Production and maintenance of ornamental plants is an important industry in the US. The value of combined floriculture and nursery production in the US is estimated at more than $14 billion (Daughtrey & Benson 2005). Two important groups of ornamentals in the Southeastern US are sycamore (Platanus spp.) and amaryllis (Amaryllis spp.). Both of these groups are attacked by insect pests that limit profitability and aesthetic value (Filer et al. 1977; Thomas 2005). Two important pests of sycamore and amaryllis are the sycamore lace bug, Corythucha ciliata (Say) (Hemiptera: Tingidae), and the weevil Stethobaris nemesis Prena & O'Brien (Coleoptera: Curculionidae), respectively.

Corythucha ciliata attacks a variety of trees in the genus Platanus and is particularly damaging to the American sycamore, Platanus occidentalis L. (Proteales: Platanaceae), (Halbert & Meeker 2001; Ju et al. 2011). This pest is native to North America and has been introduced into Europe and Asia (Horn et al. 1983; Halbert & Meeker 2001; Ju et al. 2011). The lace bugs feed on the underside of the leaves damaging the foliage and causing premature senescence (Horn et al. 1983; Halbert & Meeker 2001). A single female can lay more than 250 eggs and in the southern US the insects can have several generations per yr. Corythucha ciliata overwinter as adults under loose bark, or in nearby cracks and crevices (Horn et al. 1983; Halbert & Meeker 2001). At high infestation levels C. ciliata can kill trees, especially when associated with pathogenic fungi such as Ceratocystis fimbriata Ellis and Halst. (Ophiostomatales: Ophiostomataceae) and Apiognomonia veneta (Sacc. and Speg.) (Diaporthales: Gnomoniaceae) (Halbert & Meeker 2001).

Options for control of C. ciliata rely on chemical insecticides, yet in many populated urban areas where the pest occurs, application of chemicals is problematic due to safety issues (Halbert & Meeker 2001). Currently, no biological control option is available for suppression of C. ciliata. Entomopathogenic nematodes (EPNs) in the genera Steinernema and Heterorhabditis (Rhabditida: Steinernematidae and Heterorhabditidae) may have potential as alternative control agents for C. ciliata management (Tarasco & Triggiani 2006). These nematodes are commercially available biocontrol agents that kill insects with the aid of symbiotic bacteria (steinernematids are associated with Xenorhabdus spp. and heterorhabditids with Photorhabdus spp.) (Lewis & Clarke 2012). EPNs are used to control a wide variety of economically important pests (Grewal et al. 2005). Overwintering C. ciliata (found on the tree trunk and surrounding area) may be amenable to control with EPNs in a similar approach taken for suppression of overwintering codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae) (Lacey & Unruh 1998; Lacey & Shapiro-Ilan 2008). There have been only a few reports of EPNs as biocontrol agents in association with insects in the order Hemiptera (Leite et al. 2005, Cuthbertson et al. 2007, 2008; de Paula Batista & Auad 2010) and prior to this study there has been only one report on EPN pathogenicity to C. ciliata (Tarasco & Triggiani 2006). Our research expands on that of Tarasco and Triggiani (2006) because the prior study only considered two EPN species as potential control agents whereas we tested six EPNs in the present study.

The recently described exotic weevil, S. nemesis is a pest of various plants in the family Amaryllidaceae including Amaryllis L. spp., Hippeastrum Herb. spp., spider lily Hymenocallis Salisb. spp., swamp lily Crinum L. spp., and Amazon lily Eucharis × grandiflora Planch. & Linden (Thomas 2005; Epsky et al. 2008; Prena & O'Brien 2011). Adult insects feed primarily on the foliage, though they may also feed on the surface of bulbs (Thomas 2005). After adults lay eggs on the foliage, the larvae hatch and travel downward to the bulb to feed. Larval feeding hollows out the bulb, which can kill the plant. Larvae are believed to pupate in the surrounding soil and emerge as adults (Price 2009). The life cycle from oviposition to adult eclosion lasts approximately 47.4 ± 3.7 d and adults may live up to two years (Epsky et al. 2008; Price 2009). In the southern US several generations of S. nemesis occur and the weevils may occur throughout the yr (Epsky et al. 2008; Thomas 2005).

