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.     

Spatial pattern of parasitism in Diatraea saccharalis Fab. (Lep., Crambidae) populations at two different spatial scales in sugarcane fields in Brazil

Abstract:  The biological control of Diatraea saccharalis is regarded as one of the best examples of successful classical biological control in Brazil. Since the introduction of the exotic parasitoid, Cotesia flavipes , the decrease in D. saccharalis infestation in sugarcane fields has been attributed to the effectiveness of this agent. Native Tachinidae fly parasitoids ( Lydella minense and Paratheresia claripalpis ) have also been implicated in the success. Quantitative data confirming the actual contribution of these agents to the control of D. saccharalis are, however, rather scant. The purpose of this study was to investigate the spatial pattern of parasitism of these parasitoids in D. saccharalis populations at two large spatial scales (fields and zones). To investigate this subject, a large data set comprising information collected from a sugarcane mill located in the state of São Paulo, Brazil (São João sugarcane mill) was analysed. When regressions between the proportion parasitism against host density were computed, the percentage of significant regressions with either a positive or a negative slope was very small at both spatial scales for both parasitoid species. Regressing the densities of tachinid-parasitized hosts against host densities per field showed that these parasitoids presented a 'moderate aggregative' response to host densities, as 53.33% of the regressions were positively significant. Cotesia flavipes was 'weakly aggregated' on host densities at the field level, because only 33.33% of the regressions were positively significant. At the zone level, neither aggregative nor spatial proportion parasitism responses were evident for either parasitoid species due to the small percentage of significant regressions computed.     

Use of ablated stalks to assess field rates of parasitism of Diatraea saccharalis (Lepidoptera: Crambidae) by Cotesia flavipes (Hymenoptera: Braconidae)

We evaluated a technique that used ablated sections of corn stalks infested with larvae of Diatraea saccharalis F. to assess parasitism by Cotesia flavipes (Cameron) in a corn field. Stalkborer larvae were retrieved successfully from the artificially infested, ablated stalks for at least 96 h after deployment in the field. Levels of parasitism in ablated stalks attached to corn plants were comparable to levels of parasitism measured using whole plants that were artificially infested. Olfactometer comparisons showed preference by female C. flavipes for both whole plants and ablated stalks containing larval D. saccharalis, over uninfested stalks and plants. This technique provides a means to estimate parasitism of stalkborer larvae by C. flavipes without destructive sampling of agronomic plants.     

Spatial and temporal population interactions between the parasitoids Cotesia flavipes and Tachinidae flies: considerations on the adverse effects of biological control practice

Biological control of Diatraea saccharalis is regarded as one of the best examples of successful classical biological control in Brazil. Since the introduction of the exotic parasitoid Cotesia flavipes, the decrease of D. saccharalis infestation in sugarcane fields has been attributed to the effectiveness of this agent. Recently, the native tachinid fly parasitoids (Lydella minense and Paratheresia claripalpis) have also been implicated in the success. Here, we investigated the spatial and temporal population interactions between C. flavipes and the tachinid flies, and provide a critical analysis of the biological control practice, focusing on the undesirable effects of introductions of exotic natural enemies. To investigate these questions, a large data set comprising information from two sugarcane mills located in the state of São Paulo, Brazil (Barra and São João Mills), was analysed. Analysis of the correlation between C. flavipes and tachinid fly population densities through time revealed that such populations were inversely correlated in the São João Mill and not correlated in the Barra Mill. Logistic regressions were computed to investigate the proportion of sites occupied by the parasitoid species at both mills as a function of time. An increasing trend in the proportion of sites occupied by C. flavipes was observed, with a concomitant decrease of the sites occupied by tachinid flies. This effect was more intense in the São João Mill. Thus, there is a convincing possibility that constant releases of C. flavipes decreased the tachinid fly populations, resulting in an undesirable effect of biological control practice.     

