Diapausing Stages Research Articles

Global attractivity in tick population models incorporating seasonality and diapausing stages

The goal of this paper is to understand the interplay between seasonality and diapause on the population dynamics of ticks. To analyse this challenge, we study a mathematical model in which the population is divided into two groups: individuals with regular development and individuals undergoing diapause. A key ingredient in the model is that the common birth and death rates depend on time in order to describe the seasonal fluctuations of the environment. From a mathematical point of view, this paper offers a novel methodology to derive criteria of global attraction. In some classical models, we give delay-dependent conditions that cover the best delay-independent criteria of global attraction. We discuss how our approach improves existing results in relevant literature. Moreover, our results are simple and versatile enough to match experimental observations. To illustrate this potential, we analyse our model with parameters coming from real observations. A message of this paper is that seasonality plays a critical role in the population dynamics of ticks.

The timings of host diapause and epidemic progression mediate host genetic diversity and future epidemic size in Daphnia-parasite populations.

Epidemics commonly exert parasite-mediated selection and cause declines in host population genetic diversity. This can lead to evolution of resistance in the long term and smaller subsequent epidemics. Alternatively, the loss of genetic diversity can increase host vulnerability to future disease spread and larger future epidemics. Matters are made more complex by the fact that a great many host organisms produce diapausing life stages in response to environmental change (often as a result of sexual reproduction; e.g. plant seeds and invertebrate resting eggs). These diapausing stages can disrupt the relationship between past epidemics, host genetic diversity and future epidemics because they allow host dispersal through time. Specifically, temporally dispersing hosts avoid infection and thus selection from contemporary parasites, and also archive genetic variation for the future. We studied 80 epidemics in 20 semi-natural populations of the temporally dispersing crustacean Daphnia magna and its sterilizing bacterial parasite Pasteuria ramosa, and half of these populations experienced a simulated environmental disturbance treatment. We found that early initiation of diapause relative to the timing of the epidemic led to greater host genetic diversity and reduced epidemic size in the subsequent year, but this was unaffected by environmental disturbance.

Global stability and oscillations for mosquito population models with diapausing stages

In this paper we analyze the dynamical behavior of a mosquito population with diapausing stages through a mathematical model derived from the classical McKendrick-von Forester equations. The absence of monotonicity properties and the structure of the equations are noticeable features that hinder the use of commonly applied tools in the field. Our main contribution is to provide a novel methodology to derive criteria of global attractivity of a positive solution for periodic and discontinuous delay differential equations. From an applied perspective, we simulate the evolution of Aedes aegypti with diapausing stages. Moreover, we discuss the influence of the diapause parameters on the creation/suppression of population oscillations.

Anemochory of diapausing stages of microinvertebrates in North American drylands.

1. Dry, ephemeral, desert wetlands are major sources of windblown sediment, as well as repositories for diapausing stages (propagules) of aquatic invertebrates. Zooplankton propagules are of the same size range as sand and dust grains. They can be deflated and transported in windstorm events. This study provides the evidence that dust storms aid in dispersal of microinvertebrate propagules via anemochory (aeolian transport). 2. We monitored 91 windstorms at six sites in the southwestern U.S. over a 17-year period. The primary study site was located in El Paso, Texas in the northern Chihuahuan Desert. Additional samples were collected from the Southern High Plains region. Dust carried by these events was collected and rehydrated to hatch viable propagules transported with it. 3. Using samples collected over a six-year period, 21 m above the ground which included 59 storm events, we tested the hypothesis that transport of propagules is correlated with storm intensity by monitoring meteorological conditions such as storm duration, wind direction, wind speed, and PM10 (fine dust concentration). An air quality monitoring site located adjacent to the dust samplers provided quantitative hourly measurements. 4. Rehydration results from all events showed that ciliates were found in 92% of the samples, rotifers in 81%, branchiopods in 29%, ostracods in 4%, nematodes in 13%, gastrotrichs in 16%, and tardigrades in 3%. Overall, four bdelloid and 11 monogonont rotifer species were identified from rehydrated windblown dust samples. 5. PCA results indicated gastrotrichs, branchiopods, nematodes, tardigrades, and monogonont rotifer occurrence positively correlated with PM10 and dust event duration. Bdelloid rotifers were correlated with amount of sediment deposited. NMDS showed a significant relationship between PM10 and occurrence of some taxa. Zero-inflated, general linear models with mixed-effects indicated significant relationships with bdelloid and nematode transport and PM10. 6. Thus, windstorms with high particulate matter concentration and long duration are more likely to transport microinvertebrate diapausing stages in drylands.