Control tactics for S. nemesis are limited to chemical insecticides or removal of infested plants (Price 2009). Given that EPNs have been reported to control numerous weevil pests in soil (Shapiro-Ilan et al. 2002; Grewal et al. 2005), biocontrol using EPNs may be an option for S. nemesis as well. Other than this study, there have been no reports on the susceptibility of S. nemesis to EPNs. Thus, the objective of this study was to estimate the potential for EPNs to control C. ciliata and S. nemesis by determining pathogenicity, virulence and reproductive capacity under laboratory conditions.

MATERIALS AND METHODS

RESULTS

Differences in virulence and reproductive capacity were detected among nematode treatments when using C. ciliata as the target host. Percentage insect mortality was lower in the control than in all treatments except H. bacteriophora (Oswego) (F = 10.63; df = 6, 56; P = 0.0001) (Fig. 1). Heterorhabditis indica (HOM1) caused higher mortality than all other treatments except S. carpocapsae (Fig. 1). Mortality in the S. carpocapsae treatment was higher than mortality caused by H. bacteriophora (Oswego) but was not different from other nematode treatments (Fig. 1). Reproductive capacity was higher in the H. indica (HOM1) treatment than other treatments (F = 6.87; df = 5, 24; P = 0.0004) (Fig. 2). No other differences in reproductive capacity were detected (and no reproduction was detected in S. riobrave [355]) (Fig. 2).

Fig. 1.

Percentage mortality of Corythucha ciliata following exposure to entomopathogenic nematodes or a water-only control for 2 d. Nematode species names are abbreviated as follows: Hi = Heterorhabditis indica, Sc = Steinernema carpocapsae, Sr = S. riobrave, Hb = H. bacteriophora, and Hg = H. georgiana; strain designations are indicated following the species abbreviation. Different letters above bars indicate statistically significant differences (SNK test, α = 0.05).

Differential virulence to S. nemesis was also detected among EPN treatments (Fig. 3). One d after treatment, all nematode treatments caused higher mortality than the control, and S. carpocapsae and S. feltiae caused higher mortality than the two heterorhabditids (F = 26.46; df = 4, 34; P = 0.0001) (Fig. 3). Two d after treatments, S. carpocapsae (All) caused higher mortality than H. indica (HOM1), and the other two treatments, S. feltiae and H. bacteriophora (Hb), were intermediate. (F = 91.86; df = 4, 34; P = 0.0001). After three d no differences were detected among treatments (F = 63.26; df = 4, 30; P = 0.0001) (Fig. 3).

DISCUSSION

Biocontrol potential against both pests, C. ciliata and S. nemesis, was observed using entomopathogenic nematodes. Based on virulence and reproductive capacity our results indicate H. indica (HOM1) showed the highest potential for C. ciliata suppression. S. carpocapsae also showed high potential. Our results can be compared with reports of EPN virulence to other hemipterans. Similar to our study, Cuthbertson et al. (2008) reported high levels of S. carpocapsae virulence to the sweetpotato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae). Additionally, significant virulence among steinernematids and heterorhabditids has been reported for control of spittlebugs (Hemiptera: Cercopidae) in the genus Mahanarva (Leite et al. 2005; de Paula Batista & Auad 2010).

Fig. 2.

Number of infective juvenile nematodes (IJs) produced per infected Corythucha ciliata cadaver. Nematode species names are abbreviated as follows: Hi = Heterorhabditis indica, Sc = Steinernema carpocapsae, Sr = S. riobrave, Hb = H. bacteriophora, and Hg = H. georgiana; strain designations are indicated following the species abbreviation. Different letters above bars indicate statistically significant differences (SNK test, α = 0.05).

Fig. 3.

Percentage mortality of Stethobaris nemesis following exposure to entomopathogenic nematodes or a water-only control for 1, 2, or 3 d after treatment (DAT). Nematode species names are abbreviated as follows: Hb = Heterorhabditis bacteriophora, Hi = H. indica, Sc = Steinernema carpocapsae, and Sf = S. feltiae; strain designations are indicated following the species abbreviation. Different letters above bars indicate statistically significant differences within each DAT (SNK test, α = 0.05).

