is considered one of the most aggressive species (^{Cibrián et al., 1995}; ^{Salinas-Moreno et al., 2004}; ^{Trân et al., 2007}; Armedaríz-Toledano et al., 2014). For example; according to ^{Fonseca-González et al. (2014)}, the loss of Pinus patula Schiede ex Schltdl. et Cham. trees has been related to D. mexicanus on several occasions. Infestations of this insect and the death of trees have also been detected in the pine forests of Aguascalientes (^{Sánchez-Martínez et al., 2016}).

Bark beetles are ectothermic species that are related to water and thermal balance; consequently, the most important abiotic factors that influence the distribution and abundance of insects are water availability and temperature (^{Chown and Nicolson, 2004}; ^{De la Vega and Schilman, 2015}; ^{Clark, 2019}). There is evidence that has demonstrated that debarkers have a very important relationship between physiological characteristics and temperature, specifically tolerance and thermal preference (^{De la Vega and Schilman, 2015}).

In Mexico, the relationship between the debarking beetle abundance and temperature in pine forests has been studied (^{Hernández-Muñoz et al., 2017}; ^{Morales-Rangel et al., 2018}). However, it is necessary to generate more evidence on the population response of these insects to various climatic variables in a single site.

There are few investigations in the country that relate the abundance of specimens with humidity, despite the fact that a strong dependence of various species of beetles with this factor has been verified abroad (^{Johansson et al., 2017}; ^{Härkönen and Sorvari, 2018}). Furthermore, on many occasions, humidity influences the phenotypic expression of beetle coloration. It has been observed that in the presence of high relative humidity in the environment, beetles tend to show dark tones close to black in the exoskeleton (^{Rassart et al., 2008}; ^{Noh et al., 2016}; ^{Carrascal et al., 2017}; ^{Sun et al., 2017}). This is caused by a three-dimensional sponge-shaped porous structure located about 3 µm below the cuticular surface, which contains air in the pores, and when the insect is in conditions of high humidity, the air is replaced by water and causes a change of coloration towards dark tones (^{Noh et al., 2016}).

Climatic variability has effects on insects throughout their life history in physiological terms, such as the stage of development and diapause (^{De la Vega and Schilman, 2015}). Extreme temperatures can be critical as they provoke insects to be unable to adaptively respond to any additional temperature changes and, therefore, will be vulnerable to predation, environmental catastrophes, or extremes of temperature (^{Byrne et al., 2004}). Critical temperatures for many species are unknown, but they are very interesting in ecological and evolutionary studies (^{Mitchell et al., 1993}; ^{Lighton and Turner, 2004}).

Thus, this study analyzed the relationship between the variation in intensity of coloration of the exoskeleton of Dendroctonus mexicanus and the abundance associated with climatic variables such as humidity, temperature and aridity index that occur in four regions in which the species lives.

Materials and Methods

Location of sampling sites

In order to cover the natural distribution range of the pine-oak forest of Mexico, sampling of D. mexicanus was performed in four regions within the states of Nuevo León (1 950-2 650 masl), Hidalgo (1 342-2 611 masl), Jalisco (1 999-2 306 masl) and Oaxaca (2 113-2 889 masl) (Figure 1). Eight Lindgren Synergy Semiochemicals Corps™ insect collection traps composed of eight funnels, were placed in each region following an altitudinal transect between 1 300 and 2 900 m, which are areas suitable for the presence of barkers of the Dendroctonus genus.

Simbología = Simbology; Límites estatales = State boundaries; Estados de colecta = Collection states; Sitios de colecta = Collection sites; Bosque de coníferas = Conifer forest.

Figure 1 Location of Dendroctonus mexicanus Hopkins, 1905 collection sites within pine and pine-oak forests in the four states of Mexico. 

The eight traps were placed with an altitude separation of 100 m between each one of them (^{Macías et al., 2004}). The traps were installed in non-host trees, suspended 1.5 m from the ground (^{Rodríguez, 2009}). At the top, the trap has an attractant that contains a pheromonal formula that is designed to attract Dendroctonus barkers such as D. mexicanus, which is a combination of semiochemicals (SQ's) from Synergy Semiochemical Corps™ (^{Macías-Sámano and Niño, 2016}), which includes the Frontalina and Endo-brevicomina pheromones, plus alpha / beta-pinene as kairomona; the attractant was replaced every two months. In the lower part, the collection cup of each trap is accommodated, in which antifreeze was placed to sacrifice and facilitate the conservation of insects (^{Zylstra et al., 2010}). The collection of insects was carried out in periods of 15 days; they were subsequently kept into in Ziploc™-type bags. For the preservation of the collected individuals, a mixture of 50 % antifreeze and 70 % alcohol was used (^{Macías et al., 2004}).

