Host plant specificity of the moth species Glena mielkei (Lepidoptera, Geometridae) in northern Chile

Abstract

Background

Host plant specificity refers to the preference of insects for particular plant species that allow them to complete their life cycle. Moth species of the Geometridae family depend closely on the vegetation composition to complete their life cycles. In northern Chile, the Geometridae species Glena mielkei is the only species described of the genus Glena. So far, this species has only been associated to a single host plan species of the Asteraceae family, Trixis cacalioides. The aim of this study was to determine the suitability of five commonly occurring plant species of Asteraceae as hosts for G. mielkei.

Findings

We collected G. mielkei larvae from T. cacalioides plants occurring in the Azapa valley and reared them in the laboratory. We tested host plant suitability by exposing recently lab-reared adults of G. mielkei to the following Asteraceae species: T. cacalioides, Pluchea chingollo, Baccharis salicifolia, Grindelia tarapacana and Tessaria absinthioides. Larvae fed with G. tarapacana died of starvation within four to five days. Larvae fed with B. salicifolia fed partially on the plant but died within the first and third day. Larvae fed with both plant species did not complete their development. Larvae fed with T. cacalioides, P. chingollo and T. absinthioides developed into adult stages, producing viable progeny.

Conclusions

We found T. cacalioides, P. chingollo and T. absinthioides to be suitable hosts for G. mielkei. None of the larvae fed on G. tarapacana and B. salicifolia completed their life cycle. We conclude that this narrow range of host plants potentially threatens G. mielkei given the continuous loss of its host plants and feeding sources due to habitat loss and agricultural activities.

Findings

In terms of insect-plant relationships, the concept of host plant specificity has been developed on the fact that in order to be a host plant, the plant has to generate the necessary stimuli to allow an herbivore insect to find it and utilise it as an appropriate substrate (Awmack and Leather [2002]) to complete its development. The suitability of a host plant has also been discussed in terms of the capacity of the plant to shelter natural enemies that could potentially prevent the establishment of the phytophagous insects (Nomikou et al. [2003]) and to produce chemical compounds that may potentially be identified by the phytophagous insect (Rajapakse et al. [2006]).

In a narrower context, host plant specificity has focused on the preference of insects to feed on a particular plant species (Novotny and Basset [2005]). Hence, depending on the number of host plant species insects can feed on, phytophagous insects have been classified as monophagous, oligophagous and polyphagous (Cates [1980], Symons and Beccaloni [1999]). On the other hand, and based on the quality to sustain the development of phytophagous insects, host plants have been classified into two main types: primary host plants and secondary or incidental host plants (Manners et al. [2010]). Primary host plants refer to plants that provide all the necessary conditions for the successful completion of the life cycle of herbivores associated to them and also to those that are an appropriate feeding substrate to the herbivore species in question (Rajapakse et al. [2006], Manners et al. [2010]). In contrast, secondary or incidental host plants have only some features of the primary host plant and are normally used in lower numbers (Milne and Walter [2000], Novotny and Basset 2005, Manners et al. [2011]). Within this context, relationships between phytophagous insects and their host plants have been initially studied from distributional data collected from natural environments (e.g. tropical forests), having moved into more species-focused data obtained from specimens observed in situ or reared in the laboratory (Novotny and Basset [2005]).

The Geometridae family is a highly diverse family of Lepidoptera whose species are mainly herbivorous (Scoble [1995], Axmacher et al. [2009]) and closely related to the vegetation (Scoble [1995], Brehm and Fiedler [2005], Brehm et al. [2005]). Species belonging to this family are also known to be associated to a relatively small range of host plants (Bolte [1990], Bodner et al. [2010]). This is of particular importance when considering the conservation of vegetation and the insect fauna associated to it in highly disturbed areas (Brehm and Fiedler [2005], Sutrisno [2007]). In this regard, the vegetation of the coastal valleys of northern Chile, particularly the Arica and Parinacota Region, represent clear examples of areas where a vast amount of native vegetation has been removed and replaced by agricultural land (Luebert and Pliscoff 2006).

In northern Chile, the plant family most frequently used as host by geometrid larvae is Fabaceae (Vargas and Parra [2004], [2005], Vargas et al. [2005], Vargas [2007]); however, Anacardiaceae, Asteraceae and Nyctaginaceae have been also recorded as host in the Azapa valley (Vargas et al. [2010], Vargas [2011], Vargas [2014]). In addition to their trophic associations with the vegetation, geometrid moths also play an important role as prey items for predatory insects. An example of such association is the potter wasp Hypodynerus andeus (Packard). In the Azapa valley, the larval stages of H. andeus feed almost exclusively on geometrid larvae (Méndez-Abarca et al. [2012]).

