Why western spruce budworms travel so far for the winter

1. Conifer‐feeding budworms (Choristoneura) hibernate in sheltered locations on their host trees from late summer of 1 year to spring of the next. During this period, they do not feed but rely on sustenance provided in the egg. Overwinter survival is dependent on the rate of consumption of these limited reserves. 2. A process model was developed that quantifies the relationship between the rate of consumption and survival at variable temperatures and exposure times for western spruce budworm. The model supported physiological evidence that warm weather conditions early in the diapause period have a dominant influence on overwinter survival. Output compared favourably with field observations of poorer budworm survival at lower elevations where late‐summer and autumn temperatures were warmer compared to those overwintering at cooler, higher elevations. 3. Field experiments demonstrated these weather‐dependent rates of survival were modulated significantly by the degree of shelter experienced by hibernating budworms. 4. Dissection of whole trees harbouring overwintering western spruce budworms showed a significant portion of the population had travelled a considerable distance from the periphery of the tree canopy where eggs were laid to overwinter successfully on the tree bole where sheltered niches are common. 5. Thus, budworms will travel relatively long distances and risk increased mortality during this dispersal to find adequate shelter to overwinter.     

Distribution of Norway spruce bark and wood‐boring beetles along Alpine elevational gradients

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Two sides of a coin: host‐plant synchrony fitness trade‐offs in the population dynamics of the western spruce budworm

Conifer‐feeding budworms emerge from overwintering sites as small larvae in early spring, several days before budburst, and mine old needles. These early‐emerging larvae suffer considerable mortality during this foraging period as they disperse in search of available, current‐year buds. Once buds flush, surviving budworms construct feeding shelters and must complete maturation before fresh host foliage senesces and lignifies later in the summer. Late‐developing larvae suffer greater mortality and survivors have lower fecundity when feeding on older foliage. Thus, there is a seasonal trade‐off in fitness associated with host synchrony: early‐emerging budworms have a greater risk of mortality during spring dispersal but gain better access to the most nutritious foliage, while, on the other hand, late‐emerging larvae incur a lower risk during the initial foraging period but must contend with rapidly diminishing resource quality at the end of the feeding period. We investigate the balance that results from these early‐season and late‐season synchrony fitness trade‐offs using the concept of the phenological window. Parameters associated with the variation in the phenological window are used to estimate generational fitness as a function of host‐plant synchrony. Because defoliation modifies these relationships, it is also included in the analysis. We show that fitness trade‐offs characterizing the phenological window result in a robust synchrony relationship between budworm and host plant over a wide geographic range in southern British Columbia, Canada.     

Season and drought stress mediate growth and weight of the green spruce aphid on Sitka spruce

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Carry over effects of the entomopathogen Bacillus thuringiensis ssp. Kurstaki on Choristoneura fumiferana (Lepidoptera: Tortricidae) progeny under various stressful environmental conditions

Abstract

• 1 In the present study, we documented the lethal and sublethal effects of the entomopathogen Btk on spruce budworm and its progeny under various environmental conditions. We hypothesized that aerial spray of Bacillus thuringiensis ssp. kurstaki (Btk) could affect the biological performance of the surviving spruce budworm (Choristoneura fumiferana Clem.) populations and their progenies and that Btk sublethal effects could be widened by other types of stress (i.e. temperature conditions and changes in food suitability from year to year).
• 2 The results from a 3‐year field experiment indicated that Btk treatments decreased the fitness of the surviving larvae whatever the prevailing temperature and nutritional conditions.
• 3 The detrimental Btk effects on the parental generation carried over to the offspring. The percent of egg hatch and first‐instar survival were negatively affected by Btk whatever other stress spruce budworm parents underwent.
• 4 The present study also highlighted the fact that the effects of temperature and nutritional stress suffered by the parents could carry over to the next generation. Balsam fir flowering, which provided larvae with pollen rich in nitrogen, favoured both the parental generation and the fitness of their offspring. Spruce budworm mothers allocated to their progenies large amounts of energy reserves (triglycerides and glycogen) that greatly enhanced the survival of the early stages.
• 5 Egg hatch and the survival of first‐instar larval progeny were drastically affected when their parents had reduced larval growth as a result of exposure to cool temperatures that had desynchronized insect and bud phenology.
• 6 Budworm mothers submitted to negative impacts of previous defoliation allocated low amounts of energy reserves to their progeny. This lack of energy associated with unfavourable temperature conditions (i.e. high temperatures in late summer and in early fall and an extended cool period in spring) drastically reduced survival of diapausing second‐instar larvae.
• 7 These results highlight the importance of considering the various sources of stress when attempting to evaluate the impact of a control agent on an insect pest population and its progenies.

