Honeydew Gatherers

From Chapter 13 of The Ants :

Introduction
A great majority of the members of the three phylogenetically most advanced ant subfamilies, the Myrmicinae, Dolichoderinae, and Formicinae, attend homopterans to some extent. To employ one last term from Wasmann, the ants can be said to have entered into trophobiosis with the homopterans. As trophobionts, the homopterans resemble many of the lycaenid symbionts in a basic way: they obtain their own food and pass some of it on to their hosts. However, unlike the lycaenids that secrete substances from specialized exocrine glands, the honeydew provided for ants by homopterans is an excretion derived from a digestive process (see Plate 17).

When aphids feed on the phloem sap of plants, they pass a complex mixture of nutrients, including sugars, free amino acids, amides, proteins, minerals and vitamins, through their gut and back out through the anus. During this passage the phloem sap changes as some of its components are absorbed and others are converted or added by the aphid (for recent reviews see Ziegler and Penth 1977, Kunkel and Kloft 1977, Dixon 1985, Maurizio 1985). According to Maurizio, O.2-1.8 percent of the honeydew dry weight is nitrogen and 70-95 percent of the nitrogen are amino acids and amides. The mixture of nitrogen compounds in the honeydew is largely identical to that in the phloem sap. Measurements made on Tuberolachnus salignus by Mittler (1958) show that as much as one-half of the free amino acids are absorbed by the aphid's gut. In a few cases the honeydew contains amino acids which are not present in the phloem sap. Presumably these are metabolic products added by the aphids (Gray 1952, Ehrhardt 1962).

By far the largest percentage (90-95 percent) of the honeydew dry weight consists of carbohydrates. Sugars from the phloem sap are partly absorbed or converted, and the diverse mixtures of sugars contained in the honeydew are often species-specific. They usually comprise fructose, glucose, saccharose, trehalose and higher oligosaccharides. Trehalose, which is the blood sugar of insects, composes up to 35 percent of the total sugar amount in the honeydew. Typical honeydew sugars also include the trisaccharides fructo-maltose and melezitose, with the latter making up 40-50 percent of the total sugar. Other sugars detected in honeydew are maltose, raffinose, melibiose, turanose, galactose, mannose, rhamnose, and stachyose. In addition the honeydew contains other classes of substances, including organic acids, B-vitamins, and minerals.

When unattended by ants, many aphids dispose of the honeydew droplets by flicking them away with their hind legs or caudae, or by expelling them through contractions of the rectum or entire abdomen. The honeydew then falls upon the vegetation and ground below. Similar substances are excreted by several other groups of sap-feeding Homoptera, including scale insects (Coccidae), mealybugs (Pseudococcidae), jumping plant lice (Chermidae = Psyllidae), tree hoppers (Jassidae, Membracidae), leafhoppers (Cicadellidae), froghoppers or spittle insects (Cercopidae), and members of the “lantern-fly” family (Fulgoridae). Sometimes honeydew accumulates in large enough quantities to be usable by man. The manna “given” to the Israelites in the Old Testament account was almost certainly the excretion of the coccid Trabutina mannipara, which feeds on tamarisk. The Arabs still gather the material, which they call “man.” In Australia, chermid honeydew is collected as food by the aborigines. Referred to as “sugar-lerp,” up to three pounds can be harvested by one person in a single day. It is no surprise, therefore, to find that ants also gather honeydew of all kinds. Many, perhaps most, species collect it from the ground and vegetation where it falls. But many others have developed the capacity to solicit the honeydew directly from the homopterans themselves.

