Low Host Density Research Articles

Analysis of foraging behavior of the whitefly parasitoidEncarsia formosa (Hymenoptera: Aphelinidae) in an experimental arena: A simulation study

Foraging ofEncarsia formosa was analyzed using a stochastic simulation model of the parasitoid's behavior. Parasitoids were allowed to search during a fixed time in an experimental arena with immatures of the greenhouse whitefly,Trialeurodes vaporariorum. The model simulates host searching, selection, and handling behavior and the physiological state (egg load) of the parasitoid. The simulated number of hosts encountered, parasitized, or killed by host feeding agreed well with observations on leaf disks. The hypothesis of random host encounter seems to be correct. The number of ovipositions on the leaf at low host densities was strongly affected by the parasitoid's walking speed and walking activity, the probability of oviposition after encountering a host, and the initial egg load. At high densities, the initial and maximum egg load were most important. A strong temperature effect was found at 18°C or below. The number of encounters, ovipositions, and host feedings increased with host density to a maximum of 25, 6.5, and 1.5, respectively, during 2 h at 25°C. The shape of the curves resembled a Holling Type II, which may be the result of the “experimental” procedure, where a parasitoid was confined to a patch during a fixed time.

Changes in Oviposition Behavior of the Ectoparasitoids Nasonia vitripennis and Muscidifurax zaraptor (Hymenoptera: Pteromalidae) When Using Different Species of Fly Hosts, Prior Oviposition Experience, and Allospecific Competition

Oviposition behavior was used to determine if handling time was a reliable indicator of host suitability for oviposition by the ectoparasitic wasps, Nasonia vitripennis (Walker) and Muscidifurax zaraptor Kogan & Legner, on pupae of Sarcophaga bullata Parker, the black blow fly, Phormia regina (Meigen), and the house fly, Musca domestica L. Oviposition behavior of both parasitoids was composed of a series of stereotypic and interdependent events or phases. The duration of behavioral events preceding puparium penetration was influenced by host species, indicating that host recognition depends on some external cue associated with the fly puparium. Total handling time was stereotypic regardless of host species or the number of eggs laid on a host, suggesting that the amount of time invested by N. vitripennis and M. zaraptor in oviposition does not predict clutch size or host preference. When N. vitripennis and M. zaraptor were held together at low host densities, females of N. vitripennis reacted aggressively and interfered with all oviposition attempts by M. zaraptor . Aggression by N. vitripennis toward M. zaraptor subsided when host density was increased, presumably because this was a less competitive situation.

A Count-Down Mechanism for Host Search in the Parasitoid Venturia canescens

On the basis of his study of patch time allocation by Venturia canescens, a larval parasitoid of phycitid flour moths (Lepidoptera: Pyralidae), J. K. Waage proposed a decision mechanism for patch exploitation. This putative mechanism would be efficient in habitats, such as granaries, consisting of patches with heterogeneous host densities. However, the distribution of hosts in mummified fallen fruits, a common natural substrate, differs from their distribution in granary stores, tending to be rather uniform with patches containing a few, mostly one, host larva. This discrepancy led us to re-examine Waage's mechanism. We investigated V. canescens's decision mechanism in small patches containing low host densities. Following the previous study, we tested the relation between the probability per unit time to abandon a patch and the following variables: the concentration of the contact kairomone produced by host larvae, the time elapsed since the first patch entry and the occurrence of ovipositions. A major component of Waage's model is an increase in the tendency to remain in a patch after an oviposition. In habitats where hosts are uniformly distributed, and particularly when patches contain a single host, this behaviour would not be adaptive. Our results confirm that V. canescens spends more time on patches with higher concentrations of contact kairomone and that the probability per unit time of leaving the patch increases with patch residence time. Ovipositions, however, decrease the amount of time subsequently spent by the parasitoid on the patch. Based on these results we formulate a post-hoc 'count-down' model for the decision rule for patch leaving in V. canescens in habitats with uniform host distributions. There is no evidence for or against the possibility that this parasitoid may be capable of facultative changes in its patch exploitation rule as a function of host distribution. Although increases in patch time after oviposition have been found in several other parasitoid species, re-examination of Waage's experiments shows that his results do not unambiguously support the existence of such a mechanism in Venturia.