     

Prevalence,transmission and mortality associated with Nosema fumiferanae infections in field populations of spruce budworm Choristoneura fumiferana

• 1 The prevalence, intensity and transmission of Nosema fumiferanae (Thomson) (Microsporidae) infections and potential impacts on the survival of field populations of spruce budworm Choristoneura fumiferana (Clem.) were examined in three plots in New Brunswick, Canada, from 1983 to 1992.
• 2 The highest prevalence of N. fumiferanae infection in post‐hibernation second‐instar larvae occurred in the plot where prevalence in female pupae was the highest in the previous generation, suggesting higher rates of vertical transmission. There was little change in the prevalence of N. fumiferanae infections between the second and sixth instars in the later generations. In the two other plots, N. fumiferanae prevalence increased by approximately 25% from the second to sixth larval stadia. Coincident with the changes in N. fumiferanae prevalence were substantial declines in the populations of spruce budworms, making it difficult to determine rates of horizontal transfer of the disease.
• 3 In all plots and in all years, there were progressive increases in the intensity of N. fumiferanae infections (spore loads/individual) from the second to sixth instars and pupae.
• 4 Annual spruce budworm mortality associated with N. fumiferanae was ≤15% of all mortality in reared specimens and was positively correlated with but generally less than 30% of annual N. fumiferanae prevalence.

     

Infection of the fittest: devil facial tumour disease has greatest effect on individuals with highest reproductive output

Emerging infectious diseases rarely affect all members of a population equally and determining how individuals’ susceptibility to infection is related to other components of their fitness is critical to understanding disease impacts at a population level and for predicting evolutionary trajectories. We introduce a novel state‐space model framework to investigate survival and fecundity of Tasmanian devils (Sarcophilus harrisii) affected by a transmissible cancer, devil facial tumour disease. We show that those devils that become host to tumours have otherwise greater fitness, with higher survival and fecundity rates prior to disease‐induced death than non‐host individuals that do not become infected, although high tumour loads lead to high mortality. Our finding that individuals with the greatest reproductive value are those most affected by the cancer demonstrates the need to quantify both survival and fecundity in context of disease progression for understanding the impact of disease on wildlife populations.     

Comparisons of mountain pine beetle (Dendroctonus ponderosae Hopkins) reproduction within a novel and traditional host: effects of insect natal history,colonized host species and competitors

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Influence of host size on oviposition behaviour and fitness of Elachertus cacoeciae attacking a low-density population of spruce budworm Choristoneura fumiferana larvae

1. Oviposition behaviour and host size ? fitness relationships of a gregarious, idiobiont ectoparasitoid, Elachertus cacoeciae (Howard) (Hymenoptera: Eulophidae), were studied by implanting one fourth‐, fifth‐, and sixth‐instar spruce budworm Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae) larva per tree in a stand where the density of the wild C. fumiferana population was low. 2. Development time of E. cacoeciae larvae was quickest on fifth‐instar C. fumiferana larvae, which were the preferred hosts for oviposition. 3. Brood sex ratio (proportion of females) was related positively to increasing C. fumiferana instar, indicating that more females were laid on larger hosts. 4. Parasitoid offspring size increased with increasing C. fumiferana instar and decreased with increasing brood size on smaller hosts. Female but not male size was related positively to increasing brood sex ratio (proportion of females). 5. Under laboratory conditions, parasitoid longevity was related positively to parasitoid size and realised lifetime fecundity, and clutch size was related positively to host size. 6. These results suggest that selection of intermediate‐sized C. fumiferana larvae may be adaptive for E. cacoeciae.     