Tarasco & Triggiani (2006) conducted a field test comparing S. carpocapsae and H. bacteriophora for control of C. ciliata. In that study, pathogenicity was not detected in either nematode (though they did mention a preliminary non-refereed report in which pathogenicity was detected). Thus, this is the first refereed report to indicate EPN pathogenicity and virulence to C. ciliata (and to the family Tingidae); this is also the first report of EPN reproduction in C. ciliata.

A challenging aspect of using entomopathogenic nematodes for control of C. ciliata is that the overwintering insects occur aboveground. Aboveground applications of EPNs can be limited due to the biocontrol agent's sensitivity to ultraviolet light and desiccation (Shapiro-Ilan et al. 2006). Possibly, environmental conditions associated with aboveground applications were the cause of failed biocontrol applications in the study of Tarasco & Triggiani (2006). The efficacy of EPN aboveground applications, however, may be enhanced by using improved formulations or post-application covers that protect the nematodes from environmental stress (Shapiro-Ilan et al. 2012). For example, the use of firegel (an environmentally friendly polymer based gel) or wood foam (based on hardwood fiber and a Celvol/starch mix) improved control of lesser peachtree borer, Synanthedon pictipes (Grote & Robinson) (Lepidoptera: Sesiidae) and C. pomonella (Shapiro-Ilan et al 2010; Lacey et al. 2010). Conceivably, these improvements in formulation and application may facilitate control of C. ciliata as well.

Among the nematodes tested, S. carpocapsae (All) may have the highest potential to control S. nemesis because it was the only one in the top tier of virulence on all sample dates. The high rate of mortality observed only 1 d post-treatment when applying S. carpocapsae (All) or S. feltiae (SN) may be particularly suitable for control of S. nemesis because amaryllis growers may find the quick reduction in damage attractive. The other nematodes tested, i.e., S. feltiae (SN), H. bacteriophora (Hb) and H. indica (HOM1), also showed high levels of virulence particularly 3 d post-treatment.

Application of EPNs for control of soil dwelling weevil pests has been successful in various systems (Grewal et al. 2005; Lacey & Shapiro-Ilan 2008). Similar to our study, S. carpocapsae has exhibited high levels of virulence to certain curculionid larvae such as the banana root borer, Cosmopolites sordidus Germar (Treverrow et al. 1991). In contrast, in a number of cases S. carpocapsae virulence to curculionid larvae has been reported to be inferior to other EPN species, e.g., when targeting Diaprepes root weevil, Diaprepes abbreviatus (L.) (Shapiro & McCoy 2000), plum curculio, Conotrachelus nenuphar (Herbst) (Shapiro-Ilan et al. 2011), or pecan weevil, Curculio caryae (Horn) (though S. carpocapsae is highly virulent to the adult stage) (Shapiro-Ilan et al. 2003).

Conceivably one might argue S. carpocapsae is not appropriate for S. nemesis control because the nematode is an ambush forager and thus tends to remain near the soil surface (Lewis & Clarke 2012). However, the search strategy is unlikely to be a major hindrance because the nematodes will not need to forage deep in the soil to infect S. nemesis. Furthermore, success in comparable situations has been observed when using S. carpocapsae to control other soil dwelling pests that bore into plant, e.g., when targeting peachtree borer, Synanthedon exitiosa (Say) (Lepidoptera: Sesiidae) (Shapiro-Ilan et al. 2009).

Applications of EPNs for S. nemesis control is promising due to the high degree of virulence observed in this study and bolstered by the history of success in using EPNs to control soil-dwelling weevils. Additionally, certain aspects of amaryllis culture, such as the need for moist soil conditions (Jauron & Nelson 2007) are also conducive to the use of EPNs (Shapiro-Ilan et al. 2006), and thus the compatibility of the biocontrol application is enhanced. Moreover, the relatively high crop value per unit area will likely make use of EPNs economically feasible.

Due to varying biotic or abiotic factors, EPN efficacy in laboratory studies is not equivalent to field efficacy (Shapiro-Ilan et al. 2012). Also, for the same reasons, the EPN species or strains with the highest laboratory virulence may not turn out to be the most efficacious under field conditions (Shapiro-Ilan et al. 2012). Therefore despite the potential shown in these laboratory studies for control of C. ciliata and S. nemesis, the promising EPN treatments observed in this study need to be tested for efficacy under field conditions prior to adoption as a control tactic.