Modelling diapause in mosquito population growth.

Diapause, a period of arrested development caused by adverse environmental conditions, serves as a key survival mechanism for insects and other invertebrate organisms in temperate and subtropical areas. In this paper, a novel modelling framework, motivated by mosquito species, is proposed to investigate the effects of diapause on seasonal population growth, where the diapause period is taken as an independent growth process, during which the population dynamics are completely different from that in the normal developmental and post-diapause periods. More specifically, the annual growth period is divided into three intervals, and the population dynamics during each interval are described by different sets of equations. We formulate two models of delay differential equations (DDE) to explicitly describe mosquito population growth with a single diapausing stage, either immature or adult. These two models can be further unified into one DDE model, on which the well-posedness of the solutions and the global stability of the trivial and positive periodic solutions in terms of an index [Formula: see text] are analysed. The seasonal population abundances of two temperate mosquito species with different diapausing stages are simulated to identify the essential role on population persistence that diapause plays and predict that killing mosquitoes during the diapause period can lower but fail to prevent the occurrence of peak abundance in the following season. Instead, culling mosquitoes during the normal growth period is much more efficient to decrease the outbreak size. Our modelling framework may shed light on the diapause-induced variations in spatiotemporal distributions of different mosquito species.

Metal stress in zooplankton diapause production: post-hatching response.

Aquatic organisms commonly respond to harsh conditions by forming diapausing stages, which enable populations to survive adverse periods forming egg banks. Production of diapausing eggs is frequently observed in monogonont rotifers, previously changing from asexual to partial sexual reproduction (mixis). In despite that zooplankton are frequently used in ecotoxicological assessment because of their sensitivity to various toxicants and their important role in the ecosystems, toxicity evaluations often consider the directly exposed population produced by parthenogenetic reproduction, exclusively. We assessed experimentally effects of exposure to metals on mixis delay and fitness of hatchlings of the rotifer Brachionus plicatilis obtained from a brackish water lagoon with high metal content, especially copper. We show that sub-lethal concentrations of copper affected traits related to sexual reproduction and diapausing egg production in the rotifer. Copper addition did not delay the start of mixis, suggesting that rapid initiation of mixis is promoted in risky environments, according to the hypothesis of mixis as an escape strategy. Higher investment in mixis was obtained when individuals were exposed to metal. Addition of copper negatively affected the hatching success of diapausing eggs and performance of hatchlings. Nevertheless, these effects were greater for individuals formed in non-metal conditions, suggesting an adaptive advantage of populations from natural sediments exposed to copper. These results highlight the ecological and evolutionary consequences of the presence of metals in freshwater environments by modulating diapause adaptive efficacy and the selective process in egg banks.

The Physiology and Ecology of Diapause in Marine Copepods.

Diapause is a type of dormancy that requires preparation, typically precedes the onset of unfavorable conditions, and necessitates a period of arrest before development can proceed. Two ecologically important groups of copepods have incorporated diapausing stages into their life histories. In freshwater, estuarine, and coastal environments, species within the Centropagoidea superfamily can produce resting eggs containing embryos that may be quiescent, diapausing, or in some intermediate state. Resting eggs sink into the sediments, remain viable over months to years, and form a reservoir from which the planktonic population is reestablished. In coastal and oceanic environments, copepods within the Calanidae and Eucalanidae families can enter diapause during late juvenile (copepodid) or adult stages. These copepods accumulate large amounts of lipids before they migrate into deep water and diapause for several months. Through respiration, diapausing copepods may sequester more carbon in the deep ocean than any other biogeochemical process, and changes in diapause phenology associated with climate change (particularly reduction in diapause duration) could have a significant impact not only on regional ecosystems, but on global climate as well.

Long-term competitive dynamics of two cryptic rotifer species: diapause and fluctuating conditions.

