Introduction

The physic nut (Jatropha curcas) is native to equatorial America, where it is traditionally used for hedges, green manure, land reclaim, prevention of erosion and in traditional medicine, and more recently, for biofuel (Gubitz et al. 1999; Openshaw 2000; Fairless 2007; Nature editorial 2007; Kumar and Sharma 2008). In north Brazil, J. curcas is becoming an increasingly popular plant due to its value in the biodiesel industry. Nowadays, 40,000 ha in Brazil are planted with J. curcas, and the area is still expanding. Although the plant can be cultivated on degraded land, and thus does not compete for space with crops, farmers that produce J. curcas predominantly use management practices that include high inputs of pesticides and chemical fertilizers. Recently, however, smallholders in Brazil have started to cultivate J. curcas as a cash crop, selling the seeds for biodiesel production.

The main phytosanitary problems in cultivating J. curcas are the attack of pest mites such as the broad mite Polyphagotarsonemus latus (Banks) (Acari: Tarsonemidae) and the red spider mite Tetranychus bastosi Tuttle, Baker & Sales (Acari: Tetranychidae) (Lopes 2009; Santos et al. 2006; Kavitha et al. 2007). The broad mite is a highly polyphagous pest attacking several crops worldwide (Gerson 1992; Peña and Bullock 1994; De Moraes and Flechtmann 2008). This minute mite (0.1–0.3 mm in length) is found in the apical portion of plants, especially in shoot structures (De Moraes and Flechtmann 2008). This pest causes terminal leaves and flower buds to become malformed, and its attack may occur concomitantly with infestations of T. bastosi. The spider mite T. bastosi (0.5 mm in length) was first recorded attacking native Jatropha gossypypifolia plants in the northeastern Brazil (Tuttle et al. 1977). The mite was also found infesting weeds, such as Amaranthus viridis L., Bidens pilosa, Ipomoea sp., and Hyptis suaveolens, as well as on crops such as Manihot esculenta and Morus nigra (Tuttle et al. 1977; de Moraes and Delalibera 1992; Bolland et al. 1998). Contrary to P. latus, spider mites such as T. bastosi have a silk spinning behavior, covering their colonies with webbing, which may difficult predatory mites to penetrate on their colonies (Saito 1985; Gerson 1985; Sabelis and Bakker 1992). They are more frequently found on underside of fully-developed leaves of J. curcas in comparison with younger leaves.

In Brazil, chemical control is the main method of combating pest mites (Oliveira et al. 2007). Due to excessive use of pesticides and the associated problems of pesticide resistance and environmental pollution, there is an increasing demand for sustainable environmental-friendly control methods. Biological control of pest mites is viewed as an alternative method to chemical control. Predatory mites of the family Phytoseiidae are key natural enemies of pest mites (McMurtry and Croft 1997; De Moraes et al. 2004) and the biological control provided by such indigenous species is considered an environmental service to smallholders which normally lack the resources to control pest mites in their fields. The predatory mites Iphiseiodes zuluagai Denmark & Muma and Euseius concordis Chant (Acari: Phytoseiidae) are the most common natural enemies associated with P. latus and T. bastosi on J. curcas plants in the region where the study was conducted (Rodrigues et al. in prep). Predatory mites forage for prey guided by volatiles that are produced by plants in response to herbivore injury (Dicke and Sabelis 1988; Dicke 1994; Turlings et al. 1990; Janssen et al. 1999; Sabelis et al. 2001), and therefore, the ability of these predators to find prey infested plants is key in order to biological control to succeed (Oliveira et al. 2009).

Here, we evaluated the potential of biological control of the predatory mites I. zuluagai and E. concordis on the phytophagous mites P. latus and T. bastosi. We investigated the suitability of P. latus and T. bastosi as prey for I. zuluagai and E. concordis using predation bioassays and the oviposition rate experiments as a fitness measure. Another important trait to be considered when selecting a predator for biological control purposes is its ability to find prey-infested plants (Oliveira et al. 2009), therefore, we assessed the attraction of both predatory mites to leaves of plants infested with P. latus or T. bastosi through release-recapture experiments.

