T. S. Bellows

The Descriptive Properties of Some Models for Density Dependence

(1) The descriptive abilities of several difference equation models for density dependence have been examined, first by a qualitative comparison of their flexibility and secondly by considering their ability to describe thirty sets of data on density dependent mortality. (2) The qualitative comparison demonstrated that two of the models have forms which are particularly flexible. The quantitative comparison indicated that of these two, that due to Maynard Smith & Slatkin (1973) is better able to describe a wide range of data.

The dynamics of age-structured host-parasitoid interactions

SUMMARY (1) An age-structured simulation model is developed for interacting parasitoid and host populations characterized by the presence of overlapping stages and unequal generation times. The model is based on laboratory studies of two bruchid hosts, Callosobruchus chinensis and C. maculatus, and their pteromalid parasitoid Lariophagus distinguendus. (2) The host population model has been previously described, and to this is coupled a parasitoid model incorporating experimentally determined variations in adult searching efficiency with parasitoid age and host stage. (3) The predicted populations, using the estimated bruchid-parasitoid parameters, show erratic fluctuations in densities of both host and parasitoid, due to interactions amongst parasitism, competition and age-structure in the two populations. (4) The model results are compared to those from a long-term laboratory study by Utida (1950) using C. chinensis and another pteromalid parasitoid, Anisopteromalus calandrae. The experimental populations also showed erratic fluctuations that are qualitatively similar to those from the simulation model, suggesting that they too may arise from the internal dynamics of the interaction.

Oviposition behavior of Coccidoxenoides peregrinus, a parasitoid of Planococcus ficus

The encyrtid parasitoid, Coccidoxenoides peregrinus has been used as a biological control agent against the mealybugs Planococcus citri and Planococcus ficus. This study examined the behavior and host selection of C. peregrinus attacking P. ficus. Adult parasitoids were fed a 0.1% solution of acridine orange, a DNA binding dye used to label C. peregrinus eggs. In a choice test, adult parasitoids were offered equal numbers of first through fourth instars of P. ficus and behavior of C. peregrinus was filmed and analyzed. Acridine orange labeled ova of the parasitoids found within mealybug hosts fluoresced green under fluorescence microscopy and presence of fluorescing eggs in hosts was used to determine oviposition events. A time budget prepared for C. peregrinus indicated that this parasitoid spent the majority of its time searching (71.64%) and grooming (15.06%). The average probing duration over all instars which led to oviposition from single visits was 4.93±0.62 s. A total of 35.51% of probes from all attacks led to ovipositions, whereas 33.72% of single visits to hosts resulted in ovipositions. Detection of fluorescing acridine orange labeled eggs showed all instars of P. ficus were acceptable for oviposition by C. peregrinus. There was a significant preference to probe second, third, and fourth instars rather than first instars of P. ficus. Host feeding was not observed for this parasitoid.

Host and parasitoid recruitment for quantifying losses from parasitism, with reference to Pieris rapae and Cotesia glomerata

ABSTRACT. 1 A technique for quantifying mortality due to insect parasitoids in insect populations is presented. The method is based on the measurement of recruitment to both the host susceptible stage and the pool of parasitized hosts. 2 The technique provides estimates of generational mortality due to parasitism and, when combined with estimates of population density, provides estimates of other mortality rates acting on both host and parasitoid populations. Such estimates are available both for the generation as a whole and for separate sampling periods. 3 The method is illustrated by application to two generations of Pieris rapae L. (Lepidoptera: Pieridae) and its larval parasitoid Cotesia (Apanteles) glomerata (L.) (Hymenoptera: Braconidae).

Evaluation of biological control of Siphoninus phillyreae (Haliday) by the parasitoid Encarsia partenopea (Walker), using life-table analysis

Abstract The parasitoid Encarsia partenopea (Walker) (Hymenoptera: Aphelinidae) was released against Siphoninus phillyreae (Haliday) (Homoptera: Aleyrodidae) at two sites in southern California; two other sites served as controls. Life-tables of the whitefly were constructed for all sites by combining a photographic technique for following cohorts of whiteflies with dissection of whitefly nymphs to record parasitoid oviposition. Marginal probabilities of mortality rather than observed death rates were used to evaluate the life-tables. At release sites, mortality of whitefly nymphs was substantially higher during the second through fourth instars due to parasitism by E. partenopea . The net reproductive rates R 0 of whitefly populations at control sites were greater than unity, indicating that the populations were increasing in density. The addition of parasitism to the system resulted in R 0 values that were unity or below, suggesting that populations were declining or stable. In the absence of mortality from parasitism, calculations of R 0 for populations at the release sites would have been greater than unity, indicating populations increasing in density. Percentage superparasitism increased throughout the study, reaching 60–70%. The mean number of parasitoids per host also increased; a maximum of 11 parasitoids was found in a single host. E. partenopea suffered as much as 82% mortality between the egg and adult stage, and this phenomenon is discussed.

Adult longevity, fertility and population growth rates for Bemisia tabaci (Genn.) (Hom., Aleyrodidae) on two host plant species

Bemisia tabaci were collected in the Coachella and Imperial valleys of California and reared on cotton variety Delta Pine 61 and cucumber variety Poinsett 76 in temperature‐controlled cabinets at 20.0, 25.5, 29.0, and 32.0 °C. Data were collected on adult emergence times, longevities, preoviposition periods, total fertility, and daily fertility up to 14 days. Adult emergence was greatest in the morning. Adult longevity was greater on cotton than cucumber, greater for females than males, and decreased with increasing temperature. Female longevity ranged from 43.5–13.0 days. Preoviposition periods varied from 0–4 days and were longer at lower temperatures. Mean total fertility ranged from 252–43 eggs/female, decreasing with increasing temperature. Intrinsic rates of natural increase were calculated. Values for rm varied from 0.0884 to 0.1269 on cotton and from 0.0628 to 0.1690 on cucumber, being least at 20.0 °C, and greatest at 25.5 °C or 29.0 °C. Generation times decreased from 64 to 25 days with increasing temperature.

