Brazil produces the most sugarcane (Ravaneli et al. 2011; Vacarí et al. 2012; Simöes et al. 2012; Rossato et al. 2013), and Mahanarva fimbriolata (Stal) (Hemiptera: Cercopidae) is one of the major pests of this crop (Garcia et al. 2006; James et al. 2011; Garcia et al. 2011; Volpe et al. 2012). Nymphs and adults of M. fimbriolata can cause injuries and reduce of sugarcane productivity by 20 to 40 tonnes per hectare (Mendonça 2005). Furthermore, attacked sugarcane stalks lose quality, reducing sugar and alcohol production capacity (Dinardo-Miranda et al. 2002; Madaleno et al. 2008; Garcia et al. 2010; Carvalho et al. 2011; Korndörfer et al. 2011).

Chemical insecticides and the entomopathogenic fungus, Metarhizium anisopliae (Metschnikoff) Sorokin (Hypocreales: Clavicipitaceae), are used to control M. fimbriolata nymphs and adults on sugarcane (Dinardo-Miranda et al. 2004a, 2004 b; Loureiro et al. 2005; Li et al. 2010; Cuarán et al. 2012). Some chemical insecticides that are compatible with entomopathogenic fungi and other biological control agents may be used in various combinations to provide safe and efficient control of insect pests (Asi et al. 2010; Russell et al. 2010; Bitsadze et al. 2013).

The insecticides, imidacloprid and thiamethoxam, do not reduce the viability, vegetative growth, conidial production, and germination of M. anisopliae and this demonstrates the compatibility of these insecticides with this entomopathogen (Botelho & Monteiro 2011; Akbar et al. 2012; Silva et al. 2013).

Thus, the combination of chemical insecticides and M. anisopliae can used for the management of M. fimbriolata. The insecticide causes insect death in less time, and colonization of individuals killed by the entomopathogenic fungus increases the residual pest control effect (Dinardo-Miranda et al. 2008; Jin et al. 2011). This demonstrates the importance of understanding the combination of entomopathogenic fungi with insecticides in sugarcane fields.

The aim of this study was to evaluate the costs and efficacies of combinations of M. anisopliae with imidacloprid and thiamethoxam in M. fimbriolata control on sugarcane.

Materials and Methods

The pest control materials used in this study were as follows: Metarhizium anisopliae (Meitê®) procured from Ballagro Agro Tecnologia, Bom Jesus dos Perdões city, São Paulo state, Brazil; thiamethoxam (Actara 250 WG®) obtained from Syngenta Proteção de Cultivos Ltda, Paulínia city, São Paulo state, Brazil; and imidacloprid (Evidence 700 WG®) obtained from Bayer Crop Science, São Paulo city, São Paulo state, Brazil.

The experiment was conducted in a sugarcane field of the company “Energética Santa Helena Ltda” in Nova Andradina, Mato Grosso do Sul State, from Nov 2012 to Apr 2013. The experimental area (S 22° 16′ 73″ W 53° 18′ 23″, and 380 m) was planted with sugarcane (variety ‘SP813250’) with no flaws in the sprouting plants.

The plots included 10 rows of sugarcane spaced 1.4 m apart and 10 m long, totaling an areas of 140 m². The experiment was a randomized block design (RBD) with 10 treatments and 4 replications. The treatments included the control (untreated), thiamethoxam (250 g ha^-1), imidacloprid (700 g ha^-1), M. anisopliae (M. a.) (3 × 10¹² conidia ha^-1), Al (3 × 10¹² M. a. conidia ha^-1+ 65 g ha^-1 of thiamethoxam), A2 (3 × 10¹² M. a. conidia ha^-1+ 125 g ha^-1 of thiamethoxam), A3 (3 × 10¹²M. a. conidia ha^-1 + 187.5 g ha^-1 of thiamethoxam), A4 (3 × 10¹² M. a. conidia ha^-1 + 175 g ha^-1 of imidacloprid), A5 (3 × 10¹² M. a. conidia ha^-1 + 350 g ha^-1of imidacloprid), and A6 (3 × 10¹² M. a. conidia ha^-1 + 525 g ha^-1 of imidacloprid).

