Parasite regulation of host insect metamorphosis: a new form of regulation in pseudoparasitized larvae of Trichoplusia ni

Unsuccessful Host Stinging by Aphelinus asychis (Hymenoptera: Aphelinidae) Impacts Population Parameters of the Pea Aphid (Hemiptera: Aphididae)

The biocontrol values of natural enemies are strongly correlated to their ability to regulate the density of their host/prey. For parasitoids, apart from parasitism and host feeding, unsuccessful host stinging (i.e., stings that were aborted, abandoned, or discontinued without oviposition or host feeding) can also negatively affect their hosts and host populations. Although several studies have reported unsuccessful host stinging and its impacts on hosts, the effects of this type of attack on host life table parameters are still unclear. In the present study, we used the parasitoid Aphelinus asychis Walker (Hymenoptera: Aphelinidae) and its host Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae) to investigate the influence of unsuccessful host stinging on host populations under laboratory conditions at. Biological parameters of A. pisum were analyzed using an age stage, two-sex life table. The results of this study showed that unsuccessful host stinging was prevalent under laboratory conditions, and the frequency of this type of attack on third- and fourth-instar hosts was higher than the frequencies of parasitism and host feeding. Unsuccessful host stinging adversely impacted aphid populations, by decreasing aphid survival and reproduction, and impacts were greatest in hosts attacked at the first and fourth instars. These results indicate that unsuccessful host stinging enhances the biological control impact of A. asychis attacking A. pisum, and its effect on host populations should also be considered when selecting and mass rearing of parasitoids for biological control.

Is the host or the parasitoid in control?: effects of host age and temperature on pseudoparasitization by Microplitis rufiventris in Spodoptera littoralis

We compared the production of pseudoparasitization by Microplitis rufiventris females in most (third) and less (fourth) preferred instars of Spodoptera littoralis larvae at 20+/-1 and 27+/-1 degrees C. The parasitized hosts were classified into hosts producing parasitoids (type A hosts) and hosts producing no parasitoids, i.e., pseudoparasitized hosts (type B hosts). The latter were further classified into: (a) pseudoparasitized hosts with "well" arrested development (type B1 hosts); (b) pseudoparasitized hosts with partially arrested development (type B2 hosts); and (c) pseudoparasitized hosts that successfully pupated to apparently normal host pupae (type B3 hosts). The present series of experiments showed that parasitization by M. rufiventris was clearly affected by host instar, age within an instar and rearing temperature. Production of type B hosts was less when third instar S. littoralis larvae were exposed to the wasp females than when the host larvae were in fourth instar. The production of type A hosts was much greater when early or mid ages of an instar was stung by the wasp females comparing with stung late age of the same instar. Production of type B hosts may be due to one or overall of the following: (a) dosage dilution of M. rufiventris female's factors in the different age classes of the instar; (b) endocrine system (physiological state) at parasitization time, i.e., early vs late age of the instar; (c) growth rate of host larvae. The lowest production of type B hosts was at highest growth rate; and (d) temperature, larger proportions of type B hosts were produced at 27+/-1 than at 20+/-1degrees C. The three types host development (B1, B2 and B3) are possibly representing three levels of host resistance (host control) resulting in partial or complete failure of parasitoid control. Type A hosts represent complete success of parasitoid control. The results suggest that the impact of parasitoid factor(s) on developmental arrest is affected by host age at the time of parasitism and/or by temperature.

Wasp parasitoid disruption of host development: implications for new biologically based strategies for insect control

Wasp parasitoids use a variety of methods to commandeer their insect hosts in order to create an environment that will support and promote their own development, usually to the detriment of the host insect. Parasitized insects typically undergo developmental arrest and die sometime after the parasitoid has become independent of its host. Parasitoids can deactivate their host's immune system and effect changes in host hormone titers and behavior. Often, host tissues or organs become refractory to stimulation by tropic hormones. Here we present an overview of the manipulative capabilities of wasp-injected calyx fluid containing polydnaviruses and venom, as well as the parasitoid larva and the teratocytes that originate from the serosal membrane that surrounds the developing embryo of the parasitoid. Possibilities for using regulatory molecules produced by the parasitoid or its products that would be potentially useful in developing new, environmentally safe insect control agents are discussed.

