Spittlebugs are sucking insects that cause decrease of forage grass quality, thereby constituting a serious problem for cattle rearing throughout tropical America (Valério & Nakano 1988). The most promising biological control agent of the spittlebug is the predator Salpingogaster nigra Schiner (Diptera: Syrphidae). This species lays its eggs in the foam produced by the spittlebug nymphs. Each larva feeds on a mean of 14 spittlebug nymphs (Guppy 1913; Paez et al. 1984). This predator has a short life cycle, with 2 to 3 generations during their prey's life cycle (Guagliumi 1970).
According to Parra (2002), the establishment of a program involving the use of natural enemies requires in-depth knowledge of the life history of the species involved so that the best performance may be obtained. However, it should be noted that even with all the biological and behavioral information of the developmental stages of the predator S. nigra, the host range has not been definitively determined, although the insect feeds on spittlebugs.
Here we report a study on the biology and olfactory response of S. nigra directed at the mass rearing of the predator. Salpingogaster nigra adults were field collected with an entomological net from Oct 2016 to Mar 2017. Then they were taken to the Entomology Laboratory in Juiz de Fora, Minas Gerais, Brazil, identified by sex, and paired. They were fed on flowers of Parthenium hysterophorus L. (Asteraceae) weeds and on Emilia sonchifolia (L.) DC (Asteraceae). Plants were replaced every 3 d. Honey drops (10% v/v) were placed on the cage walls. A pot with Brachiaria decumbens Stapf (Poaceae), infested with nymphs of the foamproducing prey Mahanarva spectabilis Distant (Hemiptera: Cercopidae) was placed in the cage as an egg-laying substrate. Salpingogaster nigra females mostly lay their eggs during the rainy season. Humidity was maintained by spraying water inside the cages 3 times per d.
Eighteen pairs of S. nigra were kept in 80 × 55 × 55 cm acrylic cages with the same food and egg-laying substrate mentioned previously. To have sufficient foam, nymphs of prey were added to the cages to maintain a density of 4 prey that were producing foam on each d. These insects were maintained in an acclimatized Fitotron-type chamber at 25 ± 2 °C, 70 ± 10% relative humidity (RH), and 12:12 h (L:D) photoperiod. Salpingogaster nigra eggs in the foam were counted daily. They were placed on petri dishes lined with filter paper and moistened with distilled water to keep them hydrated. Because the eggs were kept until they hatched, mean duration and viability of the predator eggs could be recorded.
Twenty-one larvae of S. nigra were placed in each 1 L pot (containing B. decumbens infested with M. spectabilis nymphs at the fourth or fifth instar as food for the larvae) with a fine-bristle brush. This was replicated 10 times, hence the development and survival of 210 larvae was determined. Spittlebug nymphs were added daily so that food would not be lacking.
Forty-two larvae of S. nigra were obtained from pastures with B. decumbens so that the biological parameters of the pupa phase could be obtained. Thirty-three larvae pupated and were transferred to petri dishes lined with cotton and filter paper, and moistened with distilled water. Mean duration and survival of the pupa phase were assessed.
Olfactometry bioassays were performed at the Entomology Laboratory. Each arm of the Y-shaped glass olfactometer, with a continuous air flow of 1.0 L per min, was linked by silicone tubes to 2 glass chambers (42 cm H × 16 cm W) with the tested materials. Females and males from mating cages were separated approximately 2 h prior to the start of the bioassays. Olfactory responses of mated S. nigra females were then compared for the following odor combinations: (I) 3 spittlebug nymphs vs. air; (II) 3 spittlebug nymphs vs. foam of the spittlebug in the petri dish; (III) foam produced by the spittlebug in the petri dish vs. air. A bioassay also was conducted to test the bioassay apparatus without any odor in the olfactometer to confirm that the insects displayed positive anemotaxis in the arena when presented with clean air in both arms.
Insects were introduced individually in the common arm of the Ytube, and each specimen was tested once to avoid pseudo-repetitions. The response was considered to be positive when the insect moved against the air current and reached the end of one of the Y-tube arms within 10 min. All olfactometry tests occurred between 2:00 PM and 4:00 PM. Fifty S. nigra females were tested for each treatment. Response data of were analyzed by BioEstat χ2 test (Ayres et al. 2003). Insects that did not respond to the stimuli were excluded from statistical analysis.
The success biological control using lab-reared predators depends on production of healthy insects. This research revealed that S. nigra females maintained in the laboratory, and fed on honey and on P. hysterophorus and E. sonchifolia flowers, had a mean life span of 17.1 ± 1.6 d (SE) (11–30 d) and laid 2,209 eggs, with mean fecundity of 25.1 ± 6.1 (SE) eggs per female. According to Chambers (1988), syrphid flies require pollen and nectar for gametogenesis. Gomes & Lastra (2009) similarly maintained S. nigra in the laboratory and fed them on brown sugar and buds of Echinochloa colona (L.) Link (Poaceae), resulting in fecundity of 29 eggs per female.
In this study, we observed a mean egg development period of 2.7 ± 0.1 d in S. nigra, with a mean egg hatch rate of 64.0 ± 4.5%. Paez et al. (1984) also reported a 2.7 d period for the embryonic development of S. nigra at room temperature. Guppy (1913) registered a mean developmental period of 2.5 d for eggs of the same species. The short embryonic period and high rate of egg hatch suggest easy rearing of the insect in the laboratory.
The mean larval period of S. nigra was 10.5 ± 0.4 d when fed on spittlebug nymphs. These results corroborate that obtained by Paez et al. (1984), who evaluated the larval duration of S. nigra and reported a mean period of 9.7 d. However, in our study the proportion surviving the larval stage was only 10.0 ± 2.1%. This corroborates results by Paez et al. (1984), who maintained 115 S. nigra larvae in the laboratory, with only 4 completing the larval stage. The entomological literature fails to report the best conditions for maintaining larvae in the laboratory to obtain a high level of survival. The mean pupal duration was 10.7 ± 0.2 d and the proportion attaining the adult stage was 88.1 ± 5.1% in this study.
It should be noted that food provided to the predator was the same as was fed upon in the field (spittlebug). In mass production of this insect, different prey or an artificial diet would be required because spittlebug nymphs occur only during the rainy season. During the dry period of the year, spittlebugs are in diapause. Hence, for the breeding of S. nigra in the laboratory, it is necessary to conduct further research to find new prey or artificial diets.
Entomophagous insects have specialized sensory systems that allow them to use a variety of cues to find target organisms (Hatano et al. 2008). Visual cues play an important role in locating prey by different predators, such as Harmonia axyridis (Pallas) (Coleoptera: Coccinelidae) in the predation of aphids (Lambin et al. 1996). Besides visual cues, sound and chemical cues can be used to locate prey. In this study, we focused on the chemical cues used by the predator. Salpingogaster nigra females did not show a directional response (no preference for a given side independent of odor stimulus) when only clean air was presented through both arms (χ2 = 0.381; df = 1; P = 0.537). The predator did not display any preference for the odors of the foam produced by the nymph of the prey when compared to air (treatment I) (χ² = 0.32; df = 1; P = 0.572). Also, there was no preference for nymphal odors when compared to the foam produced by the prey (treatment II) (χ2 = 0.040; df = 1; P = 0.577). On the other hand, when odor sources with spittlebugs were provided, there was a significant preference for odors from the nymphs compared to air (treatment III) (χ2 = 13.52; df = 1; P = 0.001) (Fig. 1). Shonouda (1996) and Sutherland et al. (2001) demonstrated that the honeydew excreted by the bugs is an attractive source for egg-laying in the adult stage of syrphids. This report on the behavior of S. nigra females showed that attraction was not due to the foam excreted by the spittlebug but by the nymph, which most likely would function as a stimulus for egg-laying by the female predator.
These studies demonstrate that chemical cues may be involved in host finding by the predator as it searches for nymphs. Our results show that although S. nigra adults, eggs, and pupae may be kept in the laboratory, mass breeding of the predator is not feasible due to low larval viability of larvae. Further studies are required to minimize such low viability and make possible mass rearing in the laboratory.
We thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) for supporting our research.
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