Limited dispersal and its effect on population structure in the milkweed beetle Tetraopes tetraophthalmus

Summary The movement patterns of adult milkweed beetles, Tetraopes tetraphthalmus, were monitored via a mark-recapture technique. Movement or dispersal patterns were studied in two natural populations, one in which the host plant, Asclepias syriaca, was nearly continuously distributed over a 250×90 m area and another where Asclepias was distributed in 17 small discrete patches. In both populations dispersal distances resulting from the flight patterns of the adult beetles were quite short, averaging less than 40 m from the point of first encounter 10 days after marking. Males were shown to be more vagile than females. The distribution of dispersal distances collected from one of the populations was fit to three statistical distributions cited in the literature as expected from dispersal by many small-scale movements or observed in other species. It was found that an equation describing an exponential decay gave the best statistical fit to the data collected here for milkweed beetles. The data is discussed in the context of the effects of the limited dispersal power of the beetles and the distribution of suitable habitat on the population structure of Tetraopes.     

Diapause induction and termination in a commonly univoltine leaf beetle (Phratora vulgatissima)

Abstract The leaf beetle Phratora vulgatissima (Linnaeus 1758) is commonly univoltine in south‐central Sweden but may sometimes initiate a partial second generation. The current study was set out to investigate under what abiotic conditions the beetles initiate a second generation. Using climate chamber experiments, the beetles were shown to have a facultative reproductive diapause induced by declining day‐length. The critical day‐length (CDL) for diapause induction was estimated to be 18 h and 10 min. In the field, first‐generation beetles developing to adulthood before August in 2009 became reproductively active and produced a second generation, but most individuals emerged later and were in reproductive diapause. P. vulgatissima overwinter as adults and diapause was shown to be maintained until mid‐winter in 2008/2009. The cumulative temperature requirement for oviposition after diapause termination was estimated to be 222 day‐degrees with a 5.5°C temperature threshold. Three different day‐degree models that were developed to predict the phenology of female oviposition in the spring were validated by comparing model results with field data on the timing of oviposition in previous years. The study suggests that P. vulgatissima may initiate a second generation in Sweden if development of the first generation is completed before August. Warmer spring and summer temperatures due to ongoing climate change may cause advanced insect phenology and faster completion of insect life‐cycles at northern latitudes, which will affect the proportion of insects that initiate a second generation.     

Partial bivoltinism in a gregarious endoparasitoid: larval diapause as influenced by season and sharing a host

The yearly timing of the life cycle of a parasitoid is a key element of its life‐history strategy. I examine here factors influencing the expression of partial bivoltinism in Tetrastichus julis Walker (Hymenoptera: Eulophidae), a specialist parasitoid introduced to North America to attack its univoltine host, the cereal leaf beetle, Oulema melanopus (L.) (Coleoptera: Chrysomelidae). The varying tendency was assessed of individuals of this gregarious larval parasitoid to either emerge as adults in the same summer they mature, or to enter diapause to emerge the following year. Parasitized hosts were obtained by rearing cereal leaf beetles collected as mature larvae from grain fields in northern Utah (western USA) throughout the growing seasons in 2013 and 2014. Cocoons spun by these beetles were held to determine patterns over the spring and summer in the tendency of the parasitoid to forgo larval diapause. A high percentage (nearly 90% in 2013) of parasitoid individuals were found to forgo diapause and emerge in the same summer from earliest maturing hosts. This percentage rapidly declined to 20% or less of individuals forgoing diapause and emerging from cocoons as the summer advanced. The percentage of parasitoid individuals forgoing diapause increased significantly at a given time of season (early or late) as the number of conspecifics with which an individual shared a host larva increased. These results may reflect a trade‐off for individual parasitoids in which greater success in finding – and ovipositing in – host larvae the following spring vs. in summer, is countered by reduced survivorship in diapausing over the winter vs. emerging in the same summer in which the parasitoid matures. Expression of partial bivoltinism of T. julis, as affected strongly by both season and within‐host density, results in high rates of parasitism of cereal leaf beetles both early and late in the season.     