The experiment was started on 23 Nov 2012, when the level of infestation of M. fimbriolata in the experimental area reached on averaged 3.82 ± 0.23 nymphs (average ± standard error) per linear foot (30.4 cm) of furrow of sugarcane (Mendonça 2005; Dinardo-Miranda et al. 2008; Table 1). Since the application of mixtures of insecticides is prohibited in Brazil, each product was applied with a separate Jacto® sprayer (Pompéia city, São Paulo State, Brazil) that was calibrated to apply 150 L ha^-1 (Mendonça 2005). The spray was directed at the base of the stumps such that 30% of the spray volume reached the stems and 70% reached the sugarcane plant roots (Loureiro et al. 2005). The surfactant Tween® (0.01% polysorbate 80) was used in the treatments of fungal suspensions.

Table 1.

Pretreatment infestation by the spittle bug Mahanarva fimbriolata (Hemiptera: Cercopidae) on sugarcane in the experimental area.

Nymphs of M. fimbriolata were sampled every 2 weeks, up to 105 days after treatment (DAT) (Mendonça 2005) in 2 linear feet (60.8 cm) of furrow planting of sugarcane in each plot. The nymphs of M. fimbriolata on the basal internodes of plants were counted after removal of the residual straw.

Climate data (average temperature, relative humidity, and rainfall) were obtained from Inmet (Instituto Nacional de Meteorologia) (Fig. 1) to establish the relationship between the infestation of M. fimbriolata and abiotic factors. The value of total recoverable sugar (TRS) (Landell et al. 1999) was obtained from 20 sugarcane stalks removed randomly per treatment on 15 Apr 2013.

UDOP (2013) provided the value of a tonne of sugar per ha, the estimated yield (total recoverable sugar, TRS) per ha of 68 tonnes, which is the average yield of Mato Grosso do Sul, State (Unica 2013) x value of a tonne per ha], maintenance cost of sugarcane (MCS) excluding the costs of pest control materials and their application for the control of M. fimbriolata (Udop 2013), costs of pest control materials and their application and earnings per ha [(the estimated value of the TRS produced per ha) — MCS - control cost)], were calculated in dollars (US$). The values of outsourced services and purchased products to manage the sugarcane field and to control M. fimbriolata were obtained from consulting firms and agricultural database of Udop (2013) (Table 2).

The population data for nymphs of M. fimbriolata were subjected to analysis of variance, and the significant means were compared by the Scott-Knott test at 5% probability. The efficacy of these treatments was calculated using Abbott's formula (Abbott 1925).

Results

Table 3 shows that at 15 DAT the number of nymphs were significantly reduced compared to the control in all treatments except A5 (3 × 10¹² M. a. conidia ha^-1 of + 350 g ha^-1 of imidacloprid).

Fig. 1.

Cumulative precipitation (mm), average temperature (°C), relative humidity (%) and number of Mahanarvafimbriolata (Hemiptera: Cercopidae) nymphs in the experimental area at 15 to 105 days after application (DAT) of insect control treatments.

At 30 DAT, the numbers of nymphs were significantly reduced compared to the control in all treatments except A3 (M. anisopliae plus the high rate of thiamethoxam) (Table 3). The efficacies of the treatments ranged from 14.28% to 92.85% (Table 4).

At 45 DAT the numbers of M. fimbriolata nymphs per treatment were significantly reduced compared to the control in the following treatments: imidacloprid alone at 700g ha^-1, Al, A2, A3, A4 and A6 (Table 3). Again the performance of A5 was anomalous.

At 60 DAT, the numbers of nymphs were significantly reduced compared to the control in all treatments. The best efficacies occurred in A1 (3 × 10¹² M. a. conidia ha^-1 + 65g ha^-1 of thiamethoxam) and A2 (3 × 10¹² M. a. conidia ha^-1 + 125 g ha^-1 of thiamethoxam), which 77.4% and 87.1%, respectively (Table 4).

Table 2.

Maintenance costs per ha of sugarcane (MCS) at Nova And radina, Mato Grosso Do Sul State in 2012 and 2013.

At 75 DAT, the numbers of nymphs were significantly reduced to the greatest extent in the following 3 treatments: A1 (3 × 10¹² M. a. conidia ha^-1 + 65g ha^-1 of thiamethoxam), A3 (3 × 10¹² M.a. conidia ha^-1 + 187.5 g ha^-1 of thiamethoxam), and A5 (3 × 10¹² M. a. conidia ha^-1 + 350 g ha^-1 of imidacloprid). Also the numbers of nymphs were significantly reduced compared to the control in all of the remaining treatments except A2 and A6, yet the numbers of nymphs in the latter 2 treatments were numerically less than in the control. The 2 best control efficacies relative to the control at 75 DAT occurred in treatments A1 (3 × 10¹² M. a. conidia ha^-1 of + 65g ha^-1 of thiamethoxam) (82.1%) and A5 (78.6%) (3 × 10¹² M. a. conidia ha^-1 + 350 g ha^-1 of imidacloprid).