Life-history traits may have an important role in promoting species coexistence. However, the complexity of certain life cycles makes it difficult to draw conclusions about the conditions for coexistence or exclusion based on the study of short-term competitive dynamics. Brachionus plicatilis and B. manjavacasare two cryptic rotifer species co-occurring in many lakes on the Iberian Peninsula. They have a complex life cycle in which cyclical parthenogenesis occurs with diapausing stages being the result of sexual reproduction. B. plicatilis and B. manjavacasare identical in morphology and size, their biotic niches are broadly overlapping, and they have similar competitive abilities. However, the species differ in life-history traits involving sexual reproduction and diapause, and respond differently to salinity and temperature. As in the case of certain other species that are extremely similar in morphology, a fluctuating environment are considered to be important for their coexistence. We studied the long-term competitive dynamics of B. plicatilis and B. manjavacas under different salinity regimes (constant and fluctuating). Moreover, we focused on the dynamics of the diapausing egg bank to explore how the outcome of the entire life cycle of these rotifers can work to mediate stable coexistence. We demonstrated that these species do not coexist under constant-salinity environment, as the outcome of competition is affected by the level of salinity—at low salinity, B. plicatilis excluded B. manjavacas, and the opposite outcome occurred at high salinity. Competitive dynamics under fluctuating salinity showed that the dominance of one species over the other also tended to fluctuate. The duration of co-occurrence of these species was favoured by salinity fluctuation and perhaps by the existence of a diapausing egg bank. Stable coexistence was not found in our system, which suggests that other factors or other salinity fluctuation patterns might act as stabilizing processes in the wild.

Allocation patterns in modes of reproduction in two facultatively sexual cryptic rotifer species

Many zooplankters rely on diapausing stages to survive unsuitable conditions in time-varying habitats. In facultativesexualrotifers, reproductive effortallocatedtothe sexuallyproduced diapausingeggs isat the expenseofthe subitaneousparthenogenetic eggs, generatingatrade-offbetweencurrentand future population growth.Thetimingand the amountof sex (the sexual pattern) affect diapausing-egg production. This switch to sex is complex because the reproductivemode is separated in distinct females: asexual (female-producing), unfertilized sexual (male-producing) and fertilizedsexual (diapause-egg-producing). We studied sexual patterns and life-history variation of these females in two crypticspecies (Brachionus plicatilis and Brachionus manjavacas) co-occurring in Spanish ponds. Results revealed species-specific dif-ferencesinsexual pattern;B. plicatilis had an earlierand higher sex allocation. Female types allocated resources different-ly among life-history traits, revealing relationships between lifespan and reproductive traits that demonstrate the cost ofreproduction. Sexual females reproductive traits showed between-species differences. Brachionus manjavacas unfertilizedfemales produced more sons earlier and at a greater daily rate than B. plicatilis ones. Moreover, B. manjavacas fertilizedfemales had higher relative allocation per diapausing egg than those of B. plicatilis. We relate these differences to the en-vironmentaluncertaintyfacedbyeachspecies,anddiscusstheirimplicationsforcompetitiveoutcome.KEYWORDS: density threshold for sex initiation; life-table parameters; female reproductive types; competition;diapausing eggs

Zooplankton competition promotes trade-offs affecting diapause in rotifers.

Facultative diapause should be favoured by natural selection in temporary variable habitats. Diapause patterns are evolutionary constrained because producing diapause is resource demanding, which might have implications for competitive dynamics and competitor coexistence through mechanisms such as the storage effect. Besides these implications, competition intensity might affect the quality of the diapausing stages and the reproductive success of the offspring emerging from them. We experimentally analysed traits involved in diapause in the cyclically parthenogenetic rotifer Brachionus calyciflorus, in relation to the presence of its competitor, the cladoceran Moina micrura. Under competition with Moina, Brachionus produced fewer diapausing eggs, most of which possessed visual attributes indicating a higher quality. These diapausing eggs produced under a competitive environment had a higher hatching success; however, the hatchlings exhibited a lower population growth rate. We propose the existence of trade-offs between traits related to diapause: the number of produced eggs, quality of these eggs and hatchling performance. Our results show that interspecific competition may cause fine-tuned changes in the life cycle patterns of the populations. Furthermore, these changes could affect that abundance and performance of competitors and thereby drive back effects on the competitive output. These diapause-driven feedback mechanisms may have strong implications for the dynamics of the natural communities.

Can biotic resistance be utilized to reduce establishment rates of non-indigenous species in constructed waters?