Random movement pattern of fed and unfed adult Colorado potato beetles in bare‐ground habitat

• 1 Although the successful management of the Colorado potato beetle Leptinotarsa decemlineata (Say) depends on the prevention of its dispersal, its walking pattern in the landscape remains poorly understood. In the present study, post‐diapause, early summer, late summer and colony adult beetles, both fed and unfed before release, were tracked with a harmonic radar to establish their walking movement pattern in a bare‐ground field.
• 2 The random walk model successfully described the dispersal of all beetle types, whether fed or unfed.
• 3 The diverse life history of this species was manifested by an increased distance travelled and deviations of individual paths from the random model. Starved post‐diapause beetles travelled furthest and individual paths deviating from random were both local and directed, probably aiming to maximize opportunities for host colonization. Starved early summer beetles also travelled further than fed beetles but relied more on random movement to disperse in the habitat. Starving had little impact on the distance travelled or the path deviations of late summer beetles that are searching for overwintering site rather than hosts.
• 4 The increased displacement of starving beetles over fed beetles corresponded with an increased walking step and index of straightness.
• 5 The impact of starvation on travel distance was greater than expected from laboratory tests.
• 6 In conclusion, the results obtained in the present study suggest a random walking pattern to search arable land until host volatile or visual impulses trigger a more directed walk or flight.

     

小猿叶甲对寄主植物衰老和缺乏的适应

小猿叶甲Phaedon brassicae Baly是十字花科蔬菜上的一种重要害虫,具有复杂的生活史。测定小猿叶甲取食幼嫩和衰老小白菜叶片后的滞育比率和食料缺乏条件下小猿叶甲成虫的存活时间。研究结果表明,在25℃配合光周期12L∶12D条件下,小猿叶甲取食衰老期小白菜叶片后的滞育比率显著高于取食幼期小白菜叶片后的滞育比率。在25℃、12L∶12D条件下以幼期小白菜叶片饲养小猿叶甲,其成虫羽化后取食0～7d,再分别转入30℃、14L∶10D,不供食的条件下,其结果表明,雌雄成虫存活时间随取食时间延长而延长;羽化成虫取食5d以上,成虫存活时间显著延长,雌雄成虫平均存活时间均在75d以上。最后,讨论小猿叶甲对寄主植物衰老和缺乏的可能适应机制。     

Effect of photoperiod and allatectomy on reproduction and (second) diapause induction in post-diapause adult females of the Colorado potato beetle,Leptinotarsa decemlineata Say

Summary

The effect of photoperiod and allatectomy on the rates of oviposition and juvenile hormone (JH) synthesis was studied in female beetles after the break of adult diapause. The results indicate that after diapause, photoperiod has no effect on oviposition rates. However, the photoperiodic effect on diapause induction is restored within a few days after the break of the first diapause. Allatectomy performed within one day after diapause did not induce a second diapause, whereas about 40% of the beetles allatectomized after 3 or more days responded by entering a second diapause. Oogenesis in the operated animals was never completely prevented under long and short day conditions. These preliminary results suggest a change in the sensitivity of post-diapause beetles to photoperiodic and endocrine manipulation as compared with pre-diapause insects.     

Activity enhancement of Zygogramma bicolorata,a biocontrol agent of Parthenium hysterophorus,by temperature regulated diapause aversion

Diapause is a unique strategy of dormancy in insects to avoid unfavourable conditions. The exotic beetle Zygogramma bicolorata, is an effective biological control agent of Parthenium hysterophorus in India, Adults diapause in soil during December to May. As a result, there is delay in its effectiveness on the plant that reaches to flowering and seed production by the time the beetle is able to build up its population after emerging from diapause. Therefore, a study was conducted to explore possibilities of diapause aversion by temperature regulation. Results indicated that exposure of newly emerged adults to heat treatment of 35°C and to low temperature of 10°C could reduce diapause in Z. bicolorata. The low temperature can also be used as a medium for the storage of the mass reared beetles for a long time without having negative effect on their longevity and fecundity.     

Juvenile hormone-induced break and termination of diapause in the Colorado potato beetle

Topical applications of Cecropia juvenile hormone I (JH) and several analogues, 1 day after the initiation of diapause, induce a break in diapause 8 to 10 days later, characterized by emergence from the soil, feeding, mating, and reproduction. Activation is only temporary when beetles are kept in short-day photoperiods. The percentage of beetles activated and the duration of activation depend on the nature of the JH and the dose applied. JH application during prediapause only postpones diapause, whereas beetles which have spent 2 months in diapause are activated rather easily. Diapause is completely terminated when JH application is accompanied by a transfer of the beetles to long-day conditions. Thirty μg JH applied to allatectomized females in long-day conditions allows them to reproduce for over 50 days.The extent to which the endocrine system is involved in activation has been investigated. Allatectomy does not change JH-induced activation at long or short daylengths. Volumetric measurements indicate that the corpora allata remain inactive as long as measurable amounts of JH circulate in the blood. An increase in corpus allatum activity after day 8 further stimulates long-day oviposition. Gomoripositive neurosecretory cells in the brain are activated by JH within 1 day. The hypothesis is advanced that a JH-induced break or termination of diapause is mediated through the neuroendocrine system.     

