Increasing Host Density Research Articles

SEASONAL INCIDENCE OF METEORUS AUTOGRAPHAE1 ON SOYBEAN LOOPER2 LARVAE ON SOYBEAN IN SOUTH CAROLINA, AND THE INFLUENCE OF HOST DENSITY ON PARASITIZATION3

The seasonal incidence of Meteorus autographae Muesebeck (Hymenoptera: Braconidae) on larvae of the soybean looper, Pseudoplusia includens (Walker) (Lepidoptera: Noctuidae), in soybean fields in South Carolina was monitored at two locations during 1981, 1982, and 1983. In most instances, parasitism of soybean looper larvae by M. autographae was detected 7 to 14 days after soybean looper larvae were first found in the fields. During this three-year study, maximum parasitism of soybean looper larvae by M. autographae within a season ranged from 4.6 to 19.8%. In the laboratory, numbers of hosts parasitized per 24 hours by M. autographae increased with each successive increase in host density. At a host density of 40 larvae per 1.75-liter cylindrical container, an average of 26 hosts were parasitized by one M. autographae during 24 hours. Percent parasitism remained about the same (64 – 70%) at all host densities.

Biology ofEpidinocarsis [=Apoanagyrus] lopezi [Hymenoptera: Encyrtidae] an exotic parasite of cassava mealybug,Phenacoccus manihoti [Homoptera: Pseudococcidae] in Nigeria

The biology ofEpidinocarsis (=Apoanagyrus) lopezi (DeSantis) was studied under fluctuating laboratory conditions of 24–31°C and 79–90% R.H. It is a solitary internal parasite. There are 4 tailed larval instars. The total developmental time from egg to adult ranged from 11–25 days, average 18 days. The duration of each developmental stage was: egg 2 days, 1 st instar 1 day, 2nd instar 1 day, 3rd instar 2 days, 4th instar 2 days, prepupa 4 days and pupa 6 days. Males mated several times, females mated once. Mated females produced both males and females, unmated females produced only males. Mated ovipositing females lived for 13 days while non-ovipositing mated females lived for 25 days. Parasitization decreased with increasing host density from 30% at a density of 10 hosts/plant/female to 11% at a density of 50. There was virtually no pre-oviposition period. Based on 10 females, the highest mean daily progeny production was 10. Females have an average oviposition period of 11 days and an average of 67 adult progeny. The sex-ratio was 1∶2.3 in favour of females.

Parasitoid–host relationship between Trioxys (Binodoxys) indicus Subba Rao &Sharma (Hymenoptera: Aphidiidae) and Aphis craccivora Koch (Hemiptera: Aphididae): effect of host plants on the area of discovery of the parasitoid

This paper elucidates the effect of different host plants on the area of discovery of the parasitoid Trioxys (Binodoxys) indicus, which decreases linearly with the increase of parasitoid density. This decrease is minimum in aphids reared on Cajanus cajan followed by those reared on Dolichos lablab and Solanum melongena, but the killing power (k value) increases significantly in all the tested host plants, being maximum in aphids reared on C. cajan followed by those reared on D. lablab and S. melongena. However, when the host density increases, the area of discovery and k value (high in C. cajan aphids followed by D. lablab and S. melongena aphids) increases up to 50 hosts and thereafter decreases. The leaf texture of the host plants and the interference between the parasitoids at higher densities is suggested as a cause for this type of behavioural response which is explained by the population model of M. P. Hassell and G. C. Varley. For the suppression of Aphis craccivora population, it is suggested that the parasitoid and the host ratio may be 1:50 and for the mass rearing of the parasitoid, when C. cajan is not available in the field, D. lablab may be used as an alternative host plant.