Structure, organization, and response of a species-rich parasitoid community to host leafminer population dynamics.

The parasitoid community dynamics of an agromyzid honeysuckle leafminer, Chromatomyia suikazurae (Agromyzidae, Diptera) were studied between 1981 and 1990 in a natural forest in Kyoto, Japan. The parasitoid fauna composed three koinobionts (all larval-pupal solitary parasitoids) and 22 idiodiont species (11 larval solitary, nine pupal solitary and one pupal gregarious). The parasitoid community was dominated by early-attacking oligophagous braconid koinobionts at early periods, but was gradually displaced by late-attacking polyphagous eulophid idiobionts. Accordingly, the diversity index of the parasitoid community peaked at an intermediate point in the intra-generational succession. The succeeding attack-in-waves by the late-attacking idiobionts greatly reduced not only the survival rates of early-attacking parasitoid larvae but also the survival rates of hosts. The density-dependence observed in the host pupal mortality was thought to result from density-dependent host-switching by a keystone polyphagous pupal idiobiont parasitoid, Chrysocharis pubens, whereas high host pupal mortality was potentially attained by an early-attacking koinobiont braconid. Supposed aggregation of polyphagous parasitoids at high host density resulted in intense within-host competition and in an increase of host-feeding attack, both of which contributed to low emergence rates of parasitoids at high host densities. Parasitoid emergence rates were also reduced at low host densities, probably by inter- and intra-specific hyperparasitism among oligophagous parasitoids for limited hosts. The regulation effects of the species-rich parasitoid community upon the host population dynamics are thought to derive from succeeding attack-in-waves by polyphagous late-attacking idiobionts, especially by the keystone species.

Functional response of Diaeretiella rapae (McIntosh) (Hym., Aphidiidae) to Diuraphis noxia (Mordwilko) (Hom., Aphididae) hosts

AbstractFunctional‐response experiments involving the parasitoid Diaeretiella rapae (McIntosh) (Hym., Aphidiidae) and Diuraphis noxia (Mordwilko) (Homo., Aphididae) hosts were conducted. The host densities used were 10, 20, 40, 80, and 160 hosts per arena per day. Searching bouts lasted 24 h, and female parasitoids were allowed to search over two consecutive days. New hosts were offered to parasitoids after the first 24 h. Temperature was maintained at ca. 21°C, photoperiod was set at 14L:10D, and humidity ranged from 50 to 70 % RH. In general, the offspring sex ratio did not appear to vary significantly with a host density of up to 80 hosts per female per day. However, the proportion of female offspring increased when female parasites were offered 160 hosts per day. In addition, the overall proportion of female offspring produced across all host densities offered decreased on day 2 (0.582 ± 0.037) relative to day 1 (0.661 ± 0.023). Diaeretiella rapae displayed a Type 2 functional response to D. noxia hosts during the first 24 h. The functional response displayed during the second 24 h was unlike any response commonly reported in the literature, with an initial rise followed by a sharp decline in numbers of mummies produced when more than 40 hosts were offered. This was presumably due to the greater depletion of the female parasitoid's egg complement after 24‐h exposures to high host densities (80 and 160 hosts), rather than to lower host densities (10, 20 and 40 hosts). The functional response parameters for day 1 were à =1.341 arenas per day and Th = 0.017 days per mummy. The maximum attack rate per 24‐h period (i.e. 1/Th) was 59.5 mummies.

The importance of low host densities for successful parasitism of diederik cuckoos on red bishop birds

Parasitism by diederik cuckoos Chrysococcyx caprius on their main southern African host, the red bishop bird Euplectes orix, was studied. Virtually all the cases of parasitism occurred where there were low densities of red bishop nests. Data from the present study were combined with published observations to show that the incidence of parasitism is much higher in small breeding colonies of red bishops than in large colonies. Aggression of red bishops towards diederiks is described. I suggest that diederik cuckoos cannot approach red bishop nests undetected where large concentrations of red bishops are found. High levels of brood failure in small red bishop colonies caused by brood parasitism may constitute a selective force for colonial breeding in this host species.