At 90 DAT the number of nymphs in the control had increased 2.8 fold over the number at 75 DAT. However, the numbers of nymphs were significantly reduced compared to the control in the following treatments: thiamethoxam (250 g ha^-1), Al, A2, A3, A4, and A5. The 2 best control efficacies a 90 DAT occurred in treatments A1 (3 × 10¹² M. a. conidia ha^-1 + 65g ha^-1 of thiamethoxam) (85.9%), thiamethoxam (250 g ha^-1) (83.3%) and A2 (3 × 10¹² conidia ha^-1 of M. a. + 125 g ha^-1 of thiamethoxam) (82.1%).

At 105 DAT the number of nymphs in the control had increased 2 fold over the number at 90 DAT. Also at 105 DAT, the number of nymphs were significantly reduced compared to the control in all treatments. However the numbers of nymphs in all treatments were equal or greater than the number in the control , which indicated that none of the treatments could still provide an economically useful level of control.

Table 3.

Infestation Levels of Mahanarva fimbriolata (Hemiptera: Cercopidae) nymphs per linear foot [30.4 cm] in sugarcane plots treated either with thiamethoxam, imidacloprid, or metarhizium anisopliae, or with the combination of either insecticide with the fungus.

The high rainfall, average temperature, and relative humidity increased the efficiency of the combinations and the fungus M. anisopliae alone (Fig. 1 and Table 3). However, the greatest rainfall, mainly at 75 DAT, increased the population of nymphs of M. fimbriolata on the plots treated with insecticides (Fig. 1 and Table 3).

The values of TRS were higher with the combination A1 (3 × 10¹² M. a. conidia ha^-1 + 65 g ha^-1 of thiamethoxam), M. anisopliae (3 × 10¹² conidia ha^-1), thiamethoxam (250 g ha^-1), and imidacloprid (700 g ha^-1) and, consequently, the amount paid per tonne, and earnings per ha (Table 5). The costs of acquisition and application of the product in the following treatments: M. anisopliae, combination Al and thiamethoxan (250 g ha^-1) were US$ 29.27, US$ 41.02, and US$ 64.31, respectively. In other treatments, the cost of M. fimbriolata nymph control ranged from US$ 38.46 to US$ 56.84. The treatments with the highest net earnings per hectare were in treatments A1 (3 × 10¹² M. a. conidia ha^-1 + 65 g thiamethoxam) and M. anisopliae (3 × 10¹² M. a. conidia ha^-1) alone at the recommended dose. Lower earning per ha were obtained the combinations A4 (3 × 10¹² M.a. conidia ha^-1 + 175 g ha^-1 de imidacloprid ), A2 (3 × 10¹² M. a. conidia ha^-1 + 125 g ha^-1 of thiamethoxam), A5 (3 × 10¹² M. a. conidia ha^-1 + 350 g ha^-1 of imidacloprid), and A3 (3 × 10¹² M. a. conidia ha^-1 + 187.5 g ha^-1 of thiamethoxam), respectively (Table 5).

Discussion

Almost all treatments had lower numbers of spittlebugs than the control at 15 and 30 DAT, which confirms the susceptibility of M. fimbriolata nymphs to the 2 insecticides, M. anisopliae, and combinations of the insecticides plus the fungus. However some anomalies occurred. For example treatment A5 appeared to be infective at 15 DAT and 45 DAT, but this treatment was quite effective at the other times. Likewise treatment A3 appeared to be ineffective at 30 DAT, but this treatment was effective at all other times. These anomalies in the data probably are the result of the highly aggregated distribution of the spittlebugs.

Furthermore, an additive interaction between each of the chemical insecticides and the fungus was observed in this experiment like that observed for Tibraca limbatiuentris Stål (Hemiptera: Pentatomidae) (Quintela et al. 2013). However, this should not be generalized because the control efficacy depends on the weather, the mode of action of the chemical, and the M. anisopliae isolate used to control M. fimbriolata (Dinardo-Miranda et al. 2004a; Loureiro et al. 2005; James et al. 2011). On the other hand, the genetic constitution of the pest population (Quinelato et al. 2012), adaptations, and mechanisms of insecticide resistance (Dubovskiy et al. 2013) may affect the efficacy of control methods.

Table 4.

