Assessment and Models of Insect Damage to Cones and Seeds of Pinus strobiformis in the Sierra Madre Occidental, Mexico

Abstract

Insect damage to cones and seeds has a strong impact on the regeneration of conifer forest ecosystems, with broader implications for ecological and economic services. Lack of control of insect populations can lead to important economic and environmental losses. Pinus strobiformis is the most widespread of the white pines in Mexico and is widely distributed throughout the mountains of northern Mexico. Relatively few studies have examined insect damage to the cones and seeds of these pines, especially in Mexico. In this study, we therefore analyzed insect damage to cones and seeds of P. strobiformis in Mexico by using X-ray and stereomicroscopic analysis. The specific objectives of the study were (a) to characterize insect damage by measuring external and internal cone traits, (b) to assess the health of seeds and cones of P. strobiformis in the Sierra Madre Occidental, Mexico, and (c) to estimate the relative importance of the effects of different environmental variables on cone and seed damage caused by insects. We found that 80% of P. strobiformis seeds and 100% of the tree populations studied had damage caused by insects. Most seeds were affected by Leptoglossus occidentalis, Tetyra bipunctata, Megastigmus albifrons, and the Lepidoptera complex (which includes Apolychrosis synchysis, Cydia latisigna, Eucosma bobana, and Dioryctria abietivorella). The cones of all tree populations were affected by some type of insect damage, with Lepidoptera causing most of the damage (72%), followed by Conophthorus ponderosae (15%), the hemipteran L. occidentalis (7%), and the wasp M. albifrons (6%). The proportion of incomplete seeds in P. strobiformis at the tree level, cone damage by M. albifrons and seed damage in L. occidentalis were associated with various climate and soil variables and with crown dieback. Thus, cone and seed insect damage can be severe and potentially impact seed production in P. strobiformis and the reforestation potential of the species. The study findings will enable managers to better identify insects that cause damage to cone and seeds. In addition, identification of factors associated with damage may be useful for predicting the levels of insect predation on seeds and cones.

Keywords: X-ray; machine learning; regression analyses; seed production; stereomicroscopic analysis.

Figure 1

Distribution of Pinus strobiformis (brown outlined areas) and sample collection sites: populations (x40) (green triangles) in which 192 trees were sampled; distribution map based on Shirk et al. (2018). Base digital elevation map from Jarvis et al. (2008).

Figure 2

Identification guide to types of damage in Pinus strobiformis seeds.

Figure 3

Examples of insects and the damage they cause in cones of Pinus strobiformis: (A) adult Leptoglossus occidentalis (Heidemann) (Photo: Álvarez-Zagoya, 2019), (B) adult Megastigmus albifrons (Walker) (Photo: Álvarez-Zagoya, 2019), (C) damage with frass by Lepidoptera [Apolychrosis synchysis (Pogue), Cydia latisigna (Miller), or Eucosma bobana (Kearfott), Dioryctria abietivorella (Grote)], (D) damage by Conophthorus ponderosae (Hopkins).

Figure 4

Insect damage in 1,920 seeds from 40 Pinus strobiformis populations sampled and studied at the seed (black) and insect population level (gray).

Figure 5

Percentage of cones damaged by different types of insect, determined in five randomly chosen cones per tree population studied (40 populations, total 200 cones). Note that some cones were damaged by more than one insect species.