Monitoring of temperature (°C) and relative humidity (%) was evaluated using Hobos-type climate data collectors (EL-USB-2 RH/Micro DAQ^TM Data Logger); climate data was recorded every 30 minutes for a one-year period (January-December 2015); with these numbers, the aridity index was calculated (^{Sáenz-Romero et al., 2010}; ^{Soto-Correa et al., 2012}). An adjustment was made to the aridity index which consisted of the ratio of the average temperature over the average relative humidity (temperature/humidity), in which the highest values are hotter places with less humidity, while the lower values refer to colder and wetter places.

Results

There are significant differences in the color intensity of the exoskeleton of D. mexicanus between the states (red P≤ 0.0004; green P ≤ 0.000, blue P ≤ 0.0014), contrary to what was found associated with the sex of the species where no significant statistical differences occurred, as well as for the Region * Sex interaction (Table 1).

The differences in the intensity of the red, green and blue bands of the exoskeleton of D. mexicanus show high values in individuals collected in the state of Oaxaca (red = 138; green = 92; blue = 76), which makes them darker in regard to the rest of the sites; it should be noted that a high similarity in coloration was determined with individuals from the state of Jalisco, which had similar values in green and blue bands (Figure 2). For the state of Hidalgo, it was observed that the blue band is similar to the localities of the states of Jalisco and Oaxaca; however, it was relatively different from the individuals from Nuevo León, which showed lower intensity values for the three color bands of the exoskeleton with lighter and redder individuals (red = 120; green = 77; blue = 66) (Figure 2).

Longitud de coloración = Color length; Regiones = Regions; Rojo = Red; Verde = Green; Azul = Blue.

Figure 2 Tukey’s mean test (α=0.5) for color length (red, green and blue) among the studied Mexican states. 

The high pixel values for the three basic colors mean darker individuals like those observed in Oaxaca and the lower values, a lighter coloration like the specimens from the state of Nuevo León.

On the other hand, it was observed that there is a non-significant relationship between the percentage of humidity and color intensity of the species (r² = 0.7826; p = 0.11); it was estimated that in Nuevo León, the average environmental relative humidity is 66 %, a percentage lower than that of the states of Hidalgo, Jalisco and Oaxaca, and so, the insects have a lighter color. In Hidalgo and Oaxaca, the collected individuals have greater intensity where it is estimated that the average relative humidity was higher than 80 %; therefore, at higher relative humidity, specimens of D. mexicanus are darker (Figure 3, A).

Intensidad de coloración = Intensity of coloration; Humedad = Moisture; Índice de aridez = Aridity index. A) Percentage of relative humidity; B) Aridity index.

Figure 3 Relationship between intensity of coloration of the exoskeleton of Dendroctonus mexicanus Hopkins, 1905 and climatic variables of the four sampling sites in Mexico. 

Correlations between humidity and color intensity support the fact that humidity is an important environmental factor influencing changes in staining patterns of barking beetles. However, there is a relationship of greater precision when variables such as relative humidity and temperature are integrated into an aridity index (r² = 0.9861; p = 0.007); in this context, the darkest insects are those of the state of Oaxaca because the values of this index are lower than those of Nuevo León (Figure 3, B).

If moisture influences the intensity of coloration, it is also a determining factor that regulates the abundance of D. mexicanus. Under this reasoning, it can be affirmed that the northern regions of Mexico such as Nuevo León tend to present lower relative humidity, therefore, the response in the abundance of D. mexicanus would be less compared to southern regions such as Oaxaca, where high values of relative humidity and, consequently, of the abundance of the barkers are expected.

The regions with high relative humidity were Hidalgo and Oaxaca, while the opposite was registered in Nuevo León. For the first, there was a higher abundance of D. mexicanus (8 190 individuals) compared to the other three states whose average population was 2 000 total individuals during the sampling period (Table 2). The highest figure in the Hidalgo region may be the result of the combination of the different favorable climatic variables for the species of interest (average, maximum and minimum temperature, as well as average, maximum and minimum relative humidity) (Table 2).

Particularly, in the present study, relative humidity and average temperature explain the behavior of D. mexicanus abundance in the four regions; for example, Hidalgo region has an average temperature of 16.3 °C and an average humidity of 82.5 %, which are similar to those of the Jalisco and Nuevo León sampling sites, but differ from the Oaxaca region (13.9 °C); on the contrary, the average relative humidity of Hidalgo, which is close to that of Oaxaca, but it is different for the of Nuevo León and Jalisco regions (66.3 and 77.8 % respectively) (Table 2).