The Role of Disturbance, Topography, and Forest Structure in the Development of a Montane Forest Landscape

HADLEY, K. S. (Department of Geography, Western Oregon State College, Monmouth, OR 97361). The role of disturbance, topography, and forest structure in the development of a montane forest landscape. Bull. Torrey Bot. Club 121: 47-61. 1994.-Human set fires beginning in the mid 1 800s and repeated insect outbreaks of western -spruce budworm (Choristoneura occidentalis Free.) and Douglas-fir bark beetle (Dendroctonus pseudotsugae Hopk.) during the past 50 years have resulted in a dramatic change in the montane (<ca. 2900 m) forest landscape of the Colorado Front Range. Here, I examine the historical and spatial relationship between these disturbance agents and topography using stand structure and dendroecological data from 38 contiguous stands. These data suggest that aspect and relief are important factors determining the spatial and temporal patterns of disturbance, succession, and rates of stand development. The rate of postfire stand development and hence, subsequent stand susceptibility to insect outbreaks appears to be related to aspect. North-facing stands experience rapid postfire development and greater susceptibility to insect attack due to higher host tree densities, larger mean tree size, and a more uniform distribution of host trees over larger contiguous areas. Postfire stand recovery on south facing slopes appears to be slower and stand susceptibility to insect attack is less due to lower host densities, smaller mean tree size, and a less uniform distribution of host trees over smaller areas. Relief, independent of aspect, enhances the structural diversity of the forest landscape by promoting irregular burn patterns and intensities, thus creating a fire-induced mosaic of different aged stands. As these different aged stands continue to grow older, they reach a stage of development susceptible to insect outbreaks at different times. As a result, insect-induced changes in the structural characteristics of the current landscape emulate fire-induced landscape patterns that developed largely due to human activities beginning in the 1860s.

Sex ratios in natural populations ofAphelinus mali (Hym.: Aphelinidae) in relation to host size and host density

In an apple orchard at Armidale, the Northern Tablelands of NSW, population sex ratios ofAphelinus mali (Haldeman), an endoparasitoid of the woolly apple aphid,Eriosoma lanigerum (Hausmann) varied from 0.51 (proportion of males) at low host densities to female-biased at high host densities (proportion of males ranged from 0.35–0.39). This shift in sex ratio seems to be caused by the differences in allocation of sons and daughters to hosts of different sizes. In the fieldA. mali parasitizes all life stages (four nymphal instars and adult) of the woolly aphid upon encountering. According to Hughes'(1979) optimal diet model, such general host acceptance seems to be the best strategy. However, it allows the host nymphs or adults to continue to develop or reproduce until about to mummify (pupate). No mortality was observed when first or second-instar hosts were parasitized in the laboratory. Field collected small mummified hosts yielded male-biased sex ratios whereas large mummified hosts produced mainly females. In the laboratory, progeny from smaller hosts (first to third-instars) produced sex ratios which were not significantly different from 0.5 whereas progeny from larger hosts (third and fourth-instars) produced female-biased sex ratio. During winter (June–August) and early spring (September–October) when the host populations in the orchard were predominantly nymphs, the parasitoid tended to allocate equal resources to male and female offspring. In contrast, at peak population densities in summer and autumn (December–May) when larger hosts were available, the sex ratios were female-biased. The host size ofE. lanigerum andA. mali is, therefore, an important component in the dynamics of host-parasitoid interactions.

