Influence of intrapatch experiences and temperature on the time allocation of the whitefly parasitoidEncarsia formosa (Hymenoptera: Aphelinidae)

The effect of experiences, such as contact with honeydew, rejections of hosts, and ovipositions in hosts, and of temperature on the time allocation of individualEncarsia formosa female parasitoids on tomato leaflets have been studied. Behavioral records were analyzed by means of the proportional hazards model. Analyses were carried out at two levels: (1) the tendency of leaving and (2) the tendency of changing from one leaf side to another. The patch-leaving behavior ofE. formosa can be described by a stochastic threshold mechanism, which is characterized by a certain tendency (probability per time) to leave. The median time from being placed on the leaflet or, if it occurred, from the latest encounter with a host until leaving was 18.6 min. The median time for changing from one leaf side to the other was initially 11.6 min and dropped to 5.7 min after both leaf sides had been visited. The effect of temperature, ranging from 20 to 30°C, was negligible. The presence of honeydew as well as the first oviposition in an unparasitized host decreased the tendency to leave, thus increasing the giving up time (GUT) since the latest encounter with a host. Encounters with parasitized hosts did not affect the GUT since latest encounter; as a result, the total residence time increased. After the first oviposition in an unparasitized host the tendency of changing from the lower leaf side on which hosts were present to the upper side was decreased. The presence of honeydew did not affect the tendency of changing leaf sides.     

Changes in Biological Control Usage in Great Britain Between 1968 and 1995 with Particular Reference to Biological Control on Tomato Crops

This paper outlines the overall changes in the use of both conventional pesticides and biological control agents on all crops in Great Britain since 1968 and looks at usage within the tomato crop in particular. The total pesticide treated area, including repeat applications to the same crop, increased from 14 million ha in 1976 to 17 million ha in 1981 and almost doubled to over 28 million ha in 1985. In 1998, the total pesticide treated area stood at 46 million ha. Usage of insecticides followed the same pattern as that of total pesticides with increases in the use of pyrethroids and decreases in organochlorine, organophosphate and carbamates. Use of biological control agents increased from just 17 ha in 1968 to approximately 30 000 ha in 1999. Usage of biological control agents in 1995 constituted less than one percent of the total pesticide treated area. Up until 1981, only two biological control agents, Encarsia formosa and Phytoseiulus persimilis , were recorded. However, in 1995 the total number of parasites, predators and other biological control agents recorded was 23. Although both the area of tomatoes grown in Great Britain and the use of insecticides on tomatoes has significantly declined since 1968, the area treated with biological control agents continued to increase over the same period. In 1995, the biological control agent-treated area exceeded 10 000 ha and accounted for over 70% of the area of tomatoes treated.     

Role of the aphelinid parasitoid Encarsia guadeloupae in the suppression of the exotic spiralling whitefly Aleurodicus dispersus on banana in India

A field study conducted during 2000-2001 on banana plantation located at Bangalore Rural district revealed that the population of the spiralling whitefly Aleurodicus dispersus Russell declined from 116.85 per 25-cm² leaf area in March 2000 to 1.10 per 25-cm² leaf area in December 2001. During the study period, a total of seven natural enemies were observed. The aphelinid parasitoid Encarsia guadeloupae Viggiani was the only major natural enemy encountered on the spiralling whitefly causing 20.70% parasitism in January 2000, which had increased to 95.68% by December 2001. The density of the spiralling whitefly was positively correlated with maximum temperature and negatively correlated with relative humidity and the parasitism by E. guadeloupae. The multiple regression model showed that about 67.94% of the variation in the whitefly density could be accounted for by parasitism only.     

Influence of antibiotics on the offspring production of the Wolbachia-infected parthenogenetic parasitoid Encarsia formosa

Three different concentrations of the antibiotic tetracycline in honey were tested for their influence on the offspring production and longevity of the parasitoid wasp Encarsia formosa. Several earlier publications did not provide a conclusive answer on the effect that the Wolbachia have on these wasps. The results of our experiments show that at high tetracycline hydrochloride concentrations in honey (50mg/ml) the antibiotic is toxic to the females, all females died within three days after the antibiotic treatment. The concentration 5mg/ml was less toxic although the treated females also lived shorter and produced less offspring than the control females. At the lowest tested concentration of 1mg/ml there was no significant difference either in offspring production or in longevity between the control and the treated females. The antibiotic treatment at both 5 and 1mg/ml resulted in exclusively male progeny after the first two days of oviposition. These results are consistent with the theory that in species in which all individuals are infected the Wolbachia should not impose a large fitness cost.     

Competition, resource partitioning and coexistence of an endoparasitoid Encarsia perniciosi and an ectoparasitoid Aphytis melinus of the California red scale

Abstract. 1. Laboratory experiments and field studies were conducted to explain the coexistence of an endoparasitoid, Encarsia perniciosi Tower, and an ectoparasitoid, Aphytis melinus DeBach, both of which were introduced into California to control the California red scale, Aonidiella aurantii (Mask.).
2. Encarsia parasitized all scale stages but it preferred first and second instar scales. This is in contrast to Aphytis melinus , in which previous studies have shown that it parasitizes second and third instar females and second instar males but prefers third instar female scales. Encarsia developed most rapidly when it parasitized an early second instar and slowest when it parasitized the mature female scale. However, on early second instar scales it was about 80% as fecund as a wasp that emerged from a mature female scale.
3. Second instar scales parasitized by Encarsia were accepted by Aphytis as readily as unparasitized scales.
4. Encarsia did not distinguish between unparasitized hosts and those previously parasitized by Aphytis.
5. Encarsia always outcompeted by Aphytis when both species parasitize the same host.
6. Encarsia prefers scale on stems whereas Aphytis prefers those on leaves and fruits. This, too, may be a result of interspecific competition with Aphytis.
7. The partitioning of the scale resource by the two species explains why they coexist in coastal southern California but it does not explain why Encarsia disappeared from citrus groves in the inland valleys coincident with the introduction of Aphytis melinus into southern California.     