Climate‐dependent costs of reproduction: Survival and fecundity costs decline with length of the growing season and summer temperature

Costs of reproduction are expected to vary with environmental conditions thus influencing selection on life‐history traits. Yet, the effects of habitat conditions and climate on trade‐offs among fitness components remain poorly understood. For 2–5 years, we quantified costs of experimentally increased reproduction in two populations (coastal long‐season vs. inland short‐season) of two long‐lived orchids that differ in natural reproductive effort (RE; 30 vs. 75% fruit set). In both species, survival costs were found only at the short‐season site, whereas growth and fecundity costs were evident at both sites, and both survival and fecundity costs declined with increasing growing season length and/or summer temperature. The results suggest that the expression of costs of reproduction depend on the local climate, and that climate warming could result in selection favouring increased RE in both study species.     

Lepidopteran pest populations and crop yields in row intercropped broccoli

Abstract

• 1 To determine how the presence of non‐host plants impact populations of lepidopteran pests and yield of broccoli, Brassica oleracea var. italica L., broccoli was intercropped with tomato, Lycopersicon esculentum Miller, or yellow sweetclover (YSC), Melilotus officinalis L., and compared with broccoli monoculture grown at two densities (high and low).
• 2 During a 1997 study, numbers of early instar imported cabbageworm, Artogeia rapae L., were greater in broccoli monoculture compared with intercropped plantings. In contrast, late instar A. rapae numbers were significantly higher in intercropped broccoli during most of the season, indicating greater survival of early instar stages in intercropped habitats.
• 3 Cabbage looper, Trichoplusia ni Hübner, eggs and larvae were more abundant in broccoli monoculture than intercropped broccoli in 1997. In addition, T. ni populations were found at significantly lower levels in broccoli‐YSC compared with broccoli‐tomato habitat throughout the season.
• 4 During a 1998 study, numbers of early and late A. rapae instars were greater in intercropped broccoli on most sampling dates.
• 5 The mean percentages of harvested broccoli heads infested with insects and associated frass were higher in monocultures than intercropped treatments in 1997. Additionally, a significantly lower number of contaminants were found in the crowns of broccoli harvested from broccoli‐YSC (YSC) compared with broccoli‐tomato (tomato) habitat. However, broccoli heads were significantly smaller in intercropped habitats.
• 6 Variations in insect responses and challenges to using crop diversification as a management tool are discussed.

     

Reducing spruce beetle Dendroctonus rufipennis (Kirby) (Coleoptera: Curculionidae) emergence for hibernation in central British Columbia by felling infested trees

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Host stage selection of the mealybug parasitoid Anagyrus spec. nov near sinope

The mealybug parasitoid Anagyrus spec. nov near sinope (Hymenoptera: Encyrtidae) is an undescribed parasitoid of the Madeira mealybug, Phenacoccus madeirensis Green (Homoptera: Pseudococcidae). We investigated the preference of Anagyrus spec. nov near sinope for six developmental stadia (first‐ and second‐instar nymphs, third‐instar immature females, third‐ or fourth‐instar immature males, pre‐reproductive adult females, and ovipositing adult females) of P. madeirensis and the fitness consequences of the host stage selection behavior. In the no‐choice test, Anagyrus spec. nov near sinope parasitized and completed development in all host stadia except third‐instar immature males. When all host stadia were offered simultaneously, the parasitoids preferred third‐instar immature and pre‐reproductive adult females. Dissection of the stung mealybugs revealed that the clutch size (number of eggs per host) was approximately four and three in the third‐instar and pre‐reproductive females, respectively, and one egg per first‐instar nymph. Parasitoids emerged from P. madeirensis parasitized at third‐instar or pre‐reproductive adult female completed development in the shortest duration, achieved a higher progeny survival rate, larger brood and body size, and the lowest proportion of males. We showed that the continued development of mealybugs had significant influence on the fitness of the parasitoids. Although deposited as eggs in first‐ or second‐instar nymphs, parasitoids emerged from mummies that had attained third‐instar or adult development achieved similar progeny survival rate, brood size, body size, and sex ratio as those parasitoids deposited and developed in third‐instar or adult mealybugs. By delaying larval development in young mealybugs, Anagyrus spec. nov near sinope achieved higher fitness by allowing the parasitized mealybugs to grow and accumulate body size and resources. We suggest that the fitness consequence of host stage selection of a koinobiont parasitoid should be evaluated on both the time of parasitism and the time of mummification.     