Factors Affecting Supernumerary Egg Deposition by the ParasitoidLysiphlebus delhiensis(Subba Rao &amp; Sharma) (Hymenoptera: Aphidiidae) into its HostRhopalosiphum maidis(Fitch)

ABSTRACT Effects of six host densities (5 to 120 aphids Rhopalosiphum maidis (Fitch)) and three exposure periods (1h, 6h and 24h) on the egg distribution by the parasitoid Lysiphlebus delhiensis (Subba Rao & Sharma) were studied under laboratory conditions. The observations demonstrated that the probability of deposition of eggs into the parasitized host (supernumerary eggs) decreased with increase of the host-patch size and increased following longer exposure of the hosts to the parasitoids. The females distributed their eggs randomly into the hosts at lower host densities (≤ 40) at all exposure periods. However, at higher host densities (≥ 80) the distribution became non-random, i.e. more hosts than expected from random oviposition were parasitized only once and fewer than expected parasitized several times. It is suggested that L. delhiensis is able to distinguish parasitized hosts from healthy ones; however the parasitoid is motivated somehow to oviposit supernumerary eggs when confined with lower host numbers for a longer period. The significance of host discrimination by L. delhiensis is discussed.

Oryctes rhinoceros (Coleoptera: Scarabaeidae) populations suppressed by a baculovirus

Oryctes rhinoceros (L.) breeding sites and damage to coconut palms ( Cocos nucifera L.) were surveyed in the Republic of the Maldives before and after the introduction of a baculovirus disease and in untreated areas of the Indonesian provinces of North and South Sulawesi. The results and those of previous surveys showed that the baculovirus disease usually has little influence on the overall survival rate of O. rhinoceros larvae, but that it reduces dramatically the average life span of the adult beetles. This and the ability of the virus disease to persist at low host densities are the main reasons for its success as a microbial control agent. The data provide additional evidence that the presence of a low number of dead standing palms per hectare promotes the spread of the virus disease because these are highly attractive to adult beetles. It is concluded from the results that in areas where the baculovirus is indigenous, control methods should be directed against the larval population of O. rhinoceros.

Response ofChelonus blackburni [Hym.: Braconidae] to different ages and densities of potato tuber moth eggs

The response of an efficient egg-larval parasitoidChelonus blackburni Cameron to different ages and densities of potato tuber mothPhthorimaea operculella (Zeller) eggs was studied in a laboratory system. The parasitoid accepted 0–1, 1–2 and 2–3 day old eggs for parasitization. However, the eggs belonging to the first two age groups were significantly more acceptable when compared to the last age group. At low host density levels of 2 and 5 eggs per parasitoid, parasitoid emergence was zero. On increasing the host density, percent progeny production increased till a plateau was reached at a parasitoid-host ratio of 1∶50. This ratio thus proves to be ideal for obtaining maximum percent progeny production in the case of 0–1 and 1–2 day old eggs. Further increase in host density did not result in any significant increase in percent progeny production.

Glyptapanteles flavicoxis (Hymenoptera: Braconidae) Dispersal in Relation to Parasitism of Gypsy Moth (Lepidoptera: Lymantriidae)

Glyptapanteles flsavicoris Marsh is a recently imported but unestablished parasitoid of the gypsy moth, Lymantria dispar (L.). Its dispersal and parasitization was field-tested along distance gradients at high (5,307 egg masses/hal and low (750 masses/hal host density sites. Host larvae were collected in each 14.3-ha site after the last of eight parasitoid releases. Parasitism of 13.8% was recorded in 7.4 ha at the low density site, and 1.4% parasitism was recorded in 2.6 ha at the high density site. Parasitism was aggregated around the release point for each site. Each female parasitized an estimated 68 and 9 gypsy moth larvae at the low and high density sites, respectively. Parasitism in branch sleeve cages on beech and oak was higher at low host densities. In laboratory tests, the effects of female density (1, 2, 3) and exposure time (12, 24, 48 h) were tested at constant host density. G. flavicoris parasitization significantly increased with female density. Results suggest that future G. flavicoxis release points should be separated by at least 150 m and that most parasitism will occur near the point of release within 12 h. G. flavicoxis may be most successful against low-density gypsy moth populations such as pre-outbreak infestations rather than against epidemic populations.