Effects of Glyptapanteles liparidis (Hym.: Braconidae) parasitism, polydnavirus, and venom on development of microsporidia-infected and uninfected Lymantria dispar (Lep.: Lymantriidae) larvae

Effects of parasitism, polydnavirus, and venom of the endoparasitoid Glyptapanteles liparidis on Lymantria dispar larvae infected with the microsporidium Vairimorpha sp. and uninfected hosts were studied. We tested the impact on growth and development of hosts, as well as on microsporidian infection. Both parasitism and polydnavirus/venom treatment alone caused a slight increase in growth rate and relative growth rate in uninfected fourth instar hosts. This effect was more pronounced with the addition of Vairimorpha infection. With no parasitism, however, infection reduced host growth markedly. Microsporidiosis delayed larval molts of L. dispar, and additional polydnavirus/venom treatment or parasitization induced significantly earlier molting. Polydnavirus/venom treatment of uninfected L. dispar resulted in prolonged larval development due to supernumerary molts and in higher pupal mortality. Infected larvae treated with polydnavirus/venom died earlier than infected larvae that were not treated and produced more Vairimorpha spores per unit fresh mass of the host.

Developmental and immunological aspects of Drosophila-parasitoid relationships

The Drosophila-parasitic wasp (parasitoid) associations involve integrating adaptations of considerable complexity. This review focuses on some of the factors that influence these interactions including host immunity, nutrition and hormonal changes, and parasitoid virulence and mechanisms of immune suppression.

Preultimate 4th/5th instar Trichoplusia ni naturally-injected with venom/calyx fluid from Chelonus curvimaculatus precociously metamorphose, rather than obey the metamorphic size threshold that would normally compel molting to a 5th/6th instar

In normally regulated larval metamorphosis of Trichoplusia ni, a 4th, 5th or other numbered instar is a 'preultimate' instar, and will normally continue larval molting, if the larva has not yet surpassed the critical (minimal) size threshold corresponding to attainment of the 'ultimate' (metamorphic) instar. Natural injection of T. ni embryos with venom/calyx fluid of female Chelonus sp. near curvimaculatus caused 'penultimate' 4th or 5th instar larvae that would normally molt at least once more, to a 5th/6th instar, to instead precociously metamorphose without another larval molt. These effects were observed in naturally-injected insects that never contained either a parasite larva, a viable parasite embryo, or a parasite egg. These data demonstrate that this effect of venom/calyx fluid of this wasp to induce precocious metamorphosis, at an instar earlier than would otherwise have typically occurred under normal growth conditions, does not require the presence of a parasite larva. Other data did indicate the parasite larva contributes an additional effect that causes a 4th instar host (that from its size would normally require not just one, but at least two more larval molts to reach the metamorphic instar) to not grow to the size metamorphic threshold, but to instead, precociously metamorphose at an even smaller size than occurs with the venom/calyx fluid alone. Additionally, arylphorin was precociously highly expressed in parasitized hosts in a manner independent of a decline in the host JH titer. Therefore, the main target of the venom/calyx fluid activity to induce precocious metamorphosis appears to be an event upstream of the decline in JH production by the corpora allata. Pseudoparasitized hosts become developmentally arrested as precocious prepupae and express a 2.7kb polydnavirus transcript. The larger (but still subthreshold size) larvae showed less suppressed prepupal ecdysteroid titers, less developmental suppression, and a much weaker expression of that transcript. A general model for mechanisms of action of chelonine venom/calyx fluid, and larvae, to cause precocious host metamorphosis and suppressed prepupal development is presented that is based on the current 'size threshold' model of normal lepidopteran development, rather than the older, displaced 'instar count' model. By basing the model for chelonine regulation of host development on the current 'size threshold' model for normal development, the proposed model for chelonine action both accounts for observations reported on various species of that subfamily and makes useful, testable predictions.

Physiological and endocrine changes associated with polydnavirus/venom in the parasitoid-host system Chelonus inanitus-Spodoptera littoralis

As shown earlier, parasitization by the egg-larval parasitoid C. inanitus causes in its host the precocious onset of metamorphosis in the 5th instar followed by developmental arrest in the prepupal stage. Polydnavirus/venom were shown to be responsible for the developmental arrest. We investigated how polydnavirus/venom affect growth of the host larvae and found that head capsule widths were smaller from the 4th to 6th stadium and weights were lower in the 6th stadium in polydnavirus/venom-containing larvae than in non-parasitized larvae. In an attempt to identify endocrine parameters that are modified by polydnavirus/venom and might be responsible for the developmental arrest in the prepupa, we compared juvenile hormones, juvenile hormone esterase and ecdysteroids between non-parasitized and polydnavirus/venom-containing larvae from the 4th instar until pupation or developmental arrest, respectively. Obvious differences became manifest only in the 6th instar at the pupal cell formation stage, i.e. 12 days after entry of polydnavirus/venom into the host egg. Then, prothoracic glands of polydnavirus/venom-containing larvae released less ecdysteroids and ecdysteroid titres were lower than in non-parasitized larvae; this was followed by a delayed, reduced and desynchronized increase in prepupal juvenile hormones and juvenile hormone esterase and a slightly modified metabolism of ecdysone. This indicates that polydnavirus/venom affects the endocrine system of the host only after pupal commitment and that inhibition of prothoracic gland activity is the first detectable effect.

