Effect of hosts and parasite density on the egg parasiteTrichogramma pretiosum [Hym.: Trichogrammatidae]

When females ofTrichogramma pretiosum Riley were confined with host eggs at a density of 2/150 eggs, they produced 12 times more female progeny on eggs of potato tuber moth than on eggs ofHeliothis armigera (Hübner) and 13,6 times more on eggs ofSitotroga cerealella (Olivier) than on eggs ofHeliothis. At a density of 4/150 eggs, the correspondent figures were 13 and 8 times. The percentage emergence fromHeliothis eggs was from 0,29 to 0,14 times as great as from tuber moth orSitotroga. From 15 to 140 times more runts were observed amongTrichogramma fromHeliothis eggs than among those from tuber moth eggs and 8 times more thant among those fromSitotroga eggs. This may explain the low recoveries in South Africa ofT. pretiosum in eggs ofH. armigera collected in cotton fields after mass liberation of the parasite. An increase in parasite density from 1/300 eggs to 16/300 eggs resulted in a decrease from 29 to 14 in the hosts parasitised per female, a decrease in the proportion of female progeny from 72 to 39%, a decrease in the female progeny per female from 18 to 4,8, and an increase in the proportion of runts from 2,4 to 12,4%. It is suggested that in mass culture ofTrichogramma unduly high parasite densities should be avoided in order to reduce the effect of mutual interference and raise the output of female progeny.     

Effect of temperature and host species on parasitism,development time and sex ratio of the egg parasitoid Trichogrammatoidea lutea Girault (Hymenoptera: Trichogrammatidae)

The developmental biology of Trichogrammatoidea lutea Girault (Hymenoptera: Trichogrammatidae) was studied at six constant temperatures (18, 21, 24, 27, 30 and 35 °C) on eggs of three lepidopteran host species: Helicoverpa armigera (Hübner) (Noctuidae), Chilo partellus (Swinhoe) (Crambidae) and Cadra cautella (Walker) (Pyralidae). T. lutea did not complete development at 35 °C on any of the three host species. Parasitism levels were highest on H. armigera at 27 °C (58%), C. cautella at 27 and 30 °C (31% and 28%) and C. partellus between 24 and 30 °C (13–17%). Realized progeny of T. lutea per parasitized host egg was influenced by host size. The number of progeny of T. lutea per parasitized host egg was highest on H. armigera, followed by C. partellus and lowest on C. cautella. The sex ratio was female biased on C. partellus, female biased on C. cautella with the exception of 21 °C and close to 1:1 on H. armigera. The rate of development from egg to pupa and egg to adult was fastest on H. armigera and slowest on C. partellus. Lower thresholds for development and degree days (DD) of T. lutea from egg to adult were 12.8 °C and 105.4 DD on H. armigera, 11.3 °C and 141.6 DD on C. partellus and 12.9 °C and 118.2 DD on C. cautella, respectively. Based on these results, H. armigera is the most suitable host for mass rearing of T. lutea for biological control of Lepidoptera pests because of the relatively high parasitism levels, short development time, greater clutch size and balanced sex ratio. C. cautella may also be used although longer exposure times might be required due to lower parasitism levels.     

Core promoter binding by histone-like TAF complexes

A major function of TFIID is core promoter recognition. TFIID consists of TATA-binding protein (TBP) and 14 TBP-associated factors (TAFs). Most of them contain a histone fold domain (HFD) that lacks the DNA-contacting residues of histones. Whether and how TAF HFDs contribute to core promoter DNA binding are yet unresolved. Here we examined the DNA binding activity of TAF9, TAF6, TAF4b, and TAF12, which are related to histones H3, H4, H2A, and H2B, respectively. Each of these TAFs has intrinsic DNA binding activity adjacent to or within the HFD. The DNA binding domains were mapped to evolutionarily conserved and essential regions. Remarkably, HFD-mediated interaction enhanced the DNA binding activity of each of the TAF6-TAF9 and TAF4b-TAF12 pairs and of a histone-like octamer complex composed of the four TAFs. Furthermore, HFD-mediated interaction stimulated sequence-specific binding by TAF6 and TAF9. These results suggest that TAF HFDs merge with other conserved domains for efficient and specific core promoter binding.     

Effect of mannose-binding lectin from peanut and pea on the stem borer Chilo partellus

A mannose-binding lectin found in vegetative tissues of peanut, Arachis hypogaea, was compared with mannose-binding lectin from pea, Pisum sativum, for toxic effects on larvae of the stem borer Chilo partellus (Swinhoe). After 10 days, the mortality of larvae fed on artificial diet containing 0.5% (m/m) peanut lectin was 46.2%. The mortality of larvae fed on 1.0% peanut lectin was similar (48.1%) but insects were significantly smaller than those of the 0.5% treatment. Larvae of both lectin treatments stopped feeding within three days. Larval size and mortality was not significantly reduced by 0.1% peanut lectin and 1% heat-treated lectin did not show toxic effects. The mannose-binding lectin from pea was not toxic to C. partellus at concentrations up to 1%. Peanut lectin bound to the apical membranes of columnar epithelial cells in the mid-gut of C. partellus. This suggests that peanut lectin has an antinutritive action and that it may protect vegetative tissues of peanut against insect pests.     

Alu Exonization Events Reveal Features Required for Precise Recognition of Exons by the Splicing Machinery

Despite decades of research, the question of how the mRNA splicing machinery precisely identifies short exonic islands within the vast intronic oceans remains to a large extent obscure. In this study, we analyzed Alu exonization events, aiming to understand the requirements for correct selection of exons. Comparison of exonizing Alus to their non-exonizing counterparts is informative because Alus in these two groups have retained high sequence similarity but are perceived differently by the splicing machinery. We identified and characterized numerous features used by the splicing machinery to discriminate between Alu exons and their non-exonizing counterparts. Of these, the most novel is secondary structure: Alu exons in general and their 5′ splice sites (5′ss) in particular are characterized by decreased stability of local secondary structures with respect to their non-exonizing counterparts. We detected numerous further differences between Alu exons and their non-exonizing counterparts, among others in terms of exon–intron architecture and strength of splicing signals, enhancers, and silencers. Support vector machine analysis revealed that these features allow a high level of discrimination (AUC=0.91) between exonizing and non-exonizing Alus. Moreover, the computationally derived probabilities of exonization significantly correlated with the biological inclusion level of the Alu exons, and the model could also be extended to general datasets of constitutive and alternative exons. This indicates that the features detected and explored in this study provide the basis not only for precise exon selection but also for the fine-tuned regulation thereof, manifested in cases of alternative splicing.     

Parasitoids ofPlutella xylostella (Lep.: Plutellidae) in South Africa: An annotated list

Larvae and pupae of diamondback moth,Plutella xylostella (L.) (Lepidoptera: Plutellidae), were collected weekly for two years on unsprayed cabbage plots. Samples were taken to the laboratory and parasitoids that emerged were identified and their incidence determined. Parasitoids were active throughout the year and parasitism was high (reaching 90–100%) except in the winter months of June–August when it was low. Twenty one species were reared: the egg-larval parasitoidsChelonus curvimaculatus Cameron and Chelonus sp. (Braconidae); the larval parasitoidsApanteles eriophyes Nixon,Cotesia plutellae (Kurdjumov),Habrobracon brevicornis (Wesmael) (Braconidae) andPeribaea sp. (Tachinidae); the larvalpupal parasitoidsDiadegma sp.,Itoplectis sp. (Ichneumonidae) andOomyzus sokolowskii (Kurdjumov) (Eulophidae); the pupal parasitoidsBrachymeria sp.,Hockeria sp. (Chalcididae),Diadromus collaris Gravenhorst (Ichneumonidae) andTetrastichus howardi (Olliff) (Eulophidae); and the hyperparasitoidsAphanogmus fijiensis (Ferrière) (Ceraphronidae),Brachymeria sp.,Hockeria sp.Proconura sp. (Chalcididae),Mesochorus sp. (Ichneumonidae),Pteromalus sp. (Pteromalidae),Eurytoma sp. (Eurytomidae) andTetrastichus sp. (Eulophidae).     

Effect of temperature and parasite density on three species ofAphytis (Hymenoptera: Aphelinidae), parasitising California red scale

Summary Three species ofAphytis parasites of California red scale,Aonidiella aurantii (Maskell) were each confined at different densities with approximately equal numbers of scale insects at several constant temperatures to determine the effect of these factors on progeny production and distribution and the search rate. Egg location on the host body by the parasites was unaffected by temperature or parasite density.A. melinus laid its eggs both dorsally and ventrally in equal proportions whileA. lingnanensis andA. chrysomphali laid their eggs ventrally. Progeny production byA. melinus andA. lingnanensis increased at the higher temperature; that ofA. chrysomphali did not. Unlike the other species,A. chrysomphali failed to oviposit at 32°C. Although increasing parasite density reduced progeny production in bothA. melinus andA. lingnanensis, they were able to maintain an almonst constant search rate. This was due to their ability to distribute their eggs among the hosts more regularly at the higher parasite densities. While the mechanism of this process is easy to understand forA. melinus which behaved as a gregarious parasite, it is unclear forA. lingnanensis, which is almost a solitary parasite. The progeny production and the search rate ofA. chrysomphali dropped significantly with increasing parasite density.     

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.     

The thelytokous Trichogramma chilonis in Taiwan

Trichogramma chilonis Ishii (Hymenoptera: Trichogrammatidae) with a mode of reproduction different from arrhenotoky was discovered and maintained in the laboratory. With or without mating, individuals of this form were able to produce males, females, and mosaics which varied in morphological characters. Reciprocal and backcrosses between males and females from this form and the arrhenotokous population revealed that they were compatible in mating. Progeny obtained from F1 and F2 proved the existence of gene flow between these two forms which are morphologically indistinguishable. More males were produced as temperature rose to above 25°C, or when females fed on a mixture of tetracycline-hydrochloride and honey (1 mg/1 ml). The impact of parasitoids with such mode of reproduction on biological control is discussed.