Functional response to host density by the egg parasiteTrichogramma pretiosum

The effect of host density on parasitism byTrichogramma pretiosum Riley was studied by exposing groups of 150, 300, 600 or 1200 eggs of potato tuber moth to 2, 4 or 8 female parasites per group. The parasite exhibited a type 2 functional response. As host density increasedT. pretiosum parasitised more hosts, but at a decreasing rate. The attack coefficient (a′) decreased as parasite density increased, whereas the handling time (T h ) remained almost constant. The search rate (a) decreased with increasing host density.T. pretiosum responded to increasing host density by increasing the number of its encounters with hosts and the number of hosts it parasitised only up to host density of 300 when the parasite density was 2 and up to host density of 600 when the parasite densities were greater and then remained almost constant. The observed incidence of parasitism was higher than that expected on the assumption that the parasites behaved the same at higher host densities as at the lowest. When parasite density was raised from 2 to 8 females per group the percentage of female progeny fell from about 73 to about 48%. A 2-fold increase in the number of female progeny was observed when parasite density was reduced from 8 to 2 and also when the host density was raised from 150 to 1200 eggs.

Patch Time Allocation and Parasitization Efficiency of Asobara tabida, a Larval Parasitoid of Drosophila

SUMMARY (1) Patch time allocation by the parasitoid Asobara tabida Nees was studied on patches with different host densities. (2) The parasitoid increased its searching time and giving up time with increasing host density. These increases were caused by a response of the parasitoid to the number of encounters with unparasitized hosts; the amount of kairomone in a patch may have had an additional incremental effect on patch time. (3) Encounters with parasitized hosts had no effect on searching time and giving up time. (4) The percentage of hosts parasitized first increased with host density and then levelled off at densities of more than four host larvae per patch, thus producing an accelerating functional response. (5) The foraging behaviour of A. tabida is compared to that predicted by optimal foraging models.

Some effects of insecticide deposit patterns on the parasitism of Trialeurodes vaporariorum by Encarsia formosa

SUMMARYThe functional response of Encarsia formosa parasitising Trialeurodes vaporariorum on Phaseolus vulgaris was a typical type II functional response (Holling, 1959) where, with increasing host density, the numbers of hosts parasitised by a single parasitoid show a negatively accelerating rise to an upper plateau. A mean maximum parasitism of 6.6 per female per 24 h reached at host densities near 80 larvae per leaf was observed. When presented with insecticide (‘Pynosect 30 ULV’) deposits, the parasite's search pattern changed, conforming to a typical Sigmoid functional response curve. With a uniform cover spray deposit of 27.6 × 10‐7 ml/cm2 searching began only at host densities of c. 80 larvae per leaf and the mean maximum parasitism was only 1.5 per female per 24 h. When the insecticide deposit of 27.6 × 10‐7 ml/cm2 was in the form of discrete groups of 100 μm droplets, searching began at host densities of c. 40 larvae per leaf but the mean maximum parasitism was low at 2.2. However by using the same discrete deposit pattern but reducing the deposit level to 2.65 × 10‐7 ml/cm2 it was possible to obtain the same high mean maximum parasitism as with the untreated control.

Ovipositional Response of Meteorus leviventris (Hymenoptera: Braconidae) to Various Densities of Agrotis ipsilon (Lepidoptera: Noctuidae) 1

The ovipositional response of the parasitoid Meteorus leviventris (Wesmael) was examined over five host densities. Fecundity was the same for all host densities tested. Daily oviposition peaked ca. 48 h after emergence, then decreased as parasitoids aged. Both the total number of eggs deposited and the number of hosts parasitized per 12 h were significantly higher during the dark phase of the LD 12:12 cycle. The maximum number of hosts parasitized during a l2-h period for any parasitoid was 8, with an average of 3.5 hosts for parasitoids exposed to 12 hosts per 12 h. With increasing host density, more total hosts were parasitized, resulting in lower numbers of eggs per host and fewer incidences of superparasitism.

The ectoparasitization ofColeophora alticolella(Lepidoptera) in relation to its aititudinal distribution