Percent control (Abbott - %) of Mahanarva fimbriolata (Hemiptera: Cercopidae) by of thiamethoxam, imidacloprid, metarhizium anisopliae and the combination of each insecticide with the fungus at 15 day intervals post application.

Higher numbers of M. fimbriolata nymphs with thiamethoxam (60 DAT) and imidacloprid (75, 90, and 105 DAT) show that the time of application time affect the efficiency of the chemicals (Dinardo-Miranda et al. 2004a; George et al. 2007), because high rainfall can decrease their residual effects (Carvalho et al. 2011). In addition, the insecticides imidacloprid and thiamethoxam, at recommended doses, are not selective for some non-target insects (Zhao et al. 2012; Funderburk et al. 2013), which may explain the higher incidence of M. fimbriolata in the plots treated with these products. Imidacloprid can cause negative impacts on insects of the family Syrphidae (Easton & Goulson 2013) and can't be selective against the predator of M. fimbriolata nymphs, Salpingogaster nigra Schiner (Diptera: Syrphidae). On the other hand, low doses of thiamethoxam are selective against Apis melifera L. (Hymenoptera: Apidae) (El Hassani et al. 2007). This favors the maintenance of certain beneficial insects in the crop and can explain the efficacy of some combinations of M. anisopliae with this insecticide. In addition, low doses of thiamethoxam increased the susceptibility of T. limbativentris to M. anisopliae (Quintela et al. 2013).

The highest efficacy of controlling M. fimbriolata with the combinations A1, A2, and A3 at 30, 60, 75, and 90 DAT can suggest that the thiamethoxam is more suitable than imidacloprid for use with M. anisopliae. Contact of M. anisopliae with thiamethoxam did not affect the biological characteristics of the fungus, demonstrating compatibility between these two control agents (Botelho & Monteiro 2011; Akbar et al. 2012; Silva et al. 2013). Furthermore, this insecticide and the fungus M. anisopliae are most often used to control M. fimbriolata in sugarcane fields (Dinardo-Miranda et al. 2004 a, 2004 b).

The TRS was higher in group A1 (3 × 10¹² M. a. conidia ha^-1 + 65 g ha^-1 of thiamethoxam) with 96.19. The price per tonne of sugarcane is based on the TRS value, i.e., the higher the value obtained, the higher the price per tonne. The combination of 3 × 10¹² M. a. conidia ha^-1 + 65 g ha^-1 of thiamethoxam was found to be the most effective in increasing the price per tonne per ha. Similar values of TRS in the other treatments may be due to the combination M. anisopliae with the insecticides. The 2 chemical products kill insects faster than M. anisopliae (Dinardo-Miranda et al. 2002; Carvalho et al. 2011), and dead individuals are colonized by this fungus (Dinardo-Miranda et al. 2008; Jin et al. 2011). Thus, entomopathogenic fungi can increase their density in the crop by infecting healthy individuals with inoculum from carcasses (Bruck 2005). In addition, M. anisopliae has a high ability to persist in the field (Bruck & Donahue 2007; James et al. 2012), which decreases the probability of resurgence of pest (GuerreroGuerra et al. 2013).

The price of the combinations ranged only from US$ 38.46 to US$ 64.31, indicating that it is affordable to control M. fimbriolata with a combination of M. anisopliae and a chemical insecticide. However, use of the combination of 3 × 10¹² M. a. conidia ha^-1 + 65 g ha^-1 of thiamethoxam produced lower costs, more tonnes of sugarcane ha^-1, the greatest estimated gross income per ha of US$ 1242.77 and the greatest net earnings per ha of US$ 556.16 (Table 5). On the other hand, the plant age can also influence the productivity of the sugarcane (Dinardo-Miranda et al. 2008), so that plants in a more advanced developmental stage may have a higher yield of TRS. The value of the TRS was obtained with 8-month-old plants, but TRS of older plants may be higher.

Table 5.

Total recoverable sugar (trs), quotation of trs (cot. trs), price per tonne, gross income estimate per ha, maintenance cost of sugarcane fields per ha (mcs), cost control of Mahanarva fimbriolata (Hemiptera: Cercopidae) and net earnings per ha in the treatments with insecticides, Metarhizium anisopliae and certain combinations.

Thus, the combination of the entomopathogenic fungus, M. anisopliae, with thiamethoxan or imidacloprid can reduce infestations of M. fimbriolata to sufficiently low levels to protect the sugarcane. Nevertheless, factor influencing the effectiveness of combinations of M. anisopliae with either thiamethoxan or imidacloprid need to be better understood, and further relevant studies should be conducted.