The extremes of the temperature show significant differences between the sampling sites (P≤0.05) (Table 2). Although the maximum average temperatures suitable for the presence of high populations of debarkers occurred in the Nuevo León region, the minimum average humidity was very low for them (10 °C and 39.6 %), which is a limitation. When comparing Oaxaca to Hidalgo, there is a discrepancy of 2 °C less in Oaxaca in the mean temperatures and a difference of 2.6 °C in the minimum mean temperature, which may explain the abundance of D. mexicanus. This suggests that the existence of this barking beetle may be associated with extreme temperatures that are too high, and, in combination with extreme low temperatures, the abundance of this species is limited.

Regarding relative humidity, it is only in one direction, since the lower abundance of barking beetles is associated with lower humidity in the environment. The conditions that can lead to an increase in the D. mexicanus populations are mean temperatures near 16 °C with relative humidity around 80 %, with low variability in the maximum and minimum extremes of temperature with respect to the mean.

The analysis of temperature and humidity together in the aridity index facilitates the interpretation of the interaction between these variables regarding their influence on the abundance of barkers per site. The aridity averages within the Hidalgo region recorded an index of 0.20, similar to that estimated for the Jalisco and Oaxaca sites, but it differs from the Nuevo León site with a higher value (0.24).

On the other hand, it is observed that the maximum average aridity indexes for the regions of Nuevo León, Jalisco and Oaxaca reach values above 0.36, while for the region of Hidalgo, 0.30; this is indicative of a high probability that aridity indexes of maximum or greater value than 0.30 can limit the abundance of D. mexicanus. Unfortunately, there is no relationship between the insect abundance and the maximum average aridity index (r² = 0.39), abundance-minimum average aridity index (r² = 0.45) and abundance-average aridity index (r² = 0.37) (Figure 4, A, B, C). There is a relationship between the abundance and the variation between the maximum and minimum aridity indexes, the abundance of barking beetles being lower when the difference between the extremes is more pronounced; and, when the difference between the extremes of the aridity index was smaller, the average abundance was higher at the sampling sites (Figure 4, D).

Abundancia = Abundance; Índice de aridez promedio máximo = Maximum average aridity index; Índice de aridez promedio mínimo = Minimum average aridity index; Índice de aridez promedio = Average aridity index; Diferencia entre índice de aridez imx-imn = Difference between aridity index; imx = Maximum average aridity index; imn = Minimum average aridity index.

Figure 4 Total abundance of Dendroctonus mexicanus Hopkins, 1905 and average aridity index of four states in Mexico. 

Discussion

The variation in the coloration of the exoskeleton of D. mexicanus individuals from the different regions of Mexico is a response to climatic variables such as humidity, which has been observed in other beetle species, in which the relative humidity of the environment is related with the coloration of the exoskeleton of such beetles (^{Rassart et al., 2008}). This is explained by the structure of the exoskeleton that is porous, and the pores that contain water can be replaced by air, which, in turn, influences the variation in coloration of beetles (^{Carrascal et al., 2017}; ^{Noh et al., 2016}). There is evidence on how environmental humidity influences color, such as the case of Dynastes tityus Linnaeus, 1763, which exhibits dark colors in the presence of high humidity percentages (≥ 80 %) (^{Rassart et al., 2008}; ^{Sun et al., 2017}). With the above, it is possible to explain the estimate in the sites of Hidalgo and Oaxaca, where the highest relative humidity values (≥80 %) were present and the D. mexicanus individuals tend to be darker compared to those of the Nuevo León region, where clearer individuals were observed at lower relative humidity percentages than other sites in this study (66.3 %).

As relative humidity, temperature also assumes a direct relationship with the coloration intensity of the barker exoskeleton (^{Carrascal et al., 2017}; ^{Noh et al., 2016}; ^{Sun et al., 2017}), as has been confirmed in Chrysomela lapponica Linnaeus, 1758 in which it was evident that darker colored beetles were associated at low temperatures and light colored ones at elevated temperatures; these differences imply an adaptive capacity of dark beetles due to their greater thermal capacity at low temperatures (^{Zverev et al., 2018}).

A direct relationship was identified between the intensity of coloration of the exoskeleton and the relative humidity; by integrating temperature and humidity in the aridity index, the ratio is adjusted even more, in an inversely proportional relationship in which, with greater intensity of color, the aridity index is lower. The regions with the highest humidity were Hidalgo and Oaxaca, where the tones of the beetles are darker.

