Host-finding behavior, host acceptance, and host suitability of the parasiteXanthopimapla stemmator

Xanthopimpla stemmator (Thunberg)(Hymenoptera: Ichneumonidae), a solitary endoparasite of pupae of Old World lepidopteran stalkborers, was recently imported into Texas as a candidate for biological control of New World stalkborers. Information on host acceptability, host suitability and cues responsible for host finding were necessary to gain an insight into parasite/host interactions, because of the absence of a coevolutionary history.Xanthopimpla stemmator females were exposed to laboratory-reared one-to six-day-oldDiatraea saccharalis (F.) pupae. An average of 62% of host pupae were accepted and all ages of pupae were equally acceptable. Host suitability decreased with host age. One- to five-day-old host pupae averaged 31–37% suitability, whereas only 19% of 6-day-old pupae were suitable. Successful parasitization, defined as the product of the proportion accepted and the proportion suitable, decreased from 22–23% for 1-, 2- and 3-day-old pupae to 13% for 6-day-old pupae. Sex ratio (female:male) of the parasite progeny increased with host age. Females comprised 47% of total parasite progeny of 1-day-old and 84% of 6-day-old pupae. The increase in percent females was a result of a similar number of females in all age classes, coupled with a decrease in the number of males from older hosts.Xanthopimpla stemmator superparasitized 61% of acceptedD. saccharalis pupae in the laboratory. On dissection, 73% of host pupae with multiple probe wounds were found to contain parasite eggs or larvae; these hosts contained up to 10 eggs or 7 first-instar larvae. Increased numbers of probes by the parasites were associated with an increase in successful parasitization. Host seeking activity inX. stemmator was stimulated by the presence of larval frass, host odor and movement of host pupae. Results suggest thatX. stemmator is a good candidate for biological control ofD. saccharalis and possibly other factitious stalkborer hosts.     

Differential expression of the pr1A gene in Metarhizium anisopliae and Metarhizium acridum across different culture conditions and during pathogenesis

The entomopathogenic fungi of the genus Metarhizium have several subtilisin-like proteases that are involved in pathogenesis and these have been used to investigate genes that are differentially expressed in response to different growth conditions. The identification and characterization of these proteases can provide insight into how the fungus is capable of infecting a wide variety of insects and adapt to different substrates. In addition, the pr1A gene has been used for the genetic improvement of strains used in pest control. In this study we used quantitative RT-PCR to assess the relative expression levels of the pr1A gene in M. anisopliae and M. acridum during growth in different culture conditions and during infection of the sugar cane borer, Diatraea saccharalis Fabricius. We also carried out a pathogenicity test to assess the virulence of both species against D. saccharalis and correlated the results with the pattern of pr1A gene expression. This analysis revealed that, in both species, the pr1A gene was differentially expressed under the growth conditions studied and during the pathogenic process. M. anisopliae showed higher expression of pr1A in all conditions examined, when compared to M. acridum. Furthermore, M. anisopliae showed a greater potential to control D. saccharalis. Taken together, our results suggest that these species have developed different strategies to adapt to different growing conditions.     

Exploitation of mitochondrial nad6 as a complementary marker for studying population variability in Lepidoptera

The applicability of mitochondrial nad6 sequences to studies of DNA and population variability in Lepidoptera was tested in four species of economically important moths and one of wild butterflies. The genetic information so obtained was compared to that of cox1 sequences for two species of Lepidoptera. nad6 primers appropriately amplified all the tested DNA targets, the generated data proving to be as informative and suitable in recovering population structures as that of cox1. The proposal is that, to obtain more robust results, this mitochondrial region can be complementarily used with other molecular sequences in studies of low level phylogeny and population genetics in Lepidoptera.     

Sucrose hydrolases from the midgut of the sugarcane stalk borer Diatraea saccharalis

A beta-fructosidase (EC 3.2.1.26) was isolated from the midgut of larval sugar cane stalk borer Diatraea saccharalis by mild-denaturing electrophoresis and further purified to near homogeneity by gel filtration. beta-Fructosidase hydrolysed sucrose, raffinose and the fructosyl-trisaccharide isokestose, but it had no activity against maltose, melibiose and synthetic substrates for alpha-glucosidases. Two other sucrose hydrolases, one resembling a alpha-glucosidase (EC 3.2.1.20) and the other one active specifically against sucrose (sucrase) were detected in the larval midgut of D. saccharalis. All three sucrose hydrolases were associated with the midgut epithelium of larval D. saccharalis. Relative molecular mass (M(r)) of the beta-fructosidase was estimated around 45,000 (by gel filtration). The other two sucrose hydrolases had M(r) of 54,000 (alpha-glucosidase) and 59,000 (sucrase). The pH optima of the sucrose hydrolases were 5-10 for both alpha-glucosidase and sucrase and 7-8 for beta-fructosidase. Considering V(max)/K(m) ratios, beta-fructosidase preferentially cleaves isokestose rather than raffinose and sucrose. In order to evaluate the possible contribution of microorganisms isolated from the midgut to the pool of sucrose hydrolases, washed midgut epithelia were homogenised and plated onto appropriate media. Seven bacterial and one yeast species were isolated. None of the sucrose hydrolases extracted from the microorganisms corresponded to the enzymes isolated from midgut tissue homogenates. This result suggests that the major sucrose hydrolases found in the midgut of larval D. saccharalis were probably produced by the insect themselves not by the gut microflora.     