Effects of entomopathogenic bacterium Photorhabdus temperata infection on the intestinal microbiota of the sugarcane stalk borer Diatraea saccharalis (Lepidoptera: Crambidae)

Photorhabdus temperata is an entomopathogenic bacterium that is associated with nematodes of the Heterorhabditidae family in a symbiotic relationship. This study investigated the effects of P. temperata infection on the intestinal microbiota of the sugarcane stalk borer Diatraea saccharalis. Histopathology of the infection was also investigated using scanning electron microscopy. Groups of 20 larvae were infected by injection of approximately 50 bacterial cells directly into the hemocoel. After different periods of infection, larvae were dissected and different tissues were used for bacterial cell quantification. P. temperata was highly virulent with an LD₅₀ of 16.2 bacterial cells at 48 h post-infection. Infected larvae started dying as soon as 30 h post-infection with a LT₅₀ value of 33.8 h (confidence limits 32.2–35.6) and an LT₉₀ value of 44.8 h (CL 40.8–51.4). Following death of the larvae, bacteria from the midgut did not invade the hemocoel. In the midgut epithelium, P. temperata occupied the space underneath the basal lamina. The cultivable intestinal bacterial populations decreased as soon as 1 h post-infection and at 48 h post-infection, 90% of the gut microbiota had died. The role of P. temperata in control of the midgut microbiota was discussed.     

Restriction site associated DNA (RAD) for de novo sequencing and marker discovery in sugarcane borer,Diatraea saccharalis Fab. (Lepidoptera: Crambidae)

We present the development of a genomic library using RADseq (restriction site associated DNA sequencing) protocol for marker discovery that can be applied on evolutionary studies of the sugarcane borer Diatraea saccharalis, an important South American insect pest. A RADtag protocol combined with Illumina paired‐end sequencing allowed de novo discovery of 12 811 SNPs and a high‐quality assembly of 122.8M paired‐end reads from six individuals, representing 40 Gb of sequencing data. Approximately 1.7 Mb of the sugarcane borer genome distributed over 5289 minicontigs were obtained upon assembly of second reads from first reads RADtag loci where at least one SNP was discovered and genotyped. Minicontig lengths ranged from 200 to 611 bp and were used for functional annotation and microsatellite discovery. These markers will be used in future studies to understand gene flow and adaptation to host plants and control tactics.     

Susceptibility of Cry1Ab-resistant and -susceptible sugarcane borer (Lepidoptera: Crambidae) to four Bacillus thuringiensis toxins

Sugarcane borer, Diatraea saccharalis (F.), is a primary corn stalk borer pest targeted by transgenic corn expressing Bacillus thuringiensis (Bt) proteins in many areas of the mid-southern region of the United States. Recently, genes encoding for Cry1A.105 and Cry2Ab2 Bt proteins were transferred into corn plants (event MON 89034) for controlling lepidopteran pests. This new generation of Bt corn with stacked-genes of Cry1A.105 and Cry2Ab2 will become commercially available in 2009. Susceptibility of Cry1Ab-susceptible and -resistant strains of D. saccharalis were evaluated on four selected Bt proteins including Cry1Aa, Cry1Ac, Cry1A.105, and Cry2Ab2. The Cry1Ab-resistant strain is capable of completing its larval development on commercial Cry1Ab-expressing corn plants. Neonates of D. saccharalis were assayed on a meridic diet containing one of the four Cry proteins. Larval mortality, body weight, and number of surviving larvae that did not gain significant weight (<0.1 mg per larva) were recorded after 7 days. Cry1Aa was the most toxic protein against both insect strains, followed in decreasing potency by Cry1A.105, Cry1Ac, and Cry2Ab2. Using practical mortality (larvae either died or no significant weight gain after 7 days), the median lethal concentration (LC₅₀) of the Cry1Ab-resistant strain was estimated to be >80-, 45-, 4.1-, and −0.5-fold greater than that of the susceptible strain to Cry1Aa, Cry1Ac, Cry1A.105 and Cry2Ab2 proteins, respectively. This information should be useful to support the commercialization of the new Bt corn event MON 89034 for managing D. saccharalis in the mid-southern region of the United States.