A pattern of greater abundance of D. mexicanus was verified in places with high relative humidity; under this pattern, it was observed that the regions of Oaxaca, Jalisco and Hidalgo recorded more numerous populations of the barking beetles studied compared to the state of Nuevo León, whose values in this variable were lower, which coincides with other studies of beetles that also consume wood and that are favored by higher humidity (^{Johansson et al., 2017}; ^{Härkönen and Sorvari, 2018}). It should be noted that in the regions of Oaxaca and Jalisco a lower abundance was recorded with high percentages of humidity, which contradicts the previous assumption; however, this can be explained by the extreme temperatures, which is evident in ectodermal species since these variables influence many of their physiological processes (^{Fields et al., 1992}; ^{Chown and Nicolson, 2004}; ^{Fields et al., 2012}; ^{De la Vega et al., 2015}).

The average temperature recorded in the Hidalgo region (16.3 °C), is associated with a greater abundance of insects; in other studies, it is corroborated that a high abundance of D. mexicanus occurs in sites whose average temperatures are close to 16 °C (^{Hernández-Muñoz et al., 2017}; ^{Morales-Rangel et al., 2018}), which also has been recorded for D. frontalis Zimmerman, 1868 (^{Soto-Correa et al., 2019}). Other research also highlights the complexity of insect responses to thermal variability (^{Chidawanyika et al., 2017}). For example, extreme events in minimum temperatures are the limiting variables (^{Fields, 1992}; ^{De la Vega et al., 2015}) and their variability may be the response to what happened in regions such as Hidalgo and Oaxaca, where a greater abundance of debarkers in these regions with a high relative humidity in the environment and stable average temperatures. In previous works, this link has also been estimated of a lower abundance of some beetles in extreme fluctuating temperatures (^{Jian et al., 2018}).

Oaxaca registered a minimum average temperature of 13.9 °C with a lower difference of 2.6 °C compared to the Hidalgo site; this difference in the minimum temperature may explain the discrepancy in the beetles’ abundance between these sites, as observed in other insect studies in which the lethal temperature begins at 13 °C and below (^{Fields et al., 2012}). In the United States of America, minimum temperatures are considered the determining environmental factor for the exponential presence of debarkers (^{Buotte et al., 2017}).

In the Nuevo León region, the prevailing environmental conditions may be unfavorable for a larger population of D. mexicanus, with low average humidity and wide variation in extreme temperatures (39.6 % maximum on average and 10 °C minimum on average). In regard to the variation of extreme temperatures in the Nuevo León site, the difference was up to 4 °C between the average maximum and minimum temperatures in relation to the site in the Hidalgo region, where the greatest abundance of bark insects in the present study. The maximum average temperature becomes a determining factor in controlling the abundance of D. mexicanus, which may be a consequence of the extremely high average temperatures causing the drying of the tissues and the eventual death of insects (^{De la Vega and Schilman, 2015}). The extremes in temperatures limit the abundance of insects because they are ectotherms (^{Chown and Nicolson, 2004}; ^{De la Vega and Schilman, 2015}).

The proliferation of D. mexicanus is associated with high humidity, which has been explained in various laboratory studies with other beetles, since it has been recorded that high relative humidity avoids mortality due to dehydration (^{Noh et al., 2015}).

On the other hand, in the Hidalgo region variable climatic parameters were registered that favored a greater abundance of the beetles of interest, such as average temperatures around 16 °C (^{Hernández-Muñoz et al., 2017}; ^{Morales-Rangel et al., 2018}; ^{Soto-Correa et al., 2019}) and humidity of 80 %, with little variation in average minimum and maximum temperatures.

The aridity index is useful to observe and interpret how the variables temperature and humidity interact with respect to the abundance of D. mexicanus. Ectothermic species such as beetles are usually related to water balance and thermal balance, factors that influence a greater or lesser abundance of insects (^{Chown and Nicolson, 2004}). On the other hand, it was possible to estimate a relationship between the abundance of D. mexicanus and the difference between the minimum and maximum average aridity indices, where there is a greater abundance of these insects when the difference between the two indexes is less, as has been observed in studies with temperature differences (^{Jian et al., 2018}). The present study may be of great importance since it generates evidence that associated climatic factors determine the abundance of debarking populations within the evaluated forests and not only that dry conditions at the site can increase the abundance of the populations, as estimated in studies of the population dynamics of this type of barking beetle, in addition to the fact the humidity has a lot of influence in the coloration.

Conclusions

There is variation in the intensity of the coloration in the exoskeleton D. mexicanus between individuals from the regions of Nuevo León and Oaxaca, where it is observed that there are individuals with a darker coloration in regions with higher humidity, such as Oaxaca, and a less coloration. It is dark in the region with less humidity as Nuevo León.

The abundance of D. mexicanus is limited by climatic variables such as the occurrence of low humidity, extreme minimum and maximum temperatures, as well as the difference between the maximum and minimum aridity index.