Establishment ofEncarsia(=Prospaltella) perniciosi,a specific parasite of San Jose scale,on apple in Kashmir

Attempts were made to reduce populations of San Jose scaleQuadraspidiotus perniciosus (Comstock) by releasing populations of the parasiteEncarsia (=Prospaltella) perniciosi Tower. Several releases were made, between May and November in both 1985 and 1986. TheEncarsia were released into 2 apple orchards infested with scale. Increases in the numbers ofEncarsia caught on sticky traps in these orchards, coupled with an increase from 2.3% to 9.1% in the overall level of parasitism of the scales, suggested that the released parasites had become established. Other natural enemies found associated with San Jose scale on apple in Kashmir included 3 parasites (Marietta carnesi Howard,Azotus kashmirensis Narayanan andAphytis paramaculicornis De Bach) and 5 predators, 4 of which were coccinellids (Chilocorus bijugus Muls,Pharoscymus flexibilis, Exochomus sp. andCoccinella septempunctata) and the remaining one was a chrysopid (Chrysoperla carnea).      

Flexible Use of Patch-Leaving Mechanisms in a Parasitoid Wasp

Classical optimal-foraging theory predicts that a parasitoid is less likely to leave a patch after a host encounter when the host distribution is aggregated, whereas a parasitoid is more likely to leave after a host encounter when the host distribution is regular. Field data on host distributions in the area of origin of the whitefly parasitoid Encarsia formosa showed that whiteflies aggregate at several spatial scales. However, infested leaves most likely contained a single host. This suggests that a host encounter is not enough to decide when to leave. We therefore tested the effect of host distribution and parasitoid experience on patch-leaving behavior. Each parasitoid was observed for several consecutive days in a three-dimensional arena with leaflets containing on average one host per leaflet in an either regular or aggregated host distribution. A proportional hazards model showed that a host encounter decreased the leaving tendency on a leaflet with one host when the time since the latest host encounter was short, but increased the leaving tendency when the time since the latest host encounter was long, independent of host distribution. We conclude that a parasitoid can switch from decreasing to increasing its tendency to leave a patch after a host encounter. We propose two hypotheses that may explain the evolution of such a switching mechanism.     

The longevity effects of trehalulose feeding by parasitoids

Abstract.  Insects, particularly phloem-feeding Sternorrhyncha, are known to produce sugars in their honeydew (excreta) that are not found in their host plants. Of these, Bemisia tabaci , the sweet potato whitefly, is the only insect known to produce trehalulose [α- d -glucose (1,1) d -fructose] as a major component of its honeydew. The present study aims to determine whether trehalulose is comparable to sucrose as a nutrient source for three whitefly parasitoids ( Encarsia formosa , Encarsia pergandiella and Eretmocerus eremicus ). In addition, the study also examines trehalulose feeding effects on longevity for a parasitoid of muscoid Diptera, Nasonia vitripennis . Parasitoids are provided diets of either sucrose or trehalulose in varying concentrations (from 0.1% to 70%) or a water control. Sucrose and trehalulose are not significantly different in affecting survival when compared at the same concentration. This was true for all Bemisia parasitoids and N. vitripennis. Certain specific diets are significantly different in pairwise combination tests. There is a significant effect of species, diet type and the interaction of these two factors on the longevity of the three different Bemisia parasitoid species; however, within species, there is no significant increase in longevity observed for either carbohydrate diet. This result contrasts with expectations for the effects of host-modified carbohydrates on longevity. The implications are that, although carbohydrate feeding is essential for these parasitoids, these host-provided sources of carbohydrates are equally capable of extending longevity.     

Banker plants promote functional biodiversity and decrease populations of the cabbage whitefly Aleyrodes proletella

In this study, potential banker plant systems against the cabbage whitefly Aleyrodes proletella Linnaeus (Hemiptera: Aleyrodidae) were developed under controlled conditions. The two most promising banker plant systems, that is, the parasitoid Encarsia tricolor Förster (Hymenoptera: Aphelinidae) either with Aleyrodes lonicerae Walker on European columbine (columbine system) or with Trialeurodes vaporariorum Westwood (Hemiptera: Aleyrodidae) on Hokkaido squash (pumpkin system), were further evaluated in the field. Although the pumpkin system produced three times more parasitoids than the columbine system, both banker plants led to an 1.5-fold increase in A. proletella parasitism rates. However, only the pumpkin system increased the abundance of syrphid larvae on cabbage by 61.5% and reduced A. proletella populations on average by 4.4%–25.8% depending on the respective assessment date. In conclusion, the pumpkin system revealed to be a promising (supplementary) control measure against A. proletella. Options for further improvement and standardization of the pumpkin system as well as a potential implementation in cabbage production are discussed.