Materials and methods

Plant material and mite rearing

Seedlings (15 cm) of J. curcas were transplanted into individual plastic pots (500 ml) containing soil and manure in a 3:1 portion. Stock cultures of both I. zuluagai and E. concordis were started with mites collected on J. curcas plants. Stock colonies were established on arenas prepared with 6-cm diameter flexible plastic discs floating on distilled water. Castor bean pollen (Ricinus communis L.) was added on a daily-basis as food for the mites (Reis and Alves 1997). Stock colonies of P. latus and T. bastosi were started with mites collected from infested native J. curcas plants, which were subsequently transferred to uninfested plants. Both, predatory and spider mites were maintained inside a climate chamber (28 °C, 65–70% R.H. and 12 h L/12 h D photoperiod). Each experiment was carried out in separated chambers at the laboratory of Entomology of the Federal University of Tocantins, Gurupi, Tocantins, Brazil.

Predation rate relative to prey density

The predatory activity of I. zuluagai and E. concordis on P. latus and T. bastosi was evaluated on leaf discs (Ø = 3 cm) made from healthy and unsprayed J. curcas plants. Discs were kept in batches of 10 in plastic trays (30 × 20 cm) containing foam covered with wet cotton wool and soaked with water to prevent mites from escaping. The trays were kept inside a climate chamber at the same conditions described above. Leaf discs were infested with 2-day-old mated females of each pest mite species (P. latus or T. bastosi), according to each treatment on the following prey densities: 5, 10, 15 and 20 individuals per disc. Predators were starved in the presence of water for 2 h before used in the experiments. Subsequently, one young mated female of each predatory mite species (I. zuluagai or E. concordis) was released on each disc according to each treatment. After 8 h, the predation rate of each predatory mite was calculated by counting the number of mites preyed per disc. Each treatment consisted of 20 replicates.

Predatory mite oviposition

To evaluate the oviposition rate of I. zuluagai and E. concordis on P. latus or T. bastosi one 2-day-old female plus one male of each predatory species were confined on leaf discs (Ø = 5 cm) made from healthy and unsprayed J. curcas plants, infested with P. latus or T. bastosi, according to each treatment. The discs with all developmental phases (eggs, larvae, nymphs and adults) ad libitum were kept in plastic trays (30 × 20 × 8 cm) containing foam covered with wet cotton wool and soaked with water to maintain the swelling of the disc and to prevent mites from escaping. The trays were kept inside a climate chamber. Eggs of each predatory mite were quantified and discarded daily for 5 days (Rosa et al. 2005).

Release-recapture experiments

Release-recapture experiments were conducted to test the attractiveness of the predatory mites I. zuluagai and E. concordis to leaves infested with P. latus and/or T. bastosi (Pallini et al. 1997). Infested leaves were obtained by placing clean (uninfested) plants in contact with mite-infested plants for 1 week. Plants were subsequently removed and the number of mites was estimated. The number of mobile stages on each leaf ranged from 50 to 100. Six young leaves of J. curcas plants of similar age and size (circa length = 3 cm; width = 2 cm) were removed from the plants (being three from infested plants and three from control plants) and placed in a circle (Ø = 20 cm) spiked on needles that were fixed in a plastic tray (30 × 20 × 8 cm) with silicon glue. In each experiment, uninfested and infested leaves were alternated, so that each uninfested leaf had two infested neighbors, and vice versa. To prevent the predatory mites from escaping a sticky glue (Cola Entomológica, Bio Controle, São Paulo, Brazil) was applied around the circle. Then, 25 females of each predatory mite species (I. zuluagai or E. concordis) were placed in a Petri dish (Ø = 2 cm), which was placed inside a larger Petri dish filled with wet cotton wool. Predatory mites were starved for 2 h before experiments. Subsequently, the Petri dish containing the predatory mites was positioned in the centre of the circle of leaves, and the predators were allowed to access each treatment. Starting 1 h after release, the leaves were checked for predatory mites every hour for a period of 7 h, and all predatory mites found were removed. All trays were kept in a climate chamber (27 ± 2°C, R.H. 75 ± 10%) during the 7 h of the evaluation along the day. Each treatment was replicated four times.

Statistical analysis

Predation rate of I. zuluagai and E. concordis in relation to prey density was analyzed using Generalized Linear Models (GLM) with Poisson error distribution. The difference in the number of mites preyed between predator species, prey species and prey density was compared with a full model. The contrasts between the levels within the model were assessed with Wald test (Kuhn et al. 2008). Oviposition of predatory mites was analyzed using a Generalized Linear Mixed Model (LMER) with Poisson error distribution, with time as random factor to correct for pseudoreplication due to repeated measures (Crawley 2007). Oviposition rates of predatory mites were compared with models consisting of the number of eggs laid in function of predator species and prey species as fixed effects. To analyze differences in numbers of predatory mites recaptured on each of the odour sources (pest mite infested leaves vs. uninfested leaves) we used a χ2 test (expected probability of 0.5 for each treatment). All analyses were performed using the statistical software R (R Development Core Team 2006).

