Thirst or Malnutrition: The Impacts of Invasive Insect Agrilus mali on the Physiological Status of Wild Apple Trees

Field Investigation into Tree Fates from Recent Apple Tree Decline: Abrupt Hydraulic Failure Versus Gradual Hydraulic Loss

In the last decade, a sporadic tree health syndrome affecting high-density apple plantings in North America has become known as Rapid Apple Decline (RAD) or Sudden Apple Decline (SAD). The affected apple trees were typically grafted on small dwarfing rootstocks, often displayed necrosis at the graft union, and suffered from sudden mortality that occurred over 2–3 weeks amid the growing season or a gradual decline. In 2019 and 2020, we conducted a multi-site investigation in the south Okanagan, British Columbia, Canada, to assess the stem hydraulic characteristics, stomatal conductance, leaf δ13C‰, and fruit dry matter accumulation of the declining trees during disease progression. In trees that died, mortality appeared to be associated with severe disruption in xylem water transport at the damaged graft union, followed by abrupt hydraulic failure. In contrast, symptomatic trees that did not die exhibited the moderately declined plant water relations and a reduction in fruit dry matter accumulation followed by either further deterioration or eventual recovery. This pattern indicates the risk of carbohydrate depletion over gradual hydraulic decline and the importance of timely horticultural remedies. In the present study, we discuss potential horticultural practices to mitigate hydraulic dysfunctions and enhance crop tolerance.

Malice at the Gates of Eden: current and future distribution of Agrilus mali threatening wild and domestic apples

The apple buprestid, Agrilus mali Matsumura, that was widespread in north-eastern China, was accidently introduced to the wild apple forest ecosystem in mountainous areas of Xinjiang, China. This invasive beetle feeds on domesticated apples and many species of Malus and presents a serious threat to ancestral apple germplasm sources and apple production worldwide. Estimating the potential area at risk of colonization by A. mali is crucial for instigating appropriate preventative management strategies, especially under global warming. We developed a CLIMEX model of A. mali to project this pest's potential distribution under current and future climatic scenarios in 2100 using CSIRO-Mk 3.0 GCM running the SRES A1B emissions scenario. Under current climate, A. mali could potentially invade neighbouring central Asia and eventually the mid-latitude temperate zone, and some subtropical areas and Pampas Steppe in the Southern Hemisphere. This potential distribution encompasses wild apples species, the ancestral germplasm for domesticated apples. With global warming, the potential distribution shifts to higher latitudes, with the potential range expanding slightly, though the overall suitability could decline in both hemispheres. In 2100, the length of the growing season of this pest in the mid-latitude temperature zone could increase by 1-2 weeks, with higher growth rates in most sites compared with current climate in mid-latitudes, at least in China. Our work highlights the need for strategies to prevent the spread of this pest, managing the threats to wild apples in Tian Shan Mountain forests in Central Asia, and commercial apple production globally. We discuss practical management tactics to reduce the spread of this pest and mitigate its impacts.

Invasive Insect Pests of Forests and Urban Trees in Russia: Origin, Pathways, Damage, and Management

Invasive alien insects cause serious ecological and economical losses around the world. Here, we review the bionomics, modern ranges (and their dynamics), distribution pathways, monitoring, and control measures of 14 insect species known to be important invasive and emerging tree pests in forest and urban ecosystems of Russia: Leptoglossus occidentalis (Hemiptera: Heteroptera: Coreidae), Halyomorpha halys (Hemiptera: Heteroptera: Pentatomidae), Corythucha arcuata (Hemiptera: Heteroptera: Tingidae), Agrilus fleischeri, A. mali, A. planipennis, Lamprodila (Palmar) festiva (Coleoptera: Buprestidae), Ips amitinus, Polygraphus proximus (Coleoptera: Curculionidae: Scolytinae), Cydalima perspectalis (Lepidoptera: Crambidae), Acrocercops brongniardella, Cameraria ohridella, Phyllonorycter issikii, and P. populifoliella (Lepidoptera: Gracillariidae). We identified three major scenarios of tree pest invasions in the country and beyond: (1) a naturally conditioned range expansion, which results in the arrival of a pest to a new territory and its further naturalization in a recipient region; (2) a human-mediated, long-distance transfer of a pest to a new territory and its further naturalization; and (3) a widening of the pest’s trophic niche and shift to new host plant(s) (commonly human-introduced) within the native pest’s range frequently followed by invasion to new regions.

Current Distribution and Diagnostic Features of Two Potentially Invasive Asian Buprestid Species: Agrilus mali Matsumura and A. fleischeri Obenberger (Coleoptera: Buprestidae)

Simple Summary

Knowledge of the diagnostic features and native ranges of invasive pests is vital for their correct identification and monitoring. In this regard, the diagnostic characters and geographical ranges of two potentially invasive Asian buprestid species: the quarantine apple tree pest, Agrilus mali Matsumura, and the poplar pest A. fleischeri Obenberger are studied and analyzed. Based on the examination of museum collections and literature sources, the diagnostic characters to distinguish both species from their congeners are discussed, the comprehensive databases of records of the exact collecting sites are compiled, and detailed maps of their ranges are generated. Occurrence of A. mali in Japan is not confirmed. Outbreak sites of A. mali in Xinjiang most likely represent the newly forming invasion areas; their proximity to the wild apple stands in the Kazakh part of the Tien Shan is a direct threat to Kazakhstan and adjacent countries. Sites damaged by A. fleischeri in Liaoning are situated within its native range; the outbreaks were likely triggered by the switch from indigenous to introduced poplars. The results of the study will facilitate the correct identification and monitoring of the pests in case of their findings in new areas.

Abstract

Our goal is to analyze the known geographical ranges and diagnostic features of two potentially invasive Asian buprestid species: the quarantine apple tree pest, Agrilus mali Matsumura, and the poplar pest A. fleischeri Obenberger. Based on the examination of museum collections and literature sources, we compiled comprehensive databases of records of the exact collecting sites for both species and generated detailed maps of their ranges. There are 51 documented localities for A. mali in the Russian Far East and East Siberia, Mongolia, China, and the Korean peninsula, and there are 53 documented localities for A. fleischeri in the Far East and Siberia, Kazakhstan, Mongolia, China, and Japan. No evidence of the presence of A. mali in Japan was found. Outbreak sites of A. mali in Xinjiang in the 2000s most likely represent the newly forming invasion areas; their proximity to the wild apple stands in the Kazakh part of the Tien Shan is a direct threat to Kazakhstan and adjacent countries. Sites damaged by A. fleischeri in Liaoning are situated within its native range; the outbreaks were likely triggered by the switch from indigenous to introduced poplars. This situation is similar to the early stages of emerald ash borer (Agrilus planipennis Fairmaire) invasion.