Abstract.1. The parasitization of the larvae ofColeophora alticolella. feeding onJuncus squarrosus, was investigated at a series of altitudes from 15 to 520m above sea‐level in northern England during 1977 and 1978.2. Six species of primary parasitoid and one hyperparasitoid were reared from this host. Five of the primary parasitoids were ectophagous; only two specimens of the endoparasitoid,Gonotypus melanostoma, were reared.3. All of the parasitoid species were recorded at 15 m but fewer at sites of higher altitude. Only one species,Scambus brevicomis, was recorded above 305 m, and none above 395 m. The hyperparasitoid,Tetrastichus endemus, waspresent only at 15 m.4. Percentage parasitization was highest at 15 m; it was reduced from 51% to only 2% between 215 and 305 m in 1978. There was an increase in host density over this altitudinal range.5. Three species,Scambus brevicomis. Elachertus olivaceusandEuderus viridis, accounted for most of the parasitization, but their relative proportions vaned at different altitudes.6. The sex‐ratios of the parasitoids reared fromColeophora alticolellaranged from 3.2% females forScambus brevicomis, which is considered to also use larger hosts, to 99.4% females forElachertus olivaceus, which develops by thelytokous parthenogenesis.7.Euderus viridisandScambus brevicomisstarted to attack theColeophora alticolellalarvae at a later date at 245 m than at 15 m, but attack byElachertus olivacatswas not delayed at the higher site.

The dynamics of infection of Tribolium confusum by hymenolepis diminuta: the influence of exposure time and host density.

Survival of Hymenolepis diminuta eggs (as measured by their infectivity to the intermediate host) was found to be age-dependent with an expected value of 11 days when the eggs were retained within the faecal pellet at 10 degree C. The expected life-span of eggs under experimental conditions (i.e. extracted from faecal material and placed on filter paper at 30 degree C) was estimated as 33 min. The mean parasite burden of populations of Tribolium exposed to known densities of H. diminuta eggs was found to rise to a plateau with increasing exposure time. The experimental results provided a method of estimating the instantaneous rate of parasite transmission: a value of 0.0004/egg/min/host/13 cm2 was obtained. The mean parasite burden of populations of T. confusum exposed to known densities of H. diminuta eggs was found to decrease exponentially with increasing host density. A second estimate of the instantaneous rate of parasite transmission of 0.004/egg/min/host/13 cm2 was obtained from the experimental results. No differences in susceptibility to infection between 2-week-old male and female beetles were found. A marked decrease, however, was apparent with increasing beetle age up to 14 weeks post-eclosion.

Effects of variable temperatures on responses ofAphytis melinus andA. lingnanensis to host density

The effects of three variable temperature regimes (18–24, 24–29, and 29–35°C) on the responses of two species of the genusAphytis Howard (Hymenoptera: Aphelinidae), parasitic upon the California red scale, were studied, and a test to analyze parasite behavior was developed.Aphytis lingnanensis Compere was generally more efficient thanA. melinus DeBach; however, whereas a high temperature regime (29–35°C) had a negative effect on the responses ofA. lingnanensis, it improved those ofA. melinus. The two species distributed their eggs in clumps, but were able to respond to increasing host density by reducing the number of eggs laid per encounter. The combined effects of numerical, and functional responses of the two species on host population were considered, with special reference to the mechanism of competitive displacement.

Laboratory studies ofDiadegma cerophagus [Hym.: Ichneumonidae], a parasite introduced to controlPlutella xylostella [Lep.: Hyponomeutidae] in Malaysia

Laboratory studies were conducted to determine some of the attributes ofDiadegma cerophagusGravenhorst, a parasite introduced to controlPlutella xylostella (L.). Longevity studies have shown that ♂ and ♀ could live for 40 and 73 days respectively when fed on diluted honey. The parasite is capable of attacking an average of 117±10 caterpillars per ♀. The number of caterpillars attacked per ♀ is dependent on the life span of the ♀. The average area of discovery ofD. cerophagus, based on studies of single searching parasites, was found to be 1.20±0.21. The area of discovery was found to decrease with increase in parasite or host density. However, the k-value would increase with increasing parasite density and decrease with increasing host density. The life-cycle ofD. cerophagus is as follows: egg stage, 1.5 to 2 days; larval stage, 5 to 8 days and pupal stage, 6 to 9 days. There are 4 larval instars and the duration of each instar is dependent on the age of the host attacked. The duration of the larval stage averaged 6.5 days when 1- and 2-day old caterpillars were exposed toD. cerophagus and took 5 days when 3- and 4-day old caterpillars were exposed.