Results

Predation rate relative to prey density

The predation rate of I. zuluagai was higher on P. latus than on T. bastosi in all evaluated densities (Table 1; Fig. 1). There was no significant difference on the predation rate of E. concordis either upon T. bastosi or P. latus in any of the evaluated densities (Table 1; Fig. 2). The predation rate of E. concordis was almost always lower than that of I. zuluagai to both prey (Tables 1, 2; Figs. 1, 2).

Table 1 Mean predation rate (± SE) of Iphiseiodes zuluagai and Euseius concordis at four different densities of prey (Polyphagotarsonemus latus and Tetranychus bastosi) within a period of 8 h on Jatropha curcas
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Fig. 1
figure 1

Mean predation (± SE) of Iphiseiodes zuluagai preying upon different densities of Polyphagotarsonemus latus (solid lines) and Tetranychus bastosi (dotted lines). Different letters denote significant differences among treatments

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Fig. 2
figure 2

Mean predation (± SE) of Euseius concordis preying upon different densities of Polyphagotarsonemus latus (solid lines) and Tetranychus bastosi (dotted lines). Same letters denote non-significant differences among treatments

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Predatory mite oviposition

The oviposition rate of E. concordis did not differ between prey species (χ2 = 1.0089, df = 1, P = 0.3152). Furthermore, prey species did not affect the oviposition rate of I. zuluagai (χ2 = 0.1766, df = 1, P = 0.6744). The predatory mites E. concordis and I. zuluagai laid in average 0.9 ± 0.24 and 0.9 ± 0.22 eggs per day when fed on T. bastosi while they produced in average 0.68 ± 0.25 and 1.00 ± 0.28 eggs per day when fed on P. latus, respectively. Considering the same prey species there is no difference on oviposition rate between predatory mites (χ2 = 4.1 × 10−10, df = 1, P = 1 and χ2 = 1.5332, df = 1, P = 0.2156, respectively for T. bastosi and P. latus).

Table 2 Analysis of deviance for the best fitted model with 3 sources of variation (2 categorical: predator species and prey species plus one continuum variable: prey density)
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Release-recapture experiments

When we assessed the preference of E. concordis to odours of leaves infested with P. latus or uninfested leaves, 87% of all mites were recaptured from any of the leaves offered as choice to E. concordis. Infested leaves attracted on average 46% and uninfested leaves attracted 54% of the recaptured mites (Fig. 4A). When we tested the preference of E. concordis to odours emanating from leaves infested with T. bastosi or uninfested ones, 68% of the mites were recaptured from the leaves (Fig. 4B). On average, 52% of the mites were found on leaves infested with T. bastosi while uninfested leaves attracted 48% of the recaptured mites. These results represent no statistical preference of E. concordis for uninfested leaves or leaves infested with P. latus or T. bastosi (P > 0.05).

When the same options were given to I. zuluagai, the percentage of recaptured mites was also high. When it was offered a choice towards odours of leaves infested with P. latus versus uninfested leaves, 89% of all I. zuluagai released were recaptured from the leaves. From these recaptured mites, 67% were attracted to infested leaves while uninfested leaves attracted 33% mites (Fig. 4C). These difference was significant (P < 0.05). When offered a choice to I. zuluagai between odours from leaves infested with T. bastosi or uninfested ones, 84% were recaptured from the leaves. Leaves infested with T. bastosi attracted on average 71%, while uninfested leaves attracted only 29% of the recaptured mites (Fig. 4D) and this difference was statistically significantly (P < 0.05).