Understanding the mechanisms that facilitate establishment of non-indigenous species is imperative for devising techniques to reduce invasion rates. Passively dispersing non-indigenous organisms, including zooplankton, seemingly invade constructed waters (e.g., ornamental ponds, dams and reservoirs) at faster rates than natural lakes. A common attribute of these invaded water bodies is their relatively young age, leading to the assertion that low biotic resistance may lead to their higher vulnerability. Our aim was to determine if seeding of young water bodies with sediments containing diapausing stages of native zooplankton could accelerate community development, leading to greater biotic resistance to the establishment of new species. Twenty outdoor tanks were filled with water (1,400 L) and nutrients added to attain eutrophic conditions. Ten treatment tanks had sediments added, sourced from local water bodies. In the remaining ten, sediments were autoclaved, and received zooplankton via natural dispersal only. In an initial 12 month monitoring period, species richness increased at a greater rate in the treatment tanks (at 12 months average standing richness per tank = 3.8, accumulated richness = 8.2) than control tanks (2.6 and 5.0, P < 0.05). Treatment tanks developed assemblages with greater proportions of species adapted to pelagic conditions, such as planktonic cladocerans and copepods, while control tanks generally comprised of smaller, littoral dwelling, rotifers. Analysis of similarities indicated community composition differed between the control and treatment groups at 12 months (P < 0.01). Two copepod, four rotifer and one cladoceran species were intentionally added to tanks at 12 months. In the 3 month post-introduction period, five of these species established populations in the control tanks, while only two species established in the treatment tanks. The calanoid copepod Skistodiaptomus pallidus, for example, a non-indigenous species confined to constructed waters in New Zealand, established exclusively in tanks where native calanoid copepod species were absent (primarily control tanks). Our study suggests that biotic resistance could play an important role in reducing the establishment rate of non-indigenous zooplankton. It also provides evidence that seeding constructed water bodies with sediments containing diapausing eggs of native species may provide an effective management tool to reduce establishment rates of non-indigenous zooplankton.

Application of an inexpensive and high‐throughput genomic DNA extraction method for the molecular ecology of zooplanktonic diapausing eggs

We describe the application of a simple, low‐cost, and effective method of DNA extraction (hot sodium hydroxide and Tris, HotSHOT) to the diapausing propagules of continental aquatic invertebrates for its use in PCR amplification. We illustrate the use of the technique in cladocerans, rotifers, anostracans, notostracans, and copepod diapausing eggs. We compare the performance of the HotSHOT technique to the currently most widely used method for DNA extraction of zooplankton eggs and individuals, the chelating resin (or Chelex) technique. The HotSHOT technique overcomes several of the problems posed by Chelex and permits easy optimization for its use with 96‐well plates for high‐throughput DNA extraction and subsequent genetic characterization. We foresee a wide use of this technique in the future from DNA barcoding of diapausing stages to the genetic characterization of the diapausing egg banks of continental aquatic invertebrates.

Effect of diazinon on life stages and resting egg hatchability of rotifer Brachionus plicatilis

The effects of organophosphate pesticide, diazinon, on life history parameters and hatchability of resting eggs of rotifer Brachionus plicalitis were assessed. Newly hatched (<1 h-old) neonates were individually cultured in six varying concentrations (0/control, 0.1, 1.0, 2.5, 5.0 and 10.0 mg/l) of diazinon. The life history parameters such as time (h) the rotifers bear first egg and release first neonate, reproductive period, net reproductive rate, mixis, intrinsic rate of population increase, and life span were evaluated. Results showed that among the life history parameters, the time the rotifers took to release neonates is the most sensitive, giving the lowest EC50 value of 1.24 mg/l. The fecundity of maternal females, amictic and mictic daughters was also investigated. Rotifers exposed to 10.0 mg/l produced significantly fewer amictic daughters, and at this concentration, rotifers did not produce any mictic daughter. At 5.0 mg/l, the number of male offspring was significantly lower than the control. Furthermore, the hatchability of resting eggs produced by the rotifers was evaluated when exposed to diazinon: from birth until they produced resting eggs (early development); during late developmental stage of resting eggs (before diapause); and during diapausing stage. The hatchability of the resting eggs was not affected when exposure was timed at late developmental and diapausing stages. Overall results showed that even though amictic females reproduced normally in the presence of low-concentration of diazinon, sexual reproduction is severely affected, especially the hatchability of resting eggs when the exposure was timed on its early developmental stages.

Fouling of fishing line by the waterflea Cercopagis pengoi: a mechanism of human-mediated dispersal of zooplankton?