Studies on the bionomics ofTrioxys (Binodoxys) indicus [Hym.: Aphidiidae]: Effect of population densities on sex ratio

The effect of different population densities ofTrioxys (Binodoxys) indicusSubba Rao & Sharma and its hostAphis craccivoraKoch, on the sex ratio (female: male) of the parasitoid was observed. The sex ratio (favouring female sex) increased with increase of host density. Parasitoid contamination priciple and behavioural responses of female parasitoid in addition to environmental factors were applied to explain the results. Results clearly indicated that fewer parasitoid releases might be better at any immediate release site for the production of maximum female progeny in this species.

Effects of the Microsporidium, Nosema pyrausta , on Field Populations of European Corn Borers 1 in Nebraska 2

The incidence and development of the protozoan parasite, Nosema pyrausta (Paillot), in field populations of the European corn borer, Ostrinia nubilalis (Hbn.), is documented for 2 Nebraska counties over a 16-yr period. In one county, N. pyrausta reached epizootic proportions twice and in the other the relationship was mostly enzootic. The epizootic development followed periods of increasing host density. Reduced ECB populations and larval weights followed epizootic peaks attesting to the debilitating and mortal effects of N. pyrausta on the host.

Entomophthora infecting wheat bulb fly at Rothamsted, Hertfordshire, 1967–71

SUMMARY Entomophthora dipterigena, E. hylemyiae and, most commonly, E. muscae infected wheat bulb flies at Harpenden, Hertfordshire, between 1967 and 1971. The mean annual percentages of infected flies caught were 19, 0, 1, 29 and 16 respectively. These showed an increase with increasing host density. In 1970, two‐thirds of the female flies were killed by E. muscae before they laid any eggs.Conidiophores developed in most flies infected with E. muscae during the 2 weeks after peak emergence of flies. Subsequently, resting spores developed in a progressively increasing proportion of infected flies. Resting spores developed in only six of 130 infected males compared with 103 of 244 infected females in 1970 and 1971. The physiological age of infected flies probably determined whether conidiophores or resting spores developed.

Parasitoid Strategies and Community Organization

The hypothesis that parasitoid complexes develop in relation to plant succession is supported. Early colonizers have high fecundity and low competitive ability (r strategists) and later colonizers have low fecundity and high competitive ability (K strategists), and species diversity in the complex tends to increase as succession proceeds. These trends are evident also in a succession of increasing host densities from year to year and in decreasing host abundance within each host generation.

Seasonal Occurrence of the Pea Aphid and a Braconid Parasite, Aphidius smithi, in Kentucky123

The seasonal occurrence and abundance of the pea aphid, Acyrthosiphon pisum (Harris), and its parasite, Aphidius smithi Sharma & Subba Rao, were studied in alfalfa and red clover fields in Kentucky from December 1967 to January 1970. Populations of pea aphids indicated a 2-peaked curve of abundance, with a vernal peak occurring in May and June and an autumnal peak in September and October. Vernal population peaks were reached at 975 aphids per 100 sweeps in 1968 and 1395 aphids/100 sweeps in 1969. Autumnal peaks were 15,606 aphids/100 sweeps in 1968 and 11,822 aphids/100 sweeps in 1969. Summer lows in pea aphid populations were 129 aphids/100 sweeps in July 1968 and 13 aphids/100 sweeps in August 1969. Low numbers were typical during winter months. Insecticidal treatments for control of the alfalfa weevil reduced the numbers of pea aphids but did not change population trends. The degree of parasitization fluctuated each year and resulted in several peaks in the population of wasps, Parasite populations exhibited a high degree of density-dependency, with populations responding rapidly to an increase in host density and declining as host density declined. The peak percentage of parasitism was 46% in 1968 and 69% in 1969. Greater numbers of wasps were observed in autumn, but the percentage of parasitism was slightly lower than in spring and early summer. Insecticidal treatments appeared to have little effect on parasite populations. The parasites collected were 61.6% females. The female: male ratio was high during June, September, October, November, and December.