Diapause in a large aggregation of a tropical beetle

Abstract. 1_˜ Diapause is described for adults of Stenotarsus rotundus Arrow (Endomychidae, Coleoptera) in the tropical lowland forest on Barro Colorado Island, Panama.
2 . Diapause lasted up to 10 months each year, covering some 6 months of the wet season plus 4 months of the dry season. When in diapause the beetles clustered in an aggregation of up to 70 ,000 individuals on the stem of a palm tree, the same individual palm tree in at least four successive years.
3. The habitat and habits of the beetle when not in diapause are unknown, but one or two generations may be produced during this period. Beetles returned to the diapause site from June until late August.
4. Soon after arrival at the diapause site the flight muscles degenerated and did not develop again until February/March. The ovaries were undeveloped when the beetles arrived at the site and remained in that condition until the onset of yolk deposition in March. At the beginning of the rainy season in April the beetles mated and dispersed.
5. Lean dry weight of the beetles did not decrease noticeably during diapause, but fat content did. It is suggested that the beetles do not feed, or feed very little, while in diapause. During diapause, the metabolic rate is surprisingly low considering a mean ambient temperature of 25° C.
6. Juvenile hormone analogue (ZR515) stimulated flight muscle and ovary development. Daylength, when different from LD 12: 12 , initiated a partial flight muscle development, but there was no effect on the ovaries. Field observations suggest that neither temperature nor humidity had an effect on the termination of diapause.
7. Although the development of both flight muscles and ovaries were stimulated by juvenile hormone analogue and both developed around March of each year, it is suggested that the mechanisms causing this development are different.     

Monitoring diapause development in the Colorado potato beetle, Leptinotarsa decemlineata, under field conditions using molecular biomarkers

A multiplex PCR protocol was developed using five diapause-regulated genes to monitor diapause development of the Colorado potato beetle under field conditions. A total of 870 beetles from the Red River valley of North Dakota and Minnesota, USA, were screened for three consecutive years. Out of the 32 possible expression profiles, eight could be arranged in chronological order of diapause development. These eight profiles account for over 92% of the beetles surveyed. Intra-population variation in diapause phenotypes was observed in the field. Some beetles were already in the diapause initiation phase in June when the day length was greater than 17 h. Inter-seasonal variation in the timing of diapause development was also noted. The greatest differences were before the day length decreased to less than 15 h. Anomalies in the results, e.g., the presence of the diapause maintenance phase profiles in beetles collected on the potato plants, argue that laboratory results are not always equivalent with what is observed under field conditions.     

Timing of diapause in relation to temporally variable catastrophes

This paper analyses the response of the distribution of diapause switching times in an arthropod population with respect to variation in a catastrophe, which kills nondiapause individuals. For concreteness, the catastrophe will be taken to the onset of winter. The relationship between an individual's switching time and the decision whether to diapause is defined as follows: If she passes through the end of the sensitive period, during which the diapause decision is made, before her switching time, she will complete development and reproduce rather than diapause. If she passes through the sensitive period on or after her switching time, she will diapause. The model follows the evolution of the distribution of switching times for a population over a sequence of years. Random variation in the end of the season is created by sampling from a normal distribution of times at the end of the growing season. The model is for a haploid population in which the distribution of switching times that a female passes to her offspring is normally distributed having her switching time as its mean. This approximates a sexual population with strong positive assortative mating and heritability near 1. This mode of inheritance permits a rapid response to yearly changes in the end of the season as a contrast to earlier models, which incorporated a slow response. Patterns of temporal change in the median switching time are analyzed. The influences of three parameters are considered: the mean and standard deviation of the end of the season, and standard deviation of the offspring distribution. The main conclusion differs from the results of earlier models in that the end of the season must be extremely variable before the median of the distribution of switching times displays conservative behavior, i. e., before it becomes significantly earlier than the median expected when the end of the season is constant and equal to the mean of the normal distribution for the end of the season used in the simulation. Previous models predicted a conservative response even in moderately variable environments.     

