Date Published: April 23, 2019
Publisher: Public Library of Science
Author(s): Yang Ge, Liu Zhang, Zifang Qin, Yang Wang, Pingping Liu, Shuqian Tan, Zhen Fu, Olivia M. Smith, Wangpeng Shi, Yulin Gao.
Pear psylla, Cacopsylla chinensis (Yang & Li) are present as two seasonal morphotypes in pear orchards where they, suck phloem sap, defoliate pear trees, and cause fruit russet. Despite the importance of natural enemies in psyllid control, the interactions between predators and the two seasonal morphotypes of psyllids remain poorly documented. Here we determined the predation efficiencies of the Asian lady beetle, Harmonia axyridis (Pallas) on the two psyllid morphotypes. Predation of H. axyridis on both morphotypes conformed to a Type II functional response: the proportion of consumed psyllids decreased with increasing prey densities. Predation efficiency of H. axyridis against the winterforms increased with temperature when measured from 8 to 25°C. Predation rate on the summerforms was significantly higher than that of the winterforms. This was linked to smaller body size, higher soluble protein level, thinner cuticle, and lower chitin content of summerform psyllids compared to winterform psyllids. Predation capacities of H. axyridis on both morphotypes indicated its potential as a biocontrol agent in psyllids management. Predation efficiency was higher on summerforms, likely due to the difference in body size, exoskeleton fragility, and nutritional value between the two morphotypes. Due to the Type II functional response of H. axyridis to both morphotypes of pear psylla, application of H. axyridis in pear orchards under suitable temperatures could be taken into consideration for suppression of C. chinensis, although further experiments conducted in field conditions are needed to validate our findings.
Pear trees are attacked by multiple herbivorous insects , including the phloem-sucking pear psyllid, Cacopsylla chinensis (Yang & Li). C. chinensis inflict devastating damage to trees and fruits in a number of East Asian countries, including China and Japan [2–3]. Damage caused by C. chinensis includes stunting and wilting of pear trees due to toxic saliva injecting into the trees . Sooty mold fungus that lives on honeydew excreted by pear psylla reduces the photosynthesis rate of pear leaves . Additionally, C. chinensis induces infection by plant pathogens [6–7]. Recently, it was reported that C. chinensis was responsible for vectoring phytoplasma, causing pear decline disease in Taiwan of China (PDTW) . As one of the most impactful insect pests in the pear industry in East Asia, it causes great economic losses to growers . However, current management of pear psylla in many countries remains primarily dependent on heavy usage of synthetic chemical insecticides [7, 9]. Meanwhile, pear psyllids have developed resistance to synthetic chemical insecticides likely due to their host specificity and high reproductive capacity . From the socio-environmental perspective, application of chemical insecticides often raise concerns about health risks due to pesticide residues on crops and pesticide runoff into watersheds and other surrounding ecosystems. Therefore, eco-friendly alternative management strategies such as the incorporation of natural plant compounds , processed kaolin , and the introduction of natural enemies have attracted the attention of researchers and growers [8,13].
In this study, we examined the functional response of H. axyridis, a ubiquitous predator of the two morphotypes of C. chinensis. Predation of H. axyridis on winterform C. chinensis was positively related to increases in temperature from 8°C to 25°C, as is also the case in other predaceous insects . Our results have shown that both the morphotype of C. chinensis and temperature impact the functional response of H. axyridis. However, our experiments on variation in predation capacity under different temperature conditions were limited to. Further experiments on the effect of temperature on predation of summerform C. chinensis are still required.
Predation capabilities of H. axyridis on winterform C. chinensis were studied under different early-season temperatures and results indicated high consumption of C. chinensis at 8°C and 15°C. Higher predation capacity of summerform C. chinensis was found than that of winterforms, shown as higher attack rate, shorter handling time as well as higher theoretical maximum number consumed by H. axyridis. We found that the stronger predation of H. axyridis on summerforms was likely due to the higher nutritional returns, e.g. higher protein content and less foraging costs when dealing with summerforms, smaller size, and softer and thinner exoskeleton. The high predation capacities of H. axyridis on both morphotypes of psyllids indicated its great potential as a biocontrol agent of C. chinensis. Based on the inverse density-dependent predation of type II functional response and the great predation capacities of H. axyridis on the two morphotypes, the control of C. chinensis should start early in the season to suppress the winterforms by inundative release. Further study of the control efficiency of two morphotypes of C. chinensis by H. axyridis in the field is necessary to extend our findings to on-farm pear psylla management.