Discussion

Our results indicate that the predatory mites I. zuluagai and E. concordis may contribute to the biological control of the pest mites P. latus and T. bastosi on J. curcas plantations in north Brazil. The number of P. latus killed by I. zuluagai reached a maximum around 4 mites in a period of 8 h (Fig. 1). When fed on T. bastosi, the number of prey killed by I. zuluagai was lower than that obtained by this predator when preying upon P. latus (Fig. 1). Unlike the spider mite T. bastosi the broad mite P. latus does not produce web making it easier for predation, although several predatory mite species (Acari: Phytoseiidae) have been reported entering dense webbing produced by spider mites (McMurtry and Croft 1997; Lemos et al. 2010). The production of a dense and sticky web on host plants is common in spider mites of the family Tetranychidae (Saito 1979, 1983; Gerson 1985). Although there are large differences in the amount of silk produced and in the structure of the web among tetranychid mite species (Saito 1979, 1983, 1985; Gerson 1985; Morimoto et al. 2006), the general consensus is that such webs have various possible functions, one of which is defense against predators (Gerson 1985; Saito 1985; Lemos et al. 2010). The web can act against predators by repelling them (McMurtry and Johnson 1966; Takafuji and Chant 1976), by reducing their movement on the plant (Putman 1962; McMurtry et al. 1970; Trichilo and Leigh 1986), or by sticking predators, which may result in their death (McMurtry and Scriven 1964; Trichilo and Leigh 1986; Osakabe 1988; Venzon et al. 2009). Additionally, predators experience difficulty in avoiding contact with the sticky silken threads when they enter and move inside webs (Sabelis and Bakker 1992). This would explain the higher predation rate of I. zuluagai on P. latus compared to T. bastosi. Furthermore, the larger size of T. bastosi compared to P. latus, could influencing less predation, thus predators would not need to consume as many individuals of T. bastosi as they do on the minute P. latus.

At the lowest density, the number of prey consumed by I. zuluagai was lower than two, perhaps due to the difficulty faced by the predator in finding the prey (Reis et al. 2003). Although I. zuluagai and E. concordis preyed upon P. latus and T. bastosi the predation rate of E. concordis was lower for both preys (Tab. 1). The number of P. latus and T. bastosi killed by E. concordis peaked around 1 mite in a period of 8 h (Fig. 2). When fed on T. bastosi the daily oviposition of I. zuluagai did not differ from that of E. concordis (Fig. 3B). Such ovipositional rates were higher than those obtained by other predatory mites fed on tetranychid mites (Ferla and Moraes 2003) indicating that both P. latus and T. bastosi are suitable for the development of I. zuluagai and E. concordis.

Fig. 3
figure 3

Average (± SE) ovipositional rate of Iphiseiodes zuluagai (A) and Euseius concordis (B) fed on Polyphagotarsonemus latus and Tetranychus bastosi. Same letters denote non-significant differences among treatments

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Release-recapture experiments showed that females of I. zuluagai were attracted to odours from leaves of J. curcas plants that were infested with P. latus or T. bastosi when offered uninfested leaves as alternative (Fig. 4C, D). These results are in line with previous studies with other predators, indicating that I. zuluagai uses volatile cues to localize their prey habitats (Teodoro et al. 2009). However, more research is needed to determine the nature of the volatiles used by I. zuluagai to perceive the presence of P. latus and T. bastosi on J. curcas leaves. Janssen et al. (1997) showed that the predatory mite Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseiidae) is attracted to odours produced by plants infested with thrips and by plants infested by the two-spotted spider mite Tetranychus urticae. When offered a choice between these two odours, they preferred odours of plants with spider mites. Hence, predators were able to distinguish between plants with either of the two herbivores. It was argued that this behavior could be explained by different odour profiles being produced by the plant-herbivore complex, by the concentration of similar volatiles, or both. Therefore, it is possible that the presence of spider mites on the leaves of J. curcas and their feeding could have elicited the production of volatiles and I. zuluagai may have used such volatiles to locate leaves hosting its prey. When we assessed the preference of E. concordis to leaves of plants infested with P. latus or T. bastosi relative to uninfested leaves, mites did not show preference for one of the odour sources, which suggests that such odours are not attractive to or not recognizable by this predatory mite (Fig. 4A, B).

Fig. 4
figure 4

Phytoseiid predators were offered a choice between uninfested Jatropha curcas leaves and leaves infested with Polyphagotarsonemus latus or Tetranychus bastosi. Preference of Euseius concordis females when offered odours coming from uninfested leaves versus A leaves infested with T. bastosi or B leaves infested with P. latus. Preference of Iphiseiodes zuluagai females when offered odours from uninfested leaves versus C leaves infested with T. bastosi or D leaves infested with P. latus. Within each evaluated time, asterisks denote fractions that are statistically different (χ2 test; P < 0.05)

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The preference of I. zuluagai for leaves of plants infested by either P. latus or T. bastosi combined with higher predation on P. latus in comparison to that values obtained by E. concordis suggests that I. zuluagai can be more efficient than E. concordis in reducing populations of pest mites under field conditions. Our results emphasize the key importance of predatory mites as agents of natural biological control of pest mites on J. curcas crops. However, more field realistic experiments are needed to elucidate tritrophic relationships among I. zuluagai, E. concordis, their prey, and the host plant.