Sex ratios in field populations of Epidinocarsis lopezi, an exotic parasitoid of the cassava mealybug, in Africa

Abstract. In cassava fields in Africa, population sex ratios of Epidinocarsis fopezi vaned from 0.44 (males to total parasitoids) at low host densities to highly male‐biased ratios of 0.70 at high host densities. This variability is caused by the difference in allocation of sons and daughters to hosts of different sizes, through the following mechanisms: (a) small, i.e. second instar, hosts are mainly used for the production of male offspring, whereas in large, i.e. third instar, hosts a variable, female‐biased sex ratio is produced; (b) E.fopezi does not selectively oviposit into large hosts but always accepts both small and large hosts for oviposition upon encountering; (c) in the field, this parasitoid is time‐limited, and not egg‐limited. On the basis of an optimal diet model, such general host acceptance is shown to be the best strategy. Thus, sex ratio increases with host density for three reasons: the proportion of small hosts encountered in the field increases with increasing host density, small hosts are used for male production, and hosts are always accepted when encountered.

Effects of temperature and light on the population dynamics of the Asterionella-Rhizophydium association

Growth parameters of the diatom Astenonella formosa Hass, and its fungal parasite Rhizophydium planktoniacum Canter emend, were measured at five temperatures and six light intensities with a 15·9 h light:dark cycle, using laboratory cultures of both organisms. With the parameter values obtained, threshold host densities were calculated in order to estimate the effects of light and temperature on survival and epidemic development of the parasite The uninfected host reached light-saturated growth rates between 0.917 day −1 at 21°C and 0 285 day 1 at 2°C. Under light limitation the optimum growth temperature for Asterionella decreased because of a reduced growth efficiency Growth inhibition at high irradiances was only observed at 2°C The parasite reached the highest zoospore production at 2°C and saturating irradiances: 30 2 zoospores per sporangium. This value was consistently reduced by lower irradiances and higher temperatures to only 2.2 zoospores at the opposite light and temperature extremes Low light conditions depressed also the infectivity of the zoospores At very low irradiances, they became completely uninfective The light dependence of zoospore production and infectivity was restricted to light intensities that limited the growth rate of the host. The development time of the sporangia and the mfecti ve lifetime of the zoospores were not affected by light but only by temperature, and ranged from 19.0 and 12 1 days respectively at 2°C to 1.9 and 2 1 days at 21°C- These effects result in optimal conditions for the development of a Rhizophydium epidemic at 11°C and a moderate light limitation of Astenonella At temperature above 7°C, the possibilities for epidemic development are only slightly affected by light and temperature, except for very low irradiance levels, when the zoospores of the parasite become uninfective. However, below 5°C the development of an epidemic is only possible at limiting light levels. Conditions for survival of the parasite at low host densities are optimal at low temperatures and high irradiance levels

Oviposition pattern byCotesia ayerzai [Hym.: Braconidae] onAscia monuste [Lep.: Pieridae] under laboratory conditions

Various aspects of the parasitims of caterpillars ofAscia monuste orseis byCotesia ayerzai, were studied in laboratory choice tests. Individual ♂♂ were found to be extremely variable in ovipositional duration, as well as in the number of eggs oviposited. To simulate parasitoid dispersal, or low host density, we isolated ♂♂ for 60 min following initial exposure and compared our tests with ♂♂ which had not been isolated. No differences in (1) the number of eggs oviposited per host instar; (2) the conditional probability of host encounter; (3) host acceptance; and (4) the conditional probability of instar specific parasitism were found between these groups. Held ♂♂ did, however, reject hosts after ovipositor insertions in a higher proportion.

IMMATURE STAGES AND LIFE HISTORY OF ENYTUS SP. (HYMENOPTERA: ICHNEUMONIDAE), A PARASITOID OF PERTHIDA GLYPHOPA COMMON (LEPIDOPTERA: INCURVARIIDAE)

AbstractThe egg, 3 larval instars and pupa of Enytus sp. a univoltine, primary endoparasitoid on the feeding larva of the jarrah leafminer Perthida glyphopa Common, are figured and its life history described. Single eggs laid in July and August hatch rapidly, but the caudate larva develops slowly, allowing the host to complete feeding, construct its cell and aestivate in the soil. The parasitoid's final instar larva destroys the host in March, April or May of the following year, spins a cocoon and pupates in the cell where ecdysis occurs. The species was found most frequently in areas of low host density.