斑痣悬茧蜂寄生、发育和生殖特征与寄主幼虫日龄的关系

【目的】“圆屋顶形”假说认为,对单寄生性姬蜂和茧蜂适合度而言,中间龄期幼虫寄主的品质高于更早和更晚龄期幼虫。该假说得到许多研究支持,但这些研究常以寄主幼虫脱皮划分虫龄,很少观测生殖特征,从而难以确切和全面描述适合度随寄主生长发育变化而变化的关系。本研究旨在检验“圆屋顶形”假说。【方法】本研究以斜纹夜蛾Spodoptera litura不同日龄幼虫为寄主,观测斑痣悬茧蜂Meteorus pulchricornis寄生和发育特征,并测定成蜂生殖力。【结果】线性回归分析表明,雌蜂对中间日龄寄主幼虫的寄生率大于对两端日龄寄主幼虫的寄生率;蜂卵至成虫的存活、成虫体型大小及其生殖力（产卵量）等适合度相关特征均表现出中间日龄寄主幼虫处理大于两端日龄幼虫处理。【结论】研究结果支持“圆屋顶形”假说。     

Pattern analysis of eastern spruce budworm Choristoneura fumiferana dispersal

Dispersal has been proposed as an important mechanism in the broad‐scale synchronisation of insect outbreaks by linking spatially disjunct populations. Evidence suggests that dispersal is influenced by landscape structure, phenology, temperature, and air currents; however, the details remain unclear due to the difficulty of quantifying dispersal. In this study, we used data on the abundance and distribution of spruce budworm Choristoneura fumiferana larvae (potential dispersers) and adult male moths (dispersers) to make inference on the effects of air currents and host‐species abundance on dispersal. Hierarchical‐Bayesian and inverse modeling was used to explore 4 dispersal models: 1) isotropic dispersal; 2) directional‐dispersal; 3) directional‐and‐host‐species dispersal; and 4) host‐species dispersal. Despite their strong dependence on balsam fir Abies balsamea and spruce species Picea spp., the mapped basal area of these host species did not influence the pattern of dispersed moths. The model that best fit the data was the directional‐dispersal model, which showed that the prevailing dispersal direction was from the northwest (328°). We infer that the strong pattern of directional dispersal was due to a prevailing wind from the same direction. Our interpretation was corroborated by independent wind data during the period of active adult male budworm flight, particularly in the region with high larval abundance. Our results indicate that there was a relatively high probability of individuals flying at least 48 km with the wind where larvae abundance at source locations was also high. Such findings emphasize the importance of long‐distance dispersal on spatial distribution of adult male spruce budworms. Insight into the population‐level consequences of such dispersal patterns requires additional research.     

Cotesia plutellae parasitizing Plutella xylostella: Host-age dependent parasitism and its effect on host development and food consumption