Regulatory mediators in the venom of Chelonus sp.: their biosynthesis and subsequent processing in homologous and heterologous systems

Following titration of the contents of the venom gland reservoir, the rate of biosynthesis of venom proteins was sufficiently rapid over the next 6-24 hrs to restore their titer to the level initially synthesized during early adulthood. There was no evidence of processing of smaller molecular weight components from much larger forms. Although most proteins were stable in young host embryos, two specific processing products of a 32.5 kDa venom protein were found in such hosts. The natural injection of venom proteins into either very old embryos or young embryos subsequently held at 4 degrees C for six days resulted in rapid degradation to biologically inactive forms. These data are the first report of direct examination of the biosynthesis of wasp venom proteins and the first analysis of the processing of specific hymenopteran venom proteins in target tissues.

Chelonus sp.: suppression of host ecdysteroids and developmentally stationary pseudoparasitized prepupae

When eggs of the caterpillar Trichoplusia ni are stung by Chelonus sp. (near C. curvimaculatus) (Braconidae), the developing host larvae precociously spin a cocoon but then remain developmentally stationary in the prepupal stage. The latter event happens even in hosts which were stung and precociously spin cocoons but which, upon dissection, contain no obvious parasite. Injection of radiolabeled ecdysone into either pseudoparasitized or allatectomized larvae demonstrates suppressed rates of conversion of ecdysone to 20-hydroxyecdysone when compared with controls. The data indicate that the occurrence of developmentally stationary pseudoparasitized prepupae is due to less production of ecdysteroid and less conversion of ecdysone to 20-hydroxyecdysone, both probably as a result of suppressed juvenile hormone titer.

Precocious expression of the final larval instar developmental pattern in larvae of Trichoplusia ni pseudoparasitized by Chelonus spp

The present study has used a number of electrophoretic approaches to analyze the proteins in normal Trichoplusia ni larvae and those pseudoparasitized by Chelonus spp. A number of feeding-stage, hemolymph proteins appear or increase dramatically only during the final larval stadium. Other proteins highly abundant only during the penultimate stadium disappear or decrease dramatically during the final stadium. The comparative protein profiles of penultimate instar, pseudoparasitized larvae are very similar to those of last instar larvae. These changes in hemolymph proteins are seen on gels resulting from electrophoresis, isoelectric focusing (wide range and narrow range Ampholine and very narrow range Immobiline gels) and SDS-disc electrophoresis. It is concluded that the entire last instar developmental pattern of protein gene products is occurring precociously in pseudoparasitized larvae.

Multiple forms of juvenile hormone esterase active sites in the hemolymph of larvae of Trichoplusia ni

Kinetic analysis was performed on the juvenile hormone (JH) esterase activity in the hemolymph of feeding, last instar larvae of Trichoplusia ni (Lepidoptera: Noctuidae). When the results were analyzed by several different graphical and regression procedures, all approaches yielded the same conclusion that at least two forms of JH esterase active sites exist in the hemolymph. The apparent Km for one site for JH I, II and III was 8.5 X 10(-8) M, and 6.6 X 10(-8) M, respectively. The Km for the other site for JH I, II and III was 6.6 X 10(-7) M, 7.6 X 10(-7) M, 40 X 10(-7) M, respectively. When hemolymph JHE activity was subjected to high resolution isoelectric focusing (IEF), two distinct large peaks of JHE activity were observed, with pIs of 5.3 and 5.5, as well as a small peak at pI 5.1. Separate kinetic analysis of the JHE activity in each peak showed that only the higher Km active site for each substrate was present (in the 10(-7) M range). These data necessitate a change in the current model for JHE in T. ni, and some other insects, which states that a single active site is responsible for most or all of the JH esterase activity in vivo. The data also explain the different estimates of the Km of JHE in T. ni obtained by different laboratories. Studies on the purification of, and the development of inhibitors for, JHE esterase must consider the role of both JHE forms and sites in regulation of T. ni metamorphosis.