Interspecific competition between the principal larval parasitoids of the pine sawfly, Neodiprion sertifer (Geoff.) (Hym.: Diprionidae).

Interspecific competition between the 3 principal larval parasitoids of the pine sawfly, Neodiprion sertifer, is of common occurrence when total larval parasitism, and hence multiple attack rates, are high. At the intrinsic level, the ectoparasitoid Exenterus abruptorius is superior to the 2 endoparasitoids, Lophyroplectus luteator and Lamachus eques, respectively, whereas L. luteator is superior to L. eques only. During mass outbreaks of the host competition between Exenterus and Lophyroplectus is most intense, whilst Lamachus fails to build up in the presence of its 2 competitors (Fig. 1). However, due to its superior host finding ability, Lamachus dominates in areas of low host densities where the 2 other species are less efficient (Table 1). L. luteator frequently sustains heavy losses in direct competition with E. abruptorius, but these can be largely compensated in the next generation because its fertility is about 5-times higher than that of Exenterus.

Models for pest control: Complementary effects of periodic releases of sterile pests and parasitoids

Four host-parasitoid models that incorporate the simultaneous or sequential release during each generation of sterile hosts and parasitoids for control or eradication of host species are presented. The models are based on two modifications of the Nicholson-Bailey model which incorporate density regulation either in the host larvae or via parasitoid oviposition. Parasitization of host larvae and adults forms another comparison. The models indicate that the release of sterile hosts alone is more efficient than release of parasitoids alone in controlling the hosts if population regulation is in the parasitoids; otherwise, the release of parasitoids alone is more efficient. The release of both steriles and parasitoids is much more efficient than the release of either alone for either suppressing or eradicating the hosts. This greater efficiency in combination rather than separately appears to be a special case of a more general principle, which is that two pest control methods will mutually complement each other if their optimal actions in reducing host numbers are at very different host densities. This is the case for sterile releases (optimal at low host densities) and parasitoid inundation (optimal at high host densities).

Effects of Plant Diversity, Host Density, and Host Size on Population Ecology of the Colorado Potato Beetle (Coleoptera: Chrysomelidae)

Effects of host density and plant species diversity on populations of Colorado potato beetle, Leptinotarsa decemlineata (Say), were studied in experimental field plots. Hosts were grown at two densities and four levels of plant diversity: potato monoculture, potato/bean biculture, potato/bean/weed triculture, and potatoes grown in fallow plots. All life stages of the beetle were affected by both host density and plant diversity. Numbers of beetles per plant were highest in low-diversity cultures of low host density; higher host density or plant diversity was accompanied by a reduction in numbers of beetles per plant. Differences in host-plant size among treatments were partially responsible for these trends. When beetle densities were expressed as numbers per unit leaf area, treatment effects were lessened or lost. Changes in beetle density among treatments were accompanied by differences in fecundity among treatments, but not by differences in survival. Fecundity, calculated as numbers of eggs deposited per adult, was lower in high-diversity than in low-diversity plots. Survival rates (egg to late instar) and predation on eggs were not affected by plant species diversity.

Variable Patch Times and the Functional Response of Leptopilina boulardi (Hymenoptera: Eucoilidae)

The functional response of Leptopilina boulardi Barbotin et al. to variations in density of its major natural host, Drosophila melanogaster Meigen, was examined in the laboratory. A behavioral criterion was used to assess attempts by parasitoid females to leave experimental arenas; a patch visit was considered terminated whenever a searching female left the host medium for over 5 min. The results demonstrate density-dependent oviposition behavior in L. boulardi , apparently stemming from a tendency by parasitoid females to spend longer periods of time in patches containing higher densities of hosts, and to emigrate quickly from patches of low host density. However, the form of the functional response was ambiguous. Allowing parasitoid search times to vary among patches changes the spatial frame of functional response measurements and complicates their interpretation; a number of these complications are discussed.