Matilde Eizaguirre

Thermoperiodism, photoperiodism and sensitive stage in the diapause induction of Sesamia nonagrioides (Lepidoptera: Noctuidae)

Abstract Photoperiod is the major factor governing the induction of larval diapause in Sesamia nonagrioides. Constant temperature and thermoperiod were shown to be additional factors which interact with photoperiod to induce diapause. Constant temperature modified the diapause response curve to daylength from a type III (at 25°C) to a type I (at 18°C). Photoperiodic induction of diapause was enhanced by superimposed thermoperiods regardless of whether the low temperature coincided with the scotophase or with the photophase. However, constant temperature and thermoperiods per se induced diapause in the absence of photoperiodic signals. Low (18°C) constant temperatures induced 100% diapause in larvae reared under continuous darkness. Also in continuous darkness, diapause was induced in a 50% of larvae when they were reared under a thermoperiodic regime [30°C (13 h): 18°C (11 h)] in comparison with 0% diapause if larvae were reared under 25°C constant temperature. Gradually increasing and decreasing photoperiods did not modify the diapause response but the gradual decrease of daylength shortened the larval developmental time. First and second larval instars were the most sensitive to diapause induction by photoperiod. The diapause-inducing effect of short daylengths on young larvae can be reversed by exposure to long daylengths in the late instars.

Mating disruption of the corn borer Sesamia nonagrioides (Lepidoptera: Noctuidae) using sprayable formulations of pheromone

Field trials to evaluate efficacy of two sprayable formulations of a binary pheromone blend of the maize pest Sesamia nonagrioides, were carried out from 1997 to 1999 in Spain, Greece and France. Efficacy was measured through inhibition of trap catches in treated fields and reduction of pest population growth between the first and second generations. Most treated fields showed over a 60% of trap catch inhibition within the 15 days following pheromone application. Reduction of pest population increase was variable in Spain and Greece according to the year, but mostly higher than 60%, and consistently high in France where values were above 65% during the 3 years in all replicates. Pheromone release rate was high during the first 2–3 days following application: less than 20% of the pheromone components sprayed remained on or were adsorbed into leaves 4–5 days after application. The effectiveness of mating disruption to control the S. nonagrioides population was at least as good as chemical application and additionally it is compatible with biological control applied in some areas of southern Europe. r 2002 Elsevier Science Ltd. All rights reserved.

Phenological Model for Sesamia nonagrioides (Lepidoptera: Noctuidae)

Abstract The development time of different stages of Sesamia nonagrioides Léfèvbre was studied in the laboratory at constant temperatures in the range 10–36°C. The relationship between the developmental rate (1/d) and the temperature (°C) was determined using both linear and nonlinear (Logan type III) models. The fit of the linear regression in the temperature range 15–27.5°C and Logan type III equation was good (R2 > 0.97). The lower developmental threshold, calculated using the linear model, was tb = 12°C. The nonlinear model gave a higher threshold of Tmax = 36°C, and an optimal developmental temperature of Trmax = 30°C. The flights of S. nonagrioides were studied in Lleida, Spain, from 1984 to 1997, and the number of degree-days required for their occurrence was calculated. A relationship between the degree-days required for the first flight calculated by the averaging method and the degree-days required for estimated diapause termination was obtained (R2 = 0.84). This relationship allowed the prediction of the first flight occurrence in the field. The degree-days required for the second and third generations were then calculated as 730 and 491, respectively.

Juvenile hormone and diapause in the Mediterranean corn borer, Sesamia nonagrioides.

Juvenile hormone content of Sesamia nonagrioides larvae reared under different environmental conditions that induce diapause was studied using a bioassay on newly-emerged Tribolium confusum pupae. Two analytical methods were also used to measure JHIII content. Extracts from larvae that developed under different conditions of photoperiod and temperature caused different effects in T. confusum pupae. This can be related to different diapause intensities. Extracts from diapausing larvae produced a higher juvenilizing effect than the expected, considering their JHIII titer calculated by the chromatographic analysis. This indicates that hormones other than JHIII must be present in extracts of diapausing larvae of S. nonagrioides. The analytical procedures confirm that diapausing larvae have a higher JHIII titer in the haemolymph than non-diapausing larvae. This shows that JH is involved in the maintenance of diapause in this species.