The braconid Cotesia plutellae(Kurdjumov) (Hymenoptera: Braconidae) is amajor solitary, larval endoparasitoid of thediamondback moth, Plutella xylostella(L.) (Lepidoptera: Plutellidae). Parasitism oflarvae of different host instars and fourdevelopmental ages of the 4th instar ofthe pest was examined. The effects of hostinstar at initial parasitization on thedevelopment, survival, size and fecundity ofthe parasitoid were determined in thelaboratory at 25 °C. The effects ofparasitism on host development and foodconsumption were investigated at 28 °C.Cotesia plutellae could parasitize larvaeof all four instars of P. xylostella, butpreferred 2nd and 3rd instars. In achoice test, the relative parasitism indicesfor 2nd, 3rd and 4th instarswere 0.37, 0.39 and 0.24, respectively.Parasitism decreased sharply with increasinghost age in the 4th instar and approachedzero in host larvae that had gone beyond 37%of 4th stadium. The development time andthe final adult size of the parasitoid variedwith the host instar at initial parasitization.Parasitoids with initial parasitism in the4th instar hosts had the shortestdevelopment time, followed by those in the3rd instar, and then by those in the2nd instar. Parasitoids startingparasitism in 2nd instar hosts weresmaller in body size than those starting in the3rd or 4th instar. However, resultantfemales starting parasitism in 3rd instarhosts had the highest fecundity. Parasitizedlarvae exhibited longer development time andincreased food consumption compared withunparasitized ones. This study presents thefirst record that a solitary parasitoidregulates host behavior leading to an increasein food consumption by the host.     

Temptations of weevil: feeding and ovipositional behaviour of Hylobius warreni Wood on host and nonhost bark in laboratory bioassays

• 1 Warren root collar weevil Hylobius warreni Wood (Coleoptera: Curculionidae) is a long‐lived, flightless insect native to coniferous forests across northern North America. Girdling by larval feeding causes significant mortality on young trees. The insect poses considerable challenges to reforestation.
• 2 Adult weevils feed on all life stages of a variety of coniferous hosts prior to oviposition. Their relative feeding preferences, however, have not been quantified. Moreover, it is not known whether host bark influences oviposition behaviour.
• 3 Feeding preferences of adult weevils were tested in both choice and no‐choice laboratory bioassays using small branches from three conifers (lodgepole pine Pinus contorta var. latifolia, interior hybrid spruce Picea glauca×engelmannii, and Douglas‐fir Pseudotsuga menziesii) and one deciduous tree (trembling aspen Populus tremuloides). Measurements included the surface area of bark consumed, rate of consumption, the number of days of feeding, and, in the no‐choice assay, the number of eggs oviposited.
• 4 Bark consumption was greatest on pine and Douglas‐fir, followed by spruce. Little to no feeding occurred on aspen. Consumption did not vary between male versus female insects for any of the feeding metrics quantified.
• 5 The presence of aspen branches did not inhibit feeding on any of the other species in the choice bioassays.
• 6 The number of eggs laid by female insects did not differ significantly among tree species in the no‐choice assay. Eggs were laid indiscriminately in the presence of all four host types.
• 7 Results and opportunities for future research are discussed in the context of formulating new integrated pest management strategies for this insect, which is increasingly important in the period of reforestation subsequent to the mountain pine beetle epidemic in western Canada.

     

Effect of host plant species of cotton whitefly Bemisia tabaci Middle East‐Asia Minor 1 (Hemiptera: Aleyrodidae) on the development,survival and reproduction of its predaceous ladybeetle Serangium japonicum (Coleoptera: Coccinellidae)

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Consequences of stress induced by heat on fitness of Delphacodes kuscheli and its yeast‐like endosymbionts

The planthopper Delphacodes kuscheli is the main vector of Mal de Río Cuarto virus in Argentina, disease that severely affects maize production. In this study, we investigated the effects of heat stress on fitness traits and on the number of its obligate yeast‐like symbionts (YLS). The exposition of newly‐hatched nymphs to 35°C for 3 days, a well‐known procedure used to reduce the number of YLS in planthoppers, was applied. To compare different fitness components between control and heat‐treated insects, we estimated nymphal instars development time, nymphal survival, adult body length, longevity, fecundity and fertility. Also, correlates of fitness, as proportion of sexes and wing forms of the emerging adults, were evaluated. In heat‐treated group, the nymphal developmental time increased due to an increase in the fifth instar duration, and the nymphal survival, body length of adults and fecundity were reduced when compared to control. There was a significant association between treatments (control and heat‐treated insects) and wing morphs. The heat treatment successfully reduced the number of YLS in third instar nymphs of D. kuscheli. Our results revealed the negative effect of heat stress on development, survival and reproduction of D. kuscheli and on the load of its YLS endosymbionts suggesting that YLS could play a crucial role in the development and reproduction of these planthoppers.