Effect of changes in the thermoperiod on reproductive diapause in Bruchidius atrolineatus Pic (Coleoptera: Bruchidae)

Abstract. Bruchidius atrolineatus (Pic) is a tropical bruchid developing in Vigna unguiculata (Walp) pods. Adults are in reproductive diapause during the dry season. In conditions of 40:25C or 35:25C, diapause is induced when larvae develop in thermophases longer than 14 h, in thermophases shorter than 10 h and in continuous darkness. Thermophases of 12 h cause a reduction of the proportion of diapausing beetles. This proportion also depends on the thermoperiod temperatures. Development at low temperatures (23: 16C, 12: 12 h) induces a high percentage of diapausing beetles. However, at low temperatures, the developmental times of the beetles are long and show important interindividual variability. When larvae develop in decreasing temperatures, small changes in thermoperiod temperatures (35:25C to 3O:2OC) have a strong effect on diapause induction. When larvae develop at low temperatures (25:15C) and then at high temperatures (40:25C), the proportion of diapausing beetles depends on the duration of development at low temperatures. Whatever the developmental conditions, both sexually active and diapausing beetles always emerge from the seeds. This interindividual variability is important in this species which has a wide geographical distribution in Africa.     

Effects of host-plant species on diapause induction of the Kanzawa spider mite, Tetranychus kanzawai

In many herbivorous arthropods, incidence of diapause, which is considered to reflect the timing of diapause, changes depending on the host plants they utilize. Several theoretical studies suggest that the optimal timing of diapause induction depends on life‐history traits; if the development time of the arthropod is short, fecundity is high, or survival rate remains high throughout the season, the optimal timing of diapause induction would be shifted toward the end of the season. For herbivorous arthropods, these life history traits may change among their host plants. Here we examined whether a population of the Kanzawa spider mite, Tetranychus kanzawai Kishida (Acari: Tetranychidae), shows the predicted pattern of diapause induction on two host plants, the kidney bean [Phaseolus vulgaris (Leguminosae)] and Japanese Orixa [Orixa japonica (Rutaceae)], on which the mites show different performances. Rearing conditions were controlled in two ways. In the first experiment, day length and temperature were kept constant throughout the mite lifetime at either of three conditions from mid October to early November. In the second experiment, the conditions were changed from 20 °C and L11.5:D12.5 at immature stages to 18 °C and L11:D13 at adult stage in order to better approximate field conditions. In the first experiment, diapause incidence on P. vulgaris was lower than on O. japonica. This tendency became stronger in the second experiment, suggesting a difference in the timing of diapause induction among host plants. On the other hand, P. vulgaris was proven to allow high performance, i.e., greater lifetime fecundity and shorter development times, although it had no effect on the survival rate. The relationship between diapause incidence and performance is consistent with the prediction of theoretical studies that a short development time or high fecundity delays the timing of diapause induction.     

Host‐plant quality adaptively affects the diapause threshold: evidence from leaf beetles in willow plantations

1. Voltinism of herbivorous insects can vary depending on environmental conditions. The leaf beetle Phratora vulgatissima L. is univoltine in Sweden but will sometimes initiate a second generation in short‐rotation coppice (SRC) willow plantations. 2. The study investigated whether increased voltinism by P. vulgatissima in plantations can be explained by (i) rapid life‐cycle development allowing two generations, or (ii) postponed diapause induction on coppiced willows. 3. In the field, no difference was found in the phenology or development of first‐generation broods between plantations (S. viminalis) and natural willow habitats (S. cinerea). However, the induction of diapause occurred 1–2 weeks later in SRC willow plantations. 4. Laboratory experiments indicated no genetic difference in the critical day‐length for diapause induction between beetles originating from plantations and natural habitats. Development time was unaffected by host‐plant quality but critical day‐length was prolonged by almost an hour when the beetles were reared on a non‐preferred willow species (S. phylicifolia). When reared on new leaves from re‐sprouting shoots of recently coppiced willow plants, diapause incidence was significantly less than when the beetles were reared on mature leaves from uncoppiced plants. 5. The study suggests that P. vulgatissima has a plastic diapause threshold influenced by host‐plant quality. The use of host‐plant quality as a diapause‐inducing stimulus is likely to be adaptive in cases where food resources are unpredictable, such as when new host‐plant tissue is produced after a disturbance. SRC willows may allow two beetle generations due to longer growing seasons of coppiced plants that grow vigorously.     

Diapause induction and termination in the predatory mite Euseius finlandicus in peach orchards in northern Greece

The course of diapause induction as well as of diapause termination infemales of the predatory mite Euseius (Amblyseius)finlandicus Oudemans (Acari: Phytoseiidae) in Northern Greece wasdetermined by transferring females during winter and early spring from peachtrees to a short day (LD 8:16) or a long day (LD 16:8) photoperiod both at 20°C. The first diapause females were found in mid September andby the first week of October all the females were in diapause. The mean numberof days required for diapause termination under the short day photoperiod LD8:16 was gradually reduced from 93.7 days in the beginning of October to lessthan 20 days in mid February and early March. Under the long day photoperiod ofLD 16:8 diapause was terminated in less than 20 days irrespective of the dateof collection. These results indicate that by mid February photoperiodicsensitivity of females was lost and diapause was terminated. However, femaleswere found in their overwintering sites until the second half of March,probablybecause of the prevailing relatively low temperatures and lack of adequatefood.     