The fishhook waterflea Cercopagis pengoi was first reported in Lake Ontario in 1998, but subsequently spread to Lakes Michigan and Erie as well as some inland lakes. One possible mechanism of dispersal to inland lakes occurs via fouling of and subsequent transfer on sport fishing lines. Here we explore fouling of Cercopagis on different commercial brands of fishing lines while trolling on Lake Ontario. Accumulation of animals varied significantly across brands, and was lowest on Flea Flicker brand. Fouling was more intense with line set at 20 than at 10 m, and was directly related to the distance trolled. Different pound-tests of Flea Flicker did not vary significantly in Cercopagis accumulation rates. The maximum number of Cercopagis and diapausing stages on a fishing line was 1024 individuals and 106 diapausing eggs. Because diapausing eggs may remain viable for weeks or longer, their transfer on fouled fishing line to non-invaded lakes poses a risk of invasion and supports previous studies that suggested sport fishermen as possible vectors of dispersal of invasive waterfleas.

Invasion risk of active and diapausing invertebrates from residual ballast in ships entering Chesapeake Bay

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 324:57-66 (2006) - doi:10.3354/meps324057 Invasion risk of active and diapausing invertebrates from residual ballast in ships entering Chesapeake Bay Ian C. Duggan1,*, Sarah A. Bailey2, Colin D. A. van Overdijk2, Hugh J. MacIsaac2 1Centre for Biodiversity and Ecology Research, Department of Biological Sciences, The University of Waikato, Private Bag 3105, Hamilton, New Zealand 2Great Lakes Institute for Environmental Research, The University of Windsor, Windsor, Ontario N9B 3P4, Canada *Email: i.duggan@waikato.ac.nz ABSTRACT: We examined the invasion risk posed by active invertebrates and their diapausing stages (e.g. resting eggs, quiescent adults) carried in residual sediment and water of non-ballasted ships to Chesapeake Bay. Many taxa were recorded that are not native to Chesapeake Bay, supporting the contention that residual ballast represents an invasion vector of some risk to marine systems. Composition and propagule supply differed relative to that in ships entering the Laurentian Great Lakes (e.g. marine taxa dominated in Chesapeake Bay ships), indicating that risk varies geographically. Average abundances of active invertebrates in residual sediment (1002.1 ind. kg–1) and water (2.7 ind. l–1), and diapausing eggs in sediments (779.4 eggs kg–1), were typically low relative to those in ships entering the Great Lakes (1322.5 ind. kg–1, 10.9 ind. l–1 and 3650.0 eggs kg–1, respectively). However, due to high variability among ships, differences were not statistically significant. The major cause of composition and abundance differences is dissimilar trade routes between each system, with vessels entering Chesapeake Bay primarily originating from marine rather than freshwater ports, and because diapausing stages are less commonly found among marine invertebrates. Low propagule supplies, predominant intra-continental ship movements, and salinity disparity between the upper (20 to 28‰) and lower (3 to 8‰) regions of Chesapeake Bay (where ballast water is loaded and offloaded) may greatly reduce invasion risk and be a contributing factor to the bay’s low invasion rate: invasion risk from non-ballasted ships here may be low relative to hull fouling or ballast water discharge. Other marine coastal areas may be at greater risk from this vector. KEY WORDS: Ballast sediments · Nonindigenous species · Great Lakes · Resting eggs · NOBOB vessels Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 324. Online publication date: October 23, 2006 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2006 Inter-Research.

In situ hatching of invertebrate diapausing eggs from ships’ ballast sediment

ABSTRACTShips that enter the Great Lakes laden with cargo carry only residual ballast water and sediment in ballast tanks. These ships are designated ‘no ballast on board’ (NOBOB) and constitute &gt; 90% of inbound traffic. We conducted in situ experiments using emergence traps to assess the viability and the introduction potential of invertebrate diapausing stages present in ships’ ballast sediment. All trials commenced while vessels operated on the lower lakes (Erie, Ontario) and were completed 6–11 days later at ports on the upper lakes (Michigan, Lake Superior). Eight trials were conducted on four ships using five different ballast sediments. Hatching was observed on every ship, although not from all sediments on all ships. Overall hatch rates were very low (0.5 individuals per 500 g sediment), typically involving activation of &lt; 0.05% of total eggs present. Five species of rotifers and copepod nauplii were hatched from ballast sediments, although only one or two species typically hatched from any one sediment. Results of this study indicate that hatching of diapausing eggs contained in ballast sediment of NOBOB ships poses a relatively low risk of invasion to the Great Lakes. However, as reproduction may occur in tanks, and non‐indigenous species may be involved in numerous introduction events, the risk posed by this vector is small but potentially important. While dormancy is a characteristic enabling enhanced survival during transportation in ballast tanks, it becomes a hindrance for introduction.