Behavior Changes the Reproduction of Spalangia cameroni, S. endius, Muscidifurax raptor, and Nasonia vitripennis (Hymenoptera: Pteromalidae) at Increasing Fly Host Densities1

Attack rates and progeny production of several strains of 4 fly parasite species were observed under single-confined (forced) and multiple free-choice densities of Musca domestica L. puparia. Progeny production always increased with an increased host density limited by specific reproductive capacity. Female:male sex ratio increased with higher host densities when forced on single parasite females, but decreased when 10 competing ovipositing parasites were allowed free choice of densities in test cages. A parasite “contamination” principal was suspected. Two behavioral changes in Spalangia endius Walker might account for its increased rate of attack at higher densities. First, parasites spent less time examining puparia before ovipositing; and second, they laid more eggs in the same time period at a high host density than at a low. Superparasitism and probable differential sex mortality were also greater at the low density. Spalangia species demonstrated a greater searching capacity than Muscidifurax raptor Girault and Sanders or Nasonia vitripennis (Walker), and would be favored candidates for use as biological control agents. Experimental results indicated that small-number releases might be best at any immediate release site for maximum female F1 progeny production.

The Parasite Doryphorophaga doryphorae (Diptera: Tachinidae) in Relation to Populations of the Colorado Potato Beetle in Manitoba1

Doryphorophaga doryphorae (Riley) is the predominant parasite of the larva of the Colorado potato beetle, Leptinotarsa decemlineata (Say), in Manitoba. Percentage parasitism was highest in August, after the population density of the host started to decline. The parasite was not efficient at low host densities and percentage parasitism did not increase with increasing host densities; hence D. doryphorae was considered a nonregulatory mortality factor for the Colorado potato beetle.

Evaluation of Parasite or Predator Responses

The behaviour of searching parasites and predators in relation to varying populations of their hosts or prey is unlikely to be simple. Early theorists sought to describe the relationship by a single constant (Lotka 1925; Volterra 1931; Thompson 1930; Nicholson 1933; Nicholson & Bailey 1935). Solomon (1949) first suggested the terms functional and numerical for more complex responses of parasites and predators to their hosts and prey. These terms have since been used by several authors, not always as they were originally defined. They are suitable for describing the effects of the host or prey density on the behaviour and reproduction of the parasite or predator concerned, but not to describe the effects of the parasitism or predation on the host or prey population. The purpose of this paper is to review some of the uses of this terminology in the literature and to introduce a new and more precise system of classifying parasite or predator responses to host or prey density. It should be added, however, that it is almost an impossible task to find any system for classifying responses of this kind that caters for all cases, and there will certainly be instances in which the proposed system of terms is unsuitable. Solomon (1949) stated that '. . . to be density dependent, the enemy must respond to changes in the numbers of host (cf. Nicholson 1933; Varley 1947). The nature of this response is commonly twofold. First, there must be a FUNCTIONAL response to (say) an increase in the host density, because of the increased availability of victims: as host density rises, each enemy will attack more host individuals, or it will attack a fixed number more rapidly. A frequent, but not invariable result of this is an increase in the numbers of the enemy (a NUMERICAL influence) due to an increased rate of survival or reproduction, or of both; this may or may not be sufficient to produce an increase in the PROPORTION of enemies to the increasing hosts'. It is important to realize that a weak functional response may be insufficient to result in density-dependent mortality, for which there must be an increase in the proportion of hosts taken by each enemy as host density increases. Likewise, a numerical response in the enemy may not produce delayed density-dependency for which a proportionate increase in the numbers of enemy compared with those of the host or prey is necessary. Holling (1959a) elaborated these terms in his study of the predation of cocoons of Neodiprion sertifer (Geoff.) by small mammals in Canada. The functional response was defined in terms of changes in the number of prey consumed by each predator, and the numerical response as changes in the predator density at different prey densities. Holling divided the functional response into three basic types and expressed them graphically by plotting the numbers of prey taken per predator per unit time as the dependent variable against the prey density (Fig. la). Similarly, the numerical response was subdivided into three types (direct, none and inverse) and expressed as the numbers of predators per unit area against prey density (Fig. 2, A, B and C). Varley & Gradwell (1963a) used these terms in a different sense and implied that functional and numerical responses resulted in density-dependent and delayed density-dependent mortalities of the host or prey. A functional response was defined as occurring when '. . . local concentrations of prey are