A total of 30 species of the genus Glena Hulst have been described in South America (Pitkin [2002]). In the case of Chile, only one species of this genus has been described: Glena mielkei Vargas. G. mielkei is a species whose distribution has been found to be mostly restricted to the Azapa and Chaca valleys in the Arica Province. So far, this species has been known to be associated only to a single host plant species of the Asteraceae family, Trixis cacalioides (Kunth) (Vargas [2010]). The aim of this note is to determine the host plant specificity of G. mielkei in five plant species of the Asteraceae family in laboratory conditions.

G. mielkei larvae were collected from individuals of T. cacalioides plants naturally occurring in the Azapa valley (18°31′36.38″S-70°9′59.68″O). Larvae collection was carried out manually between March and December 2012. Samples were transported to the laboratory at the Faculty of Agricultural Sciences, Universidad de Tarapacá to be reared in the laboratory under ambient temperature, humidity and normal day-night photoperiod. In order to obtain adults of G. mielkei, a total of 50 larvae were kept in 200 mL plastic vials and fed with fresh leaves of T. cacalioides until adults were obtained. We carried out three simultaneous replicates of this experiment. Once emerged, adults were kept in plastic bags with leaves of T. cacalioides to allow mating and egg lying. We selected the following plant species to test their suitability as host plants for individuals of G. mielkei: T. cacalioides, Pluchea chingollo (Kunth), Baccharis salicifolia (Ruiz and Pav.), Grindelia tarapacana (Phil) and Tessaria absinthioides (Hook. and Arn.). These selected plant species are all of common occurrence in the Azapa valley (Katinas [2011]; Luebert [2004]; Ferrú and Elgueta [2011]; Muñoz Ovalle [2010]). Larvae were carefully handled with a fine brush that allowed silk threads to stick to it and avoid any kind of damage to the larvae. Five larvae were located in 20 × 20 cm transparent plastic bags to feed on leaves of each plant species. Larvae were kept in the laboratory at ambient temperature, humidity and normal day-night photoperiod.

Larvae fed on all species of plant offered except for G. tarapacana, whose leaves remained untouched. Larvae fed with this plant died of starvation within 4 to 5 days. Larvae fed with B. salicifolia partially accepted the feeding substrate but died within the 1st and 3rd day. Larvae fed with these two plant species did not complete their development. On the other hand, larvae fed with T. cacalioides, P. chingollo and T. absinthioides developed into adult stages, producing viable progeny (see Table 1). The experiment lasted for a month.

Table 1 Life -cycle development of G. mielkei larvae fed with five Asteraceae plant species

As stated in Vargas ([2010]), larvae of G. mielkei fed with leaves of the native shrub T. cacalioides developed normally producing viable and fertile progeny. So far, there have been no records of larvae of G. mielkei feeding on other Asteraceae species, apart from T. cacalioides, in the coastal valleys of the Arica Province. Our findings expand the range of hosts that can be used for G. mielkei as potential food sources. We found that, besides T. cacalioides, two other Asteraceae species appeared to be suitable feeding sources for G. mielkei: P. chingollo and T. absinthioides. Larvae reared in both plant species completed their development into adults and produced viable progeny. Measured larval size was 3(±0.2) mm for first instar larvae. The recorded pupal size was 20(±0.2) mm (n = 150). The development period recorded for each larval instar lasted 5(±1) days from egg to first instar, 5(±1) days from first to second instar, 3(±1) days from second to third instar, 3(±1) days from third to fourth instar, 4(±1) days from fourth to fifth instar, 5(±1) days from fifth instar to pupa and 15(±1) days from pupae to adults (n = 150). This can be considered a reliable indicator of their suitability as food sources for G. mielkei.

The distribution of G. mielkei is known to be restricted to the valleys of Azapa and Chaca in the Arica Province (Vargas [2010]). Intensive agriculture has caused fragmentation, destruction and replacement of the native vegetation in these coastal valleys (Latorre [2013]; Luebert and Pliscoff, [2006]; Valenzuela et al., [2004]). Geometridae larvae are in general more sedentary and tend to be less capable of dispersing when their habitat is severely disturbed (Thomas [2002]). Hence, a species like G. mielkei, with a narrow range of host plants, could be easily threatened by the continuous loss of its host plants and feeding sources, such as the case of the species Glena cognataria (Guenée), a species that strongly depends on shrubland habitats in southern New England and south-eastern New York, USA (Wagner et al. [2003]). It is well known that in some cases host plant selection behaviour in the laboratory may differ from the host plant selection in the field (Stoeva et al. [2012]) as many other environmental factors, such as altitude, latitude and temperature, can influence host plant selection (Scriber [2002]). A logical step towards improving the understanding of the relationships of G. mielkei with its host plants should be to study host plant use in the field, particularly in scenarios where potential host plants could occur simultaneously.