Population dynamics of Siphoninus philyreae in California in the presence and absence of a parasitoid, Encarsia partenopea

Abstract. 1. Densities of the whitefly, Siphoninus phillyreae Haliday, remained at low levels at two sites where a parasitoid, Encarsia parknopea (Walker), was released on whitefly‐infested ash, Fraxinus sp., and pomegranate, Punica granatum. Populations of S.phillyreae at control sites, where parasitoids were absent, were increasing in density at the beginning of the summer.

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

Functional‐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 parasitoids 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.

Host-Plant Effects on the Behavior of Eretmocerus sp. nr. Californicus Females Raised from Melon

The behaviors of female Eretmocerus sp. nr. californiens raised from Bemisia argentifolii Bellows & Perring on melon, Cucumis melo L., were analysed on four different host plant species (cotton, melon, sweet potato and Abutilon theophrasti Medic). Comparison with previously published results of similarly treated females reared from sweet potato plants showed performance related differences leading to oviposition. In the present study, the generalized behavioral pathway (walking, host- encounter, antennation, probing and oviposition) did not vary among host plant species for melon- reared parasitoid females. Host assessment by antennation leading to host acceptance for probing varied from 10.5- 12.3 sec among the host plant species with no host stage- related differences in duration. Initial and repeated (multiple probes on the same host nymph) probing events varied from 52.4 to 77.3 sec among host plant species and no stage- related differences were detected. Oviposition occurred under 62 % of the nymphs on cotton, 100 % of the nymphs on melon and 51 % of the nymphs on sweet potato under which the ovipositor was exserted. On cotton, melon and sweet potato proportionally fewer first instar nymphs were encountered than present, a larger proportion of the second instars that were encountered were subsequently antennated, and a larger proportion of second instars on cotton were then subsequently probed. For cotton, melon, and sweet potato, the overall frequency of oviposition was lower than reported in previous studies. Results from these studies showed that the greatest factor in influencing overall parasitism in laboratory experiments was the propensity of females to alight and remain to search for hosts on a particular host plant. The role of “pre- conditioning” parasitoids to be better- suited to a particular host or host plant species is discussed relative to the findings herein.RésuméLe comportement des femelles d’Eretmocerus sp. nr. californiens, élevées sur Bemisia argentifolii Bellows & Perring sur melon, Cucumis melo L., a été étudié avec 4 plantes hÔtes différentes (coton, melon, patate douce et AbutÏlon theophrasti Medic). La comparaison avec des résultats publiés antérieurement concernant des femelles élevées sur des plants de patate douce, montre des différences dans les séquences comportementales précédant l’oviposition. Dans la présente étude, l’ensemble des séquences comportementales (locomotion, rencontre de l’hÔte, exploration à l’aide des antennes, piqûre et oviposition) ne présente pas de différences entre plantes hÔtes pour les parasitoÏdes femelles élevés sur melon. L’examen de l’hÔte à l’aide des antennes, qui permet l’acceptation puis la piqûre varie de 10, 5 à 12, 3 s entre les plantes hÔtes, sans différence de durée liée au stade larvaire. La piqûre initiale et la piqûre répétée (piqûres multiples sur le mÊme hÔte au stade nymphe) durent de 52, 4 à 77, 3 s selon la plante, sans relation avec le stade de l’hÔte. Sur coton, l’oviposition a lieu dans 62 % des nymphes, sur melon 100 % et sur patate douce 51 % des nymphes dont le parasitoide a retiré son ovipositeur. Sur coton, melon et patate douce, moins de larves de premier stade que prévu sous une hypothèse de répartition au hasard ont été trouvées ; une plus grande proportion de larves de deuxième stade a été l’objet d’un examen antennaire et une plus grande proportion de larves de second stade a été ensuite piquée sur coton. Pour le coton, le melon et la patate douce, la fréquence totale d’oviposition était plus basse que celle notée dans des travaux antérieurs. Ces recherches montrent qu’en laboratoire, le facteur principal qui influence le parasitisme global est la faculté des femelles à se poser et rester sur une plante hÔte donnée pour y rechercher un hÔte. Le rÔle du pré-conditionnement du parasitoÏde pour une meilleure adéquation à un hÔte ou à une plante hÔte donnés est discuté dans le cadre des résultats obtenus dans la présente étude.

Searching efficiency and multiparasitism in Aphidius smithi and A. ervi (Hym., Aphidiidae), parasites of pea aphid, Acyrthosiphon pisum (Hom., Aphididae)

Aphidius smithi and A. ervi were introduced into the United States to control aphid pests of alfalfa. Laboratory experiments showed that A. smithi was more efficient in searching for hosts (i.e., more hosts parasitized) than A. ervi, when measured separately. However, when the two species searched together for the same hosts, a lesser percentage of A. smithi adults than A. ervi emerged from the mummies. Studies on multiple parasitism which involved hosts parasitized by one species exposed to the other species indicated that larvae of A. ervi were more competitive and survived better to adulthood than larvae of A. smithi. These results might help explain for the decline of A. smithi in California.