Hatching of cladoceran resting eggs: temperature and photoperiod

Summary 1. We identified temperature and photoperiod conditions under which the hatching of 45 cladoceran species could be elicited. Identification of appropriate hatching cues is of primary importance for the exploration of the ties between active and diapausing stages.2. Incubation temperature affected the hatching success of resting eggs isolated from Danish, Belgian/Dutch and Spanish sediments. In general, most hatchlings and species were retrieved at 15 °C. Danish and Belgian/Dutch resting eggs hatched more successfully under a long day photoperiod than in continuous illumination.3. Most species could be retrieved after incubation of resting eggs isolated from a limited amount of sediment (0.4 kg) under a single, well chosen combination of temperature and photoperiod. Processing additional sediment samples under seven more incubation regimes only allowed detection of 21% (Spain) to 34% (Denmark) more species.4. The incubation period for resting eggs to hatch was strongly influenced by incubation temperature. Our results show that hatching experiments aimed at assessing cladoceran species richness and conducted at 15 °C should be continued for a period of at least 2 weeks, after which a random subset of hatchlings (e.g. n = 100) can be selected from the total hatchling assemblage.

Embryonic external morphogenesis of Rhammatocerus conspersus (Bruner) (Orthoptera: Acrididae: Gomphocerinae) and determination of the diapausing embryonic stage

The grasshopper Rhammatocerus conspersus (Bruner) (Orthoptera: Acrididae: Gomphocerinae) is an occasional pest in pasturelands of Rio Grande do Sul State. It is a univoltine species with an embryonic diapause. Nymphal and adult stages occur during the warmer months (November-March). Eggs were dissected periodically for characterization of embryonic external morphogenesis in 1994 and 1995. Ten embryonic stages were illustrated. Two diapausing stages were verified in R. conspersus: one at 25% and another at 50% of total embryonic development.

Factors affecting carbohydrate and free amino acid content in overwintering larvae of Enosima leucotaeniella

Amounts of several metabolites were measured in overwintering larvae of Enosima leucotaeniella acclimated to temperatures between −5 and 15°C for 30 days. In the diapausing stage, cold hardiness, as shown by the survival rate, began rising below 15°C. Glycogen content decreased as the temperature decreased from 10 to 0°C. Trehalose content rose as the temperature decreased from 15 to 5°C, but remained unchanged as the temperature decreased from 5 and 0°C. Twenty-eight free amino acids were detected in the haemolymph; levels of proline, glutamine and glutamic acid increased at high temperatures, but alanine increased at low temperatures, especially as temperature decreased from 5 to 0°C. Lipid content was unchanged by the different acclimation temperatures. The effects of temperature, diapause and aerobic conditions on the levels of carbohydrates and amino acids in overwintering larvae were analyzed. Alanine levels rose at low temperature only when the larvae were in the diapausing stage. The level of trehalose rose at low temperature in both the diapausing and post-diapausing stages, although it was higher at aerobic conditions in the post-diapausing stage. These results suggest that efficient trehalose synthesis occurs under the combination of low temperature and aerobic conditions of the post-diapausing stage, so that cold hardiness in overwintering E. leucotaeniella larvae may rise to a high level in winter.

Tolerance of the diapausing stages of Plodia interpunctella and Ephestia cautella (Lepidoptera: Pyralidae) to methyl bromide and its correlation with diapause intensity

The duration of diapause at 20°C in eight stocks of Plodia interpunctella and two stocks of Ephestia cautella was compared with the tolerance of diapausing larvae to methyl bromide, the fumigant used for the control of infestations of these moths in the food industry. The tolerance of the P. interpunctella stocks ranged from quite low to very high, the most tolerant individuals requiring a ct product of 280 mg hr/l. for complete kill. Some changes were evident in the individual tolerances of six stocks of this species over a 6 yr period, probably as a result of a set laboratory rearing procedure. The two stocks of E. cautella both showed a moderate degree of tolerance, all larvae dying at concentration time (ct) products of 150 mg hr/l. or above at 15°C. Taking all results together, a significant direct correlation was established between either mean or maximum duration of diapause and the LD 50, 90 or 99, obtained for methyl bromide. Thus the longer the duration of diapause in these moths, the higher was the tolerance shown to methyl bromide. An increase in the mean duration of diapause from <50 to >80 days was accompanied by an increase in the LD 99 from <130 to >210 mg hr/l.