Most aphid species associated with ants insert their stylets into the phloem of the host plant (Kloft 1953, 1959a, 1960a,b,c; Kunkel 1967). Although they can suck up limited amounts of liquid, the aphids appear to depend chiefly on the turgor pressure of the phloem, which forces sap up their stylets (Mittler 1957, Kunkel and Kloft 1985). To process a large volume of phloem sap and discard the excess as honeydew evidently costs the aphid fewer calories than a more nearly total extraction from smaller quantities of sap. The amounts of honeydew produced by individuals are often prodigious. First instar nymphs of Mittler's Tuberolachnus extracted honeydew at the rate of seven droplets per hour, each droplet containing 0.065 ul, and the total output per aphid was 133 percent of the aphid's weight every hour. Other species that have been analyzed are slightly more modest, their hourly output ranging from 1.9 to 13.3 percent of body weight per hour (Auclair 1963).

Most myrmecophilous homopterans, especially aphids, have special structural and behavioral adaptations for life with ants (Way 1963, Kunkel 1973, Kunkel and Kloft 1985). Aphids frequently associated with ants tend to have poorly developed cornicles, a reduced cauda, and at most a thin coating of wax filaments. However, a few ant-attended species have large cornicles and others are densely covered with wax. In the case of one such species, Prociphilus fraxini, the ants simply remove wax from the bodies of the aphids (Zwölfer 1958, Kunkel 1973). And where most pseudococcids are covered with wax, the ants often clean the symbionts bare. Kunkel (1973) notes that myrmecophilous aphids generally have more setae on the dorsal body surface and tibiae. Anal setae in particular are very numerous in myrmecophilous aphids. They form a basket (“trophobiotic organ”) that holds the honeydew droplet until it is imbibed by the ants (Zwölfer 1958, Kunkel 1973).

Contempory Perspectives
Novgorodova (2015a) - Trophobiont insects play an important role in the life of many ants. Their excreta (the honeydew) are rich in carbohydrates and form one of the main trophic resources for the ant colony (Dlussky, 1967; Delabie, 2001); therefore, protection of the trophobionts from various competitors is a fairly important task for ants. This is especially true of the species whose colonies include tens of thousands and more workers and require great amounts of carbohydrate food (Oliver et al., 2008). The efficiency of protection of trophobionts from their natural enemies varies significantly depending on the ant species (Itioka and Inoue, 1999; Gibernau and Dejean, 2001; Katayama and Suzuki, 2003; Novgorodova and Gavrilyuk, 2012). As a result, different representatives of the multi-species ant assemblage differently affect the survival of their symbionts (Addicott, 1978; Bristow, 1984; Buckley and Gullan, 1991).

Representatives of the genus Formica, living in large colonies (104 - 106 ind.) and dominating in multispecies ant associations, provide the most promising object for studying the interactions between ants and aphidophages. Species of this group are characterized by the highest level of “professional” specialization among the honeydew collectors (Reznikova and Novgorodova, 1998a; Novgorodova, 2008) and the highest level of aggression towards aphidophages (Novgorodova and Gavrilyuk, 2012). Due to these traits, they offer their symbionts the most efficient protection from natural enemies (Gavrilyuk and Novgorodova, 2007; Novgorodova and Gavrilyuk, 2012). In addition, the aggressive behavior of the honeydew collectors of these species towards aphidophages is to a greater extent related to protection of symbionts from potential competitors, rather than to simple predation. For example, as compared to ants with a simpler variant of honeydew collection (performed by non-specialized foragers with only partial division of labor), the honeydew collectors of ants of the subgenus Formica s. str., acting as obligatory dominants in multi-species ant assemblages, do not switch over to collecting proteinaceous food (Novgorodova, 2005a).

Novgorodova (2015b) - Aphid honeydew is one of the main energy sources for various ants in the temperate zone, nevertheless relatively little is known about the organization of the work of honeydew foragers (aphid milkers). This study focuses on the honeydew collecting strategies used by different ants in steppe and forest multi-species communities in Western Siberia. The behaviour of marked foragers of 12 species (Formica - 7, Lasius - 2, Camponotus - 1, Myrmica - 2) was recorded. Depending on the degree of the aphid milker specialization and degree of protection of the aphids five honeydew collecting strategies of various complexity were distinguished: unspecialized foragers in (I) "unprotected" aphid colonies (attended by ants <60% of time) and (II) "protected" colonies (