Electroantennogram, wind tunnel and field responses of male Mediterranean corn borer, Sesamia nonagrioides, to several blends of its sex pheromone components.

The electrophysiological and behavioural responses of male Sesamia nonagrioides Lefèbvre (Lepidoptera: Noctuidae) to several synthetic blends of its sex pheromone components were investigated. Electroantennogram (EAG) recordings showed the greatest response to a 77:8:10:5 blend of the four compounds found in the female glands: (Z)‐11‐hexadecen‐1‐yl acetate, (Z)‐11‐hexadecen‐1‐ol, (Z)‐11‐hexadecenal and dodecyl acetate. A logistic equation was fitted to the dose‐response data and differences in relative EAG responses were analysed by deriving five parameters of this equation. Flight responses of males were also evaluated in a wind tunnel where the proposed blend (77:8:10:5) also elicited the best response. Field tests confirmed the laboratory findings as the 77:8:10:5 blend captured more S. nonagrioides males than other pheromone mixtures and showed more selectivity for male S. nonagrioides over other noctuid species, Discestra trifolii Rottenberg and Lacanobia oleracea L., but not for Mythimna unipuncta Haworth, a species with a sex pheromone composition almost identical to that of S. nonagrioides.

Pheromone antagonism in the European corn borer moth Ostrinia nubilalis.

Mixing the sex pheromones of the Mediterranean corn borer, Sesamia nonagrioides, and the European corn borer, Ostrinia nubilalis, results in significantly lower captures of O. nubilalis when compared to traps loaded with its pheromone alone. Rubber septa loaded with a constant concentration of the pheromone of O. nubilalis and different percentages of the S. nonagrioides pheromone (from 1 to 100%) causes dose-dependent antagonism in the field. Electroantennograms of O. nubilalis males showed high antennal responses to its own pheromone components, followed by smaller responses to the major, [(Z)-11-hexadecenyl acetate (Z11-16:Ac)], and two minor components [dodecyl acetate (12:Ac) and (Z)-11-hexadecenal (Z11-16:Ald)] of the S. nonagrioides pheromone. There was almost no response to the S. nonagrioides minor component (Z)-11-hexadecenol (Z11-16:OH). Field tests that used traps baited with the O. nubilalis pheromone plus individual components of S. nonagrioides showed that Z11-16:Ald causes the antagonism. Adding 1% Z11-16:Ald to the pheromone of O. nubilalis reduced oriented flight and pheromone source contact in the wind tunnel by 26% and 83%, respectively, and trap captures in the field by 90%. The other three pheromone components of S. nonagrioides inhibited pheromone source contact but not oriented flight of O. nubilalis males and did not inhibit capture in the field. Cross-adaptation electroantennogram suggests that Z11-16:Ald stimulates a different odor receptor neuron than the pheromone components of O. nubilalis. We conclude that Z11-16:Ald is a potent antagonist of the behavioral response of O. nubilalis.

Juvenile hormone esterase activity in the pupating and diapausing larvae of Sesamia nonagrioides.

The development of the Mediterranean corn borer, Sesamia nonagrioides, under long-day (LD) photoperiod is associated with juvenile hormone (JH) decline and pupation in the 5th or 6th larval instar. The larvae grown under short-day (SD) conditions maintain a moderate JH titer and enter diapause during which they undergo several extra larval molts. Both types of larvae exhibit similar levels of juvenile hormone esterase (JHE) activity that increases in each instar during the period of low ecdysteroid titer and drops when the titer rises to a molt-inducing peak. A suppression of JHE activity within 24h after application of an ecdysteroid agonist suggests that the drop of activity is a rapid and possibly direct response to ecdysteroids or their agonist. Esterase inhibitor 3-octylthio-1,1,1-trifluoro-2-propanone (OTFP) suppressed more than 98% of the JHE activity without affecting pupation timing and adult development. The data indicate that JHE is not crucial for the switch between larval development, diapause, and metamorphosis in S. nonagrioides.

Low susceptibility of non-target Lepidopteran maize pests to the Bt protein Cry1Ab.

Transgenic Bt maize expressing the Cry1Ab toxin is poorly effective for suppressing populations of two non-target Lepidoptera, Mythimna unipuncta and Helicoverpa armigera. In order to determine the mechanisms that may be involved in this poor effectiveness, last instar larvae of the two Lepidoptera were fed with a diet containing lyophilized leaves with Bt vs non-Bt toxin for different periods; additionally, some larvae fed on Bt diet were transferred to non-Bt diet for an additional period. In the experimental larvae, we measured the growth (weight) gain from just before treatment to after the end of the treatment, and the Cry1Ab contents in the hemolymph, the peritrophic membrane and its contents and midgut epithelium. Effects of the treatments on the midgut epithelium were observed by light and transmission electron microscopy. It was seen that multiple mechanisms can be involved in the low susceptibility of the two Lepidoptera. The low content of the toxin within the peritrophic membrane 48 h after ingestion indicates a high rate of toxin elimination in this space. Moreover, M. unipuncta larvae fed on the Bt diet displayed a similar growth gain index to those fed on the non-Bt diet, and showed an increasing elimination rate during the experiment. Little toxin reached the midgut epithelium, indicating a low permeability of the peritrophic membrane or a low affinity at the binding sites. Larvae fed on the Bt toxin showed rapid recovery in weight gain and in the midgut epithelium, and also showed overcompensation mechanisms.

Dispersal capacity in the Mediterranean corn borer, Sesamia nonagrioides

Corn (Zea mays L.) borers are the primary target of Bacillus thuringiensis Berliner (Bt) transgenic maize. Management of corn borer resistance to Bt requires information on larval and adult dispersal capacities, a feature that is particularly unknown in Sesamia nonagrioides Lefèbvre (Lepidoptera: Noctuidae), the most damaging corn borer in Spain. Larval dispersal was studied over a 3 year period by infesting plants with egg masses and dissecting the neighbouring plants 7, 14, and 32 days later to measure larval dispersal at several ages. The number and age of larvae were recorded in the dissected plants. Only mature larvae dispersed in significant numbers; they moved at least to rows adjacent to those containing the infested plant, and down the row five plants. The percentage of larvae that dispersed from the infested plant was density‐dependent. Adult dispersal was studied with directional light and pheromone uni‐traps over 5 and 3 year periods, respectively. Directional light traps were placed in the margins between Bt and non‐Bt maize fields, half oriented towards each of the two kinds of maize field. Pheromone traps were placed in the Bt and non‐Bt fields at increasing distances (0–100 m) from the border. The numbers of males and females caught in directional light traps were not different in traps oriented towards Bt or non‐Bt fields, but the number of males caught in the third flight in Bt fields was lower than in non‐Bt fields. These results suggest that males from adjacent Bt and non‐Bt fields mate indiscriminately with females emerging in any of the two kinds of maize fields. However, male movement in the third flight may not be sufficient to randomly distribute males between the two fields.

Mating behaviour of female Tuta absoluta (Lepidoptera: Gelechiidae): polyandry increases reproductive output

Tuta absoluta (Meyrick) is an important pest of tomato. Mass trapping of males has seldom proved an effective control measure, probably due to the pest’s reproductive biology. There are few studies on female mating behaviour. For this reason, this study aimed to determine female mating frequency and its effects on reproductive output and female longevity; the effects of male mating status on female reproductive output and longevity and the fate of the spermatophore inside the bursa copulatrix. The studied population of T. absoluta mated only once a day at the beginning of the photophase, females remated frequently and can be classified as polyandrous. The refractory period of both T. absoluta males and females was very short and was not observed in many cases. The lack of refractory period could be due to the female’s rapid metabolism of the spermatophore thus allowing the female to reduce the intermating interval and mate again. Remating had direct benefits for T. absoluta females increasing their fecundity, fertility and longevity. These benefits were greater when females could remate to virgin males instead of to the same male. In this species, therefore, polyandry has direct benefits for the female. The occurrence of polyandry together with other known or unknown reasons can explain the failure of control methods based on pheromones in the open greenhouses in some tomato-growing areas.