Aspects of the induction of diapause in a laboratory strain of the mite Tetranychus urticae

Some diapause characteristics were studied in a strain of the spider mite. Tetranychus urticae. which had been reared on bean plants in the laboratory for over 15 yr. The diapause induction response curve was of the long-day type, showing a sharply defined critical daylength of 13 hr 50 min. In constant darkness no diapause induction occurred, but with a photoperiod of 1L:23D diapause incidence was already complete. A thermoperiod with a 5°C amplitude induced diapause in combination with a short-day photoperiod only when the low phase of the thermoperiod coincided with the scotophase. The same thermoperiod did not induce any diapause in constant darkness. The photoperiodic reaction of the laboratory strain used in these experiments appeared to remain constant over a very long period of time and to be independent of the diapause history of previous generations of mites.Although photoperiodic sensitivity was demonstrated during the whole postembryonic development, sensitivity was maximal at the end of the protonymphal instar and declined rapidly during the deutonymphal instar. Only 2 inductive cycles of 10L:14D were required to induce up to 62% diapause if the mites were kept in continuous darkness during the remainder of their development. Long days or continuous light could reverse the inductive effect of a sequence of short-day cycles previously applied to the mites.Light breaks of 1 hr duration applied at different times during the dark period of a 10L:14D photoperiod generated a sharp bimodal response curve with two discrete points of sensitivity to the light breaks at 10 hr after ‘dusk’ and 10 hr before ‘dawn’, thus showing a remarkable similarity with the results obtained in light break experiments with some species of insects.     

Molecular cloning and characterization of a putative proline dehydrogenase gene in the Colorado potato beetle,Leptinotarsa decemlineata

Leptinotarsa decemlineata adults exhibit a season-dependent activity. In spring, post-diapause beetles often fly a long distance from overwintering sites to potato fields. In summer and autumn, the flight ability is sharply reduced. Proline is the main energy substrate ofL. decemlineata during flight and proline dehydrogenase （ProDH） catalyzes the first step in proline catabolism. Here we identified a putative LdProDHgene; it had three cDNA isoforms which shared the same 5＇UTR and coding region, but differed in the lengths of 3＇UTRs （515, 1 092 and 1 242 bp for isoforms-1, -2 and -3, respectively）. LdProDH encoded a 616 amino acid protein that showed high sequence similarity to ProDH-like proteins from other insect species. LdProDHwas expressed in the third and fourth instars larvae and adults, but not in pupae. Dietary ingestion of bacterially expressed LdProDH- dsRNA by adults significantly decreased its messenger RNA （mRNA） level, and caused an elevation of free proline content in the hemolymph. Further observation revealed that three canonical polyadenylation signals （AATAAA） were tandemly located in the 3＇UTR of isoform-3. The first, second and third polyadenylation sites gave rise to isoforms-1, -2 and -3, respectively. Analysis of the genomic DNA uncovered that the three isoforms resulted from alternative polyadenylation. The mRNA level of isoform-1, which expressed at low levels in pre-diapause adults, became abundant in post-diapause beetles. It is indicated that the LdProDH expression is fine-tuned through 3＇UTR to control proline catabolism for the season-dependent activity ofL. decemlineata adults.     

Diapause termination in the spotted stem borer, Chilo partellus (Lepidoptera: Pyralidae) in the laboratory

The effect of temperature, photoperiod, artificial diet and water on the termination of diapause by larvae of the stem borer, Chilo partellus (Lepidoptera: Pyralidae), was studied in the laboratory. Termination of diapause as indicated by pupation was affected mainly by a combination of high temperature and a long day photoperiod. Total darkness did not prevent termination of diapause and pupation occurred also in larvae which were never exposed to water. Long days accelerated pupation, but, under 16 h daylength, termination of diapause was faster than under constant illumination. Provision of artificial diet had no effect or slowed down pupation but water decreased the time to pupation. Under 28°C, 16 h daylength and availability of water, C. partellus diapausing larvae terminated diapause and pupated in about 9 days.     

Time of the season: the effect of host photoperiodism on diapause induction in an insect herbivore,Leptinotarsa decemlineata

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