Overwintering, Cold Tolerance and Supercooling Capacity Comparison Between Liriomyza Sativae and L. Trifolii, Two Successively Invaded Leafminers in China

Liriomyza sativae Blanchard and Liriomyza trifolii (Burgess) are two highly polyphagous pests that successively invaded China in the 1990s and 2000s, respectively, threatening vegetable and horticultural plants. Competitive displacement of L. sativae by L. trifolii occurred during the expansion process of the latter in southern China. However, whether L. trifolii can expand their range to northern China and, if so, how they compete with L. sativae in northern China remains unclear. Overwintering and cold tolerance capacity largely determine the species distribution range and can affect species displacement through overwintering and phenology. In this study, we compared the overwintering potential, cold tolerance and supercooling point (SCP) between these two leafminer species. Our results showed that L. trifolii can overwinter at higher altitudes than L. sativae. In addition, we found that they can both successfully overwinter in greenhouses in northern China, and the overwintering capacity of L. trifolii was higher than that of L. sativae. Moreover, the extreme low-temperature survival of L. trifolii was significantly higher than that of L. sativae, and the SCP of the former was lower than that of the latter. We thus conclude that the overwintering and cold tolerance capacity of L. trifolii is much better than that of L. sativae. Our findings indicate that L. trifolii has the potential to displace L. sativae and expand its range to northern China. Moreover, our results have important implications for predicting overwinter ranges and developing management strategies for invasive leafminers in China.

Factors Influencing Cold Hardiness during Overwintering of Streltzoviella insularis (Lepidoptera: Cossidae)

Streltzoviella insularis (Staudinger) (Lepidoptera: Cossidae) is a woodboring pest that severely damages urban and plain afforestation trees in northern China. Cold hardiness is an important strategy for the insect to survived during low winter temperatures. Understanding the strategy of S. insularis might provide insights for pest management approaches. To assess the key factors affecting cold hardiness, we measured the supercooling point, freezing point, total water content, total fat content, glycogen content, and total protein content of overwintering larvae. The relationships between supercooling points, temperature, body size, and nutrients were analyzed. The results showed that the supercooling point and freezing point of the larvae decreased first, reached the lowest point in January, and then increased during the rest of the overwintering period. The supercooling point positively correlated with the daily average temperature and the daily minimum temperature. Total lipid content negatively correlated with the supercooling point, while glycogen content had a significant positive correlation with the supercooling point. The temperature may have a major impact on cold hardiness, whereas individual body size may have no significant influence over cold tolerance. During the overwintering process, glycogen and total lipid contents may directly affect cold hardiness. Therefore, the lipid and carbohydrate metabolism may play a role in the cold tolerance of S. insularis larvae. This study provides a physiological and biochemical basis for future metabolic studies on S. insularis larva and the research of overwintering strategies.

Assessing the supercooling of fresh-cut onions at −5°C using electrical impedance analysis

We supercooled fresh-cut onion at −5°C for 12 h. After supercooling, the electric impedance properties of the samples were evaluated by electrical impedance spectroscopy over the frequency range of 42 Hz − 5 MHz. The time-temperature profiles of samples indicated that the freezing point and supercooling point were −2.3°C ± 0.7°C and −6.9°C ± 1.0°C, respectively. The results indicated that 34 of the 36 supercooled samples exhibited a definite circular arc in the Cole-Cole plot, which suggested that the cell membrane remained intact during supercooling. In the other two samples which did not exhibit a definite circular arc, the cell membrane had sustained serious damage during supercooling. Furthermore, there was large difference in drip loss percentage between supercooled samples exhibited a definite circular arc in the Cole-Cole plot and samples not exhibiting a definite circular arc. Our results suggest that fresh-cut onions can be supercooled at −5°C.

Temperature Limits for the Brown Wheat Mite, in Colorado

Brown wheat mites, Petrobia latens (Müller 1776, Acari: Tetranychidae), are sporadic yet economically damaging pests of winter cereals. In Colorado, their life history is closely tied to the development of winter wheat, where they are present in the field from crop planting in late September through harvest in early June. In order to withstand winter months, these mites are able to survive cold temperatures. However, the mechanisms of cold hardening and their temperature limits are unknown. This research documents the seasonal supercooling points of the brown wheat mite. Their seasonal average supercooling point stayed consistent throughout the year, never varying more than a degree from the overall average supercooling point of −17°C. The greatest variation in supercooling point was seen in the spring, during which supercooling point temperatures ranged from −9.2 to −25.5°C. We also documented the upper and lower lethal temperatures for the brown wheat mite. When comparing small nymphs to large nymph and adult stages, small nymphs were slightly more cold tolerant (lethal temperature estimates required to kill 99% of the population [LT99] were −30.8 and −30.6°C, respectively), but less heat tolerant (LT99 was 50 and 56°C, respectively).

Extreme cold weather causes the collapse of a population of Lambdina fiscellaria (Lepidoptera: Geometridae) in the Laurentian Mountains of Québec, Canada

In 2012, an unexpected outbreak of Lambdina fiscellaria (Guenée) (Lepidoptera: Geometridae) occurred in the Laurentian Mountains, Québec, Canada, known for its harsh climate. We wondered whether the eggs were sufficiently cold hardy to survive there and, if so, how long this outbreak would last. Therefore, we assessed the capacity of the eggs to supercool, to tolerate short exposures to low sub-zero temperatures, or to successfully overwinter in the field. The same assays were performed with eggs from the island of Newfoundland, Newfoundland and Labrador, Canada. The mean supercooling point of eggs from the two populations increased from −40.2 °C in mid-February to −33.7 °C in mid-May. These eggs may also die at sub-zero temperatures above their supercooling point, depending on exposure durations. In the fall of 2012 when eggs were put out in the field, < 10% survived in the Laurentian Mountains, whereas > 70% survived further south. In the spring of 2013, no parasitism was detected in the population. However, the two cold waves that swept across the Laurentian Mountains the preceding winter were likely responsible for the collapse of the population. This study demonstrates that L. fiscellaria eggs may succumb to sub-zero temperatures above their supercooling point under field conditions.

Climatic Variation of Supercooling Point in the Linden Bug Pyrrhocoris apterus (Heteroptera: Pyrrhocoridae)

Cold tolerance is often one of the key components of insect fitness, but the association between climatic conditions and supercooling capacity is poorly understood. We tested 16 lines originating from geographically different populations of the linden bug Pyrrhocoris apterus for their cold tolerance, determined as the supercooling point (SCP). The supercooling point was generally well explained by the climatic conditions of the population’s origin, as the best predictor—winter minimum temperature—explained 85% of the average SCP variation between populations. The supercooling capacity of P. apterus is strongly correlated with climatic conditions, which support the usage of SCP as an appropriate metric of cold tolerance in this species.