Shuguang Hao

Heavy Livestock Grazing Promotes Locust Outbreaks by Lowering Plant Nitrogen Content

Locust Heaven Locust outbreaks have severe consequences for agriculture, but the conditions that promote an outbreak are unknown. Cease et al. (p. 467) investigated aspects of the locust diet and found that increased nitrogen content of cereal grasses reduced the size and viability of a herbivorous locust species. This locust prefers low N plants, which result from heavy grazing by livestock and erosion. High-protein plants inhibit locust swarming, which explains why grazed systems are more prone to outbreaks. Current paradigms generally assume that increased plant nitrogen (N) should enhance herbivore performance by relieving protein limitation, increasing herbivorous insect populations. We show, in contrast to this scenario, that host plant N enrichment and high-protein artificial diets decreased the size and viability of Oedaleus asiaticus, a dominant locust of north Asian grasslands. This locust preferred plants with low N content and artificial diets with low protein and high carbohydrate content. Plant N content was lowest and locust abundance highest in heavily livestock-grazed fields where soils were N-depleted, likely due to enhanced erosion. These results suggest that heavy livestock grazing and consequent steppe degradation in the Eurasian grassland promote outbreaks of this locust by reducing plant protein content.

Are color or high rearing density related to migratory polyphenism in the band-winged grasshopper, Oedaleus asiaticus?

Locusts represent an impressive example of migratory polyphenism, with high densities triggering a switch from a solitarious, shorter dispersal range, and sometimes greenish phenotype to a gregarious and sometimes darker form exhibiting behavioral, morphological and physiological traits associated with long-distance migratory swarms. While such polyphenism has been well documented in Locusta migratoria and Schistocerca gregaria, the extent to which other grasshoppers exhibit this type of migratory polyphenism is unclear. Anecdotally, the Chinese grasshopper, Oedaleus asiaticus, forms migratory swarms comprised mostly of a darker, brown-colored morph, but also exhibits a non-migratory green-colored morph that predominates at low densities. In a population in Inner Mongolia not currently exhibiting migratory swarms, we found that while green and brown O. asiaticus are found concurrently across our sampled range, only brown grasshoppers were found in high densities. Differences between field-collected brown and green forms matched some but not key predictions associated with the hypothesis that the brown form is morphologically and physiologically specialized for gregarious migration. Controlling for body mass, brown forms had more massive thoraxes, abdomens and legs, and higher metabolic rates, but not more flight muscle or lipid stores. Further, the brown and green grasshoppers did not differ in gregarious behavior, and neither would fly in multiple lab and field trials. Lab or field-rearing at high densities for one-to-multiple juvenile instars caused grasshoppers to exhibit some morphological traits predicted to benefit migration (larger wings and a shift in relative mass from abdomen to thorax), but did not change color or induce flight behavior. One hypothesis to explain these data is that a migratory form of O. asiaticus is partially triggered by high field densities, but that existing ecological conditions blocked full expression of such traits (and outbreak swarms). Alternatively, color variation in this species may more tightly linked to other functions in this species such as crypsis or disease resistance, and mechanisms other than late-juvenile rearing density (e.g. genetic variation, maternal effects) may be more critical for promoting variation in color and/or migratory polyphenism.

Seasonal variation in cold-hardiness of the Japanese pine sawyer Monochamus alternatus (Coleoptera: Cerambycidae)

Abstract The cold-hardiness of fourth-fifth instars of the Japanese pine sawyer, Monochamus alternatus Hope, collected from 25 to 28 April in spring, 28 June to 1 July in summer, 29 October to 1 November in autumn, and 28 December to 1 January in winter in Anhui Province, China, was compared. Parameters compared were supercooling point (SCP), lethal temperature causing 50% mortality (LT50), upper limit of cold injury zone above which temperature does not cause mortality even after an ecologically meaningful period of time (ULCIZ), sum of injurious temperature that is the duration of effective chilling that results in 50% mortality (SIT), and acclimation efficiency. The mean SCP values of summer, autumn, winter, and spring larvae were −6.2, −10.9, −15.4, and −12.3°C, respectively. The cold-hardiness of larvae collected in different seasons was consistent with seasonal air temperature in the wild. There was a significant positive correlation between SCP and LT50 (r = 0.979, P < 0.05), between SCP and monthly mean air temperature (r = 0.990, P < 0.01), and between SCP and monthly mean absolute minimum air temperature (r = 0.995, P < 0.01). ULCIZ and SIT for autumn, winter, and spring larvae were 4.9, 9.7, and 4.7°C and 1.356, 16.142, and 5.323 DD, respectively. Autumn larvae displayed significant acclimation efficiency at 5 and 0°C; however, exposure to −5°C did not enhance the cold-hardiness of larvae from any season. Cold-hardiness of M. alternatus larvae seems to increase in autumn, peak in winter, and decline in spring. Results suggest that M. alternatus drops SCP through acclimation in autumn to avoid freezing and reduces ULCIZ or increase SIT to increase its chilling tolerance at low temperature in winter.

Postdiapause Development and Hatching Rate of Three Grasshopper Species (Orthoptera: Acrididae) in Inner Mongolia

Abstract The postdiapause development, hatching characteristics, and survival of the overwintering eggs of the grasshopper species Oedaleus asiaticus Bei-Bienko, Angaracris barabensis (Pallas), and Chorthippus dubius (Zubowsky) from the Inner Mogolian steppe grasslands were studied at six constant temperatures (15, 20, 25, 30, 35, and 40°C). The results show that the three species had different postdiapause embryonic developmental rates, survival curves, and cumulative hatching probabilities. O. asiaticus had the highest developmental threshold at 13.1°C and the least effective accumulated thermal unit, 191.7 degree-days (DD). A. barabensis had the lowest developmental threshold at 11.1°C and the most effective accumulated thermal unit, 329.7 DD. C. dubius, a later hatching species, had a moderate developmental threshold of 11.4°C and an effective accumulated thermal unit of 230.3 DD. The effective thermal unit at which 50% of postdiapause eggs hatched was 202.7 DD for O. asiaticus, 340.8 DD for A. barabensis, and 251.6 DD for C. dubius. The greatest percentage hatch of O. asiaticus (71.06%), A. barabensis (75.28%), and C. dubius (87%) occurred at 27.3, 27.8, and 27.4°C, respectively. Thermal death points of the three grasshopper species were 41.0°C, 40.5, and 44.8°C, respectively. The optimal temperature ranges of each species were different, 21.9 to 32.7°C for O. asiaticus, 22.0 to 33.5°C for A. barabensis, and 17.8 to 37.1°C for C. dubius. These results suggest that postdiapause embryonic developmental rate and accumulated heat cannot help to explain the different hatching sequences of these species. Much variation in springtime emergence could be attributed to the species-specific overwintering egg stage. In addition, results also indicate that C. dubius has a wider adaptive temperature range than O. asiaticus and A. barabensis.

Specificity responses of grasshoppers in temperate grasslands to diel asymmetric warming.

Background Global warming is characterized by not only an increase in the daily mean temperature, but also a diel asymmetric pattern. However, most of the current studies on climate change have only concerned with the mean values of the warming trend. Although many studies have been conducted concerning the responses of insects to climate change, studies that address the issue of diel asymmetric warming under field conditions are not found in the literature. Methodology/Principal Findings We conducted a field climate manipulative experiment and investigated developmental and demographic responses to diel asymmetric warming in three grasshopper species (an early-season species Dasyhippus barbipes, a mid-season species Oedaleus asiaticus, and a late-season species Chorthippus fallax). It was found that warming generally advanced the development of eggs and nymphs, but had no apparent impacts on the hatching rate of eggs, the emergence rate of nymphs and the survival and fecundity of adults in all the three species. Nighttime warming was more effective in advancing egg development than the daytime warming. The emergence time of adults was differentially advanced by warming in the three species; it was advanced by 5.64 days in C. fallax, 3.55 days in O. asiaticus, and 1.96 days in D. barbipes. This phenological advancement was associated with increases in the effective GDDs accumulation. Conclusions/Significance Results in this study indicate that the responses of the three grasshopper species to warming are influenced by several factors, including species traits, developmental stage, and the thermal sensitivity of the species. Moreover, species with diapausing eggs are less responsive to changes in temperature regimes, suggesting that development of diapausing eggs is a protective mechanism in early-season grasshopper for avoiding the risk of pre-winter hatching. Our results highlight the need to consider the complex relationships between climate change and specificity responses of invertebrates.

Nutritional imbalance suppresses migratory phenotypes of the Mongolian locust (Oedaleus asiaticus)

For many species, migration evolves to allow organisms to access better resources. However, the proximate factors that trigger these developmental changes, and how and why these vary across species, remain poorly understood. One prominent hypothesis is that poor-quality food promotes development of migratory phenotypes and this has been clearly shown for some polyphenic insects. In other animals, particularly long-distance bird migrants, it is clear that high-quality food is required to prepare animals for a successful migration. We tested the effect of diet quality on the flight behaviour and morphology of the Mongolian locust, Oedaleus asiaticus. Locusts reared at high population density and fed low-N grass (performance-enhancing for this species) had enhanced migratory morphology relative to locusts fed high-N grass. Furthermore, locusts fed synthetic diets with an optimal 1 : 2 protein : carbohydrate ratio flew for longer times than locusts fed diets with lower or higher protein : carbohydrate ratios. In contrast to the hypothesis that performance-degrading food should enhance migration, our results support the more nuanced hypothesis that high-quality diets promote development of migratory characteristics when migration is physiologically challenging.

Cold tolerance and cold hardening strategy of the Japanese pine sawyer Monochamus alternatus (Coleoptera: Cerambycidae)

The Japanese pine sawyer, Monochamus alternatus, is an important pine forest pest and vector transmitting the pine wilt nematode that causes pine wilt disease. Low temperatures in autumn, winter and spring often differentially affect mortality of M. alternatus larvae. In this paper, we mainly compared the differences of mortality and cold hardening of larvae from different seasons, based on supercooling point (SCP) and cumulative probability of individuals freezing (CPIF). The cold hardening of the larvae from autumn, winter and spring seasons were largely different. Correlations between mortality and CPIF of autumn and spring larvae were highest on day 1/4, and gradually decreased with prolonged exposure duration. This beetles death mainly resulted from freezing in short exposure duration. However, the correlation between mortality and CPIF of winter larvae increased gradually with the prolonged exposure duration. Death did not mainly result from freezing in long exposure duration. Autumn larvae are more susceptible and adaptable than winter and spring larvae. Winter larvae have a slight freeze‐tolerance trend. Our research showed that M. alternatus came into complex cold‐hardening strategies under natural selection. Freeze avoidance is the primary strategy; with prolonged exposure duration to above SCP or < 0 °C, chill tolerance is more important; this is followed by freeze tolerance during harsh winters.