The Asian gypsy moth (Lymantria dispar L.) populations: resistance of eggs to extreme winter temperatures

ABSTRACTGypsy moth Lymantria dispar (GM) is a polyphagous insect and one of the most significant pests in the forests of Eurasia and North America. Accurate information on GM cold hardiness is needed to improve methods for the prediction of population outbreaks, as well as for forecasting possible GM range displacements due to climate change.As a result of laboratory and field studies, we found that the lower lethal temperature (at which all L. dispar asiatica eggs die) range from –29.0 °C to –29.9 °C for three studied populations, and no egg survived cooling to –29.9 °C. These limits agree to within one degree with the previously established cold hardiness limits of the European subspecies L. dispar dispar, which is also found in North America. This coincidence indicates that the lower lethal temperature of L. dispar is conservative.Thus, we found that the Siberian populations of GM inhabit an area where winter temperatures go beyond the limits of egg physiological tolerance, because temperature often fall below –30 °C. Apparently, it is due to the flexibility of ovipositional behavior that L. dispar asiatica survives in Siberia: the lack of physiological tolerance of eggs is compensated by choosing warm biotopes for oviposition. One of the most important factors contributing to the survival of GM eggs in Siberia is the stability of snow cover.SummaryWithin the geographical range of Siberian gypsy moth populations, extreme temperatures go beyond the limits of the physiological tolerance of wintering eggs (–29.9 °C), and their survival depends on the choice of warm biotopes for oviposition.

Behaviour and cold hardiness of the purple stem borer in winter, colonizing more northerly latitudes

To escape or alleviate low temperatures in winter, insects have evolved many behavioral and physiological strategies. The rice pest insect, the purple stem borer, Sesamia inferens (Walker) is currently reported to be expanding their northern distributions and causing damage to summer maize in Xinxiang, China. However, their method of coping with the lower temperature in the new northern breeding area in winter is largely unknown. This paper investigates the overwinter site of S. inferens, and identifies the cold hardiness of larvae collected from a new breeding area in winter and explores a potential distribution based on low temperature threshold and on species distribution model, MaxEnt. The results show that the overwintering location of the S. inferens population is more likely to be underground with increasing latitude and,in the north, with the temperature decreasing, the larvae gradually moved down the corn stalk and drilled completely underground by February 18th. Those who were still above ground were all winterkilled. The cold hardiness test shows the species is a moderate freeze-tolerant one, and Supercooling Points (SCP), Freezing Points (FP) and mortality rate during the middle of winter (January, SCP: −7.653, FP: −6.596) were significantly lower than early winter (October) or late winter (March). Distribution in the new expansion area was predicted and the survival probability area was below N 35° for the Air Lower Lethal Temperature (ALLT50) and below N 40° for the Underground Lower Lethal Temperature (ULLT50), The suitable habitat areas for S. inferens with MaxEnt were also below N 40°. This study suggests the overwinter strategies have led to the colonization of up to a five degree more northerly overwintering latitude. This behavior of S. inferens could help maize producers to propose a control method to increase pest mortality by extracting the maize stubble after harvest.

Cold tolerance of the spring-feeding larvae of the eyespotted bud moth,Spilonota ocellana(Lepidoptera: Tortricidae)

In this study, we explore the low-temperature thresholds of a pest of apple (Malus pumilaMiller; Rosaceae), the eyespotted bud moth,Spilonota ocellanaDenis and Schiffermüller (Lepidoptera: Tortricidae), in the context of spring frost.Spilonota ocellanaoverwinters as a larva, resuming activity and feeding early in the growing season when it could be vulnerable to unexpected freezing temperatures. We determined that the mean supercooling point of spring instars did not differ for larvae within or outside leaf shelters and ranged from −9.1±0.2 °C for fourth instars, to −7.9±0.2 °C for sixth instars. Larval weight increased with instar and was positively related to the supercooling point. As some insects are freeze tolerant and able to recover from freezing, we also exposed larvae to brief freeze events between −4.5 °C and −9.5 °C and found that the median lower lethal temperature, was −7.3±0.4 °C across all instars; indicating thatS. ocellanaspring instars are susceptible to freezing temperatures above their supercooling point. These low-temperature thresholds suggest that in the spring, S. ocellanalarvae are chill susceptible, and a hard frost (<−7 °C) would be necessary to cause significant larval mortality.

Low temperature survival of post-eclosion stages of the potato rot nematode Ditylenchus destructor Thorne (Tylenchida: Anguinidae)

The potato rot nematode, Ditylenchus destructor, may experience extreme environmental stress caused by freezing temperatures when overwintering in the field. A series of laboratory experiments were conducted in order to clarify overwintering strategies of the nematode. The current study aimed to examine the cold tolerance of this species in various aqueous environments as well as in plant tissues in order to determine its ability to survive 24 h exposure to subzero temperatures. Cold tolerance of adults and fourth-stage juveniles was significantly lower than that of younger juveniles. The lower lethal temperature of adults was –15°C. Although external ice formation affects all life stages, a few second-stage juveniles were able to survive temperatures as low as –30°C. The results expand the current understanding of freezing survival in the potato rot nematode and will lead to better comprehension of its ability to withstand subzero temperature conditions.