INTRODUCTION

Extensive culture of Urochloa (syn. Brachiaria) caused outbreaks of spittlebug populations (Hemiptera: Cercopidae), the main pests in the pastures of tropical America (^{COSENZA, 1981}; ^{VALÉRIO; NAKANO, 1988}; ^{SUJII et al., 2001}). According to ^{AUAD et al. (2009}), signalgrass (U. decumbens) is more prone to attacks by the spittlebug when compared to other Brachiaria species. In addition to the genus Mahanarva, the species Deois flavopicta, Deois schach, Deois incompleta and Notozulia entreriana are extremely common in the South-Central region of Brazil among the spittlebugs that attack pastures (^{SILVEIRA NETO et al., 1992}; ^{RESENDE et al., 2013}). Further, the species D. flavopicta was reported in signalgrass in the northwestern region of the state of São Paulo, Brazil (^{PEREIRA et al., 2011}). Female spittlebugs usually lay their eggs on the ground or in vegetal remains. After eclosion, nymphs fix themselves at the base of the signalgrass and maintain themselves protected by characteristic foam (^{PEREIRA; PEREIRA, 1985}; ^{VALÉRIO, 2009}).

Spittlebug nymphs suck the sap of the root and stalk on the soil’s surface. Adults feed on the aerial section of the grass and inject it with toxins, giving it a yellowish color and causing it to wither. They also reduce crude protein, fat and essential minerals. Dry matter increases and the grass becomes less tasty. Consequently, the animal feed is reduced, decreasing the milk and beef production (^{HEWITT, 1988}; ^{VALÉRIO; NAKANO, 1988}). In other words, decrease of pest population is a must. Liabilities are estimated in hundreds of millions of dollars a year. In the case of the Brazilian savannah, with an area planted with signalgrass reaching 15 million hectares, losses may range between 99 and 819 millions of dollars a year (^{MACEDO, 2005}; ^{HOLMANN; PECK, 2002}; ^{VALÉRIO, 2012}).

Management and control methods for spittlebug in pastures include biological control by the fungus Metarhizium anisopliae, a substance with insecticides and resistant varieties (^{VALÉRIO; KOLLER, 1993}; ^{PEREIRA et al., 2008}). However, high costs impair control measures, as well as lack of information and scarcity of specific insecticides against the spittlebug, scanty tolerant or resistant foragers, difficulties in acquisition and inconsistent results of biological control (^{TOWNSEND et al., 2001}). Insecticides are frequently and mistakenly applied after the yellowing of pastures, as the symptom appears approximately three weeks after the spittlebug attack (^{SOUZA et al., 2008}).

Seed treatment by insecticides, fungicides and nematicides for protection against pest-insects and phytopathogens is highly relevant for the development of robust and healthy plants (^{PARISI; MEDINA, 2014}). One of the most important characteristics in the treatment of seeds by insecticides is the systemic effect on the plant, enhanced by low vapor pressure and solubility in water of the main active substances. In fact, the ingredient releases itself slowly and is absorbed by the roots. The plant is thus protected against ground and aerial insects (^{SILVA, 1998}).

Current analysis evaluates the dry mass yield and the survival and injuries caused by adult spittlebugs D. flavopicta in growing U. decumbens and U. brizantha pastures, submitted to seed treatments with insecticides fipronil (Amulet^®) and thiamethoxam (Cruiser 350 FS^®).

MATERIALS AND METHODS

Infestation of adult D. flavopicta

Spittlebug adults (D. flavopicta) were collected in remnant pastures for the standardization and uniformity of infestation in the experimental units. Insects were then selected in the laboratory and 20 specimens were inoculated per parcel (in a 0.40x0.40x0.70 m cage), after 49 post-sowing days (Fig. 1). Population level and infestation period followed recommendations by ^{VALÉRIO; NAKANO (1988}) and ^{RESENDE et al. (2013}).

Figure 1: Infestation by D. flavopicta in the experimental units. 

RESULTS AND DISCUSSION

Analysis of variance showed that there was no significant interaction among the signalgrass species and insecticide-treated seeds on the survival of D. flavopicta in all evaluations (Table 1).

D. flavopicta population did not differentiate significantly the forage species at 2, 4, 6 and 8 days after infestation (DAI). However, overtime (on the 10^th and 12^th DAI) a greater percentage of spittlebugs survived in U. decumbens than in U. brizantha. The latter is resistant to spittlebugs, as reported by ^{COSENZA (1981}) and ^{AUAD et al. (2009}). The treatment of signalgrass seeds by insecticides fipronil and thiamethoxam failed to decrease D. flavopicta population at the 2^nd and 4^th DAI, when compared to grass without seed treatment (control). Therefore, on the 6^th DAI, the systemic insecticide thiamethoxam significantly reduced the percentage of the surviving insects when compared to foragers without any chemical protection of the seeds. It also reduced those submitted to seed treatment with fipronil. Impact of thiamethoxam on cercopid adults remained during the other evaluations at 8^th, 10^th and 12^th DAI. The above may be related to the systemic activity of thiamethoxam with great efficaciousness in the control of sucker insects (^{GAZZONI, 2008}). The study reveals that the treatment of signalgrass seeds with thiamethoxam (Cruiser 350 FS^®), specifically employed for the control of termites in pastures (^{ANDREI, 2013}), may be an asset in the population decrease of the spittlebug D. flavopicta in young pastures.

The number of U. decumbens and U. brizantha stands did not differ significantly among the different grass species and between the areas submitted or not to seed treatment with thiamethoxam and fipronil. In fact, the products (applied according to manufacturer’s instructions) did not reveal any phytotoxicity to seeds and, consequently, to plant emergence (Table 2).

Injuries attributed to the forager attacked by spittlebug adults in the susceptible grass (U. decumbens) were comparatively greater than those in the resistant one, regardless of the treatment of seeds by insecticides (Fig. 2). The above corroborates the resistance of U. brizantha to spittlebugs (^{VALÉRIO et al., 1997}).

Figure 2: Injuries caused by D. flavopicta adults in U. decumbens (a) and U. brizantha (b). 

Fipronil and thiamethoxam, applied in the treatment of signalgrass seeds, failed to reduce significantly injuries by D. flavopicta, even though there was a higher yellowing rate in pasture without any chemical protection to seeds. Since there was a decrease in survival rates in grasses from seeds treated with thiamethoxam, it may be surmised that similarity in injuries perceived among the experimental units was probably due to high infestation rates (20 adults 0.16 m^-2) of the spittlebugs in the cages and to the fact that defensive activities on the spittlebug occurred only after six days of infestation. During evaluation of the pest-insect, yellowing of the grass was perceived on the 2^nd and 4^th day after infestation. This fact differs from the report by ^{SOUZA et al. (2008}), who revealed that symptoms appeared about three weeks after the attack by the spittlebug.

Dry matter yield was significantly similar among species U. decumbens and U. brizantha, and between the areas submitted or not to seed treatment by insecticides. However, the yield of the resistant signalgrass was 9.15% times higher than that of the susceptible one. According to ^{MARTUSCELLO et al. (2009}), this fact may be related to the multi-stalk development of U. brizantha, which, aside from being a resistant variety to pest-insects, has high stalk production when compared to the growth of U. decumbens.

CONCLUSION

The treatment of signalgrass seeds with insecticide thiamethoxam may be an asset in decreasing the population of D. flavopicta in young pasture. However, it does not affect injuries caused by the pest insect and dry matter yield.