The Nearctic-Caribbean species Leptotrachelus dorsalis (Fabricius, 1801): Larval descriptions with a diagnosis of immature Ctenodactylini and natural history notes on the genus and tribe (Coleoptera, Carabidae)

Adults and larvae of Leptotrachelus dorsalis (Fabricius), the Sugarcane Savior Beetle, live in association with grasses, the larvae in the appressed leaf axils. Both adult and larval Leptotrachelus dorsalis eat larvae of the Sugarcane Borer, Diatraea saccharalis (Fabricius), and perhaps other insects living in the confines of the leaf sheaths of that and other grass-like species. The geographic range of Leptotrachelus dorsalis extends from Kansas in the west to the Atlantic seaboard, north as far as Ontario, Canada and south to Cuba; it is an eastern species of North America and the Caribbean. Larval character attributes that are shared with a related ctenodactyline, Askalaphium depressum (Bates), provide a preliminary basis for characterization of the immatures of tribe Ctenodactylini.     

Development of the braconid wasp Cotesia flavipes in two Crambids, Diatraea saccharalis and Eoreuma loftini: Evidence of host developmental disruption

Cotesia flavipes is an important gregarious larval endoparasitoid of several crambid stem borers, including Diatraea saccharalis. The suitability of two crambid species, Eoreuma loftini and D. saccharalis, pests of sugarcane and rice in Texas, for C. flavipes development was tested. The effect of parasitization by C. flavipes on encapsulation response was assessed in vivo in both D. saccharalis and E. loftini. The results indicated that the parasitoid developed and emerged successfully in D. saccharalis larvae. Although E. loftini larvae were readily parasitized by C. flavipes parasitoids, no wasp larvae hatched from the eggs in this host because eggs were encapsulated by the host's hemocytes. The developmental fate of the E. loftini larvae with encapsulated parasitoids was variable. Most died as abnormal fifth instars or as post-wandering prepupae, while a few developed normally to the pupal stage. In vivo experiments, there was a significant reduction in the percent of beads encapsulated in parasitized larvae in both hosts. However, the percent of beads showing melanization decreased significantly in parasitized D. saccharalis larvae but did not differ significantly in parasitized or unparasitized E. loftini larvae. Our results showed that D. saccharalis is a suitable host for C. flavipes whereas E. loftini is an unsuitable host. This study indicated that lepidopteran stem borers that are taxonomically, behaviorally, and ecologically very similar can differ in their ability to encapsulate a parasitoid species.     

Possibly similar genetic basis of resistance to Bacillus thuringiensis Cry1Ab protein in 3 resistant colonies of the sugarcane borer collected from Louisiana,USA

The sugarcane borer, Diatraea saccharalis (F.), is a major maize borer pest and a target of transgenic maize expressing Bacillus thuringiensis (Bt) proteins in South America and the mid‐southern region of the United States. Evolution of resistance in target pest populations is a great threat to the long‐term efficacy of Bt crops. In this study, we compared the genetic basis of resistance to Cry1Ab protein in 3 resistant colonies of sugarcane borer established from field populations in Louisiana, USA. Responses of larvae to the Cry1Ab protein for the parental and 10 other cross colonies were assayed in a diet‐incorporated bioassay. All 3 resistant colonies were highly resistant to the Cry1Ab protein with a resistance ratio of >555.6 fold. No maternal effect or sex linkage was evident for the resistance in the 3 colonies; and the resistance was functionally nonrecessive at the Cry1Ab concentrations of ≤ 3.16 μg/g, but it became recessive at ≥10 μg/g. In an interstrain complementation test for allelism, the F₁ progeny from crosses between any 2 of the 3 resistant colonies exhibited the similar resistance levels as their parental colonies, indicating that the 3 colonies most likely shared a locus of Cry1Ab resistance. Results generated from this study should provide useful information in developing effective strategies for managing Bt resistance in the insect.     

Evidences of non-additive effects of multiple parasitoids on Diatraea saccharalis Fabr. (Lep., Crambidae) populations in sugarcane fields in Brazil

Biological control is a relatively benign method of pest control. However, considerable debate exists over whether multiple natural enemies often interact to produce additive or non‐additive effects on their prey or host populations. Based on the large data set stored in the São João and Barra sugarcane mills (state of São Paulo, Brazil) regarding the programme of biological control of Diatraea saccharalis using the parasitoids Cotesia flavipes and tachinid flies, in the present study the author investigated whether the parasitoids released into sugarcane fields interfered significantly with the rate of parasitized D. saccharalis hosts. The author also observed whether there was an additive effect of releasing C. flavipes and tachinids on the rate of parasitized hosts, and looked for evidence of possible negative effects of the use of multiple parasitoid species in this biological control programme. Results showed that C. flavipes and the tachinids were concomitantly released in the Barra Mill, but not in the São Jão Mill. Furthermore, in the Barra Mill there was evidence that the parasitoids interacted because the percentage of parasitism did not increase after the release of either C. flavipes or tachinids. In the São João Mill, when both parasitoid species were released out of synchrony, both the percentage of parasitism by C. flavipes as well as that of the tachinids increased. When large numbers of tachinids were released in the Barra Mill, they caused a significant lower percentage of parasitism imposed by C. flavipes. The implications of the results as evidence of non‐additive effects of C. flavipes plus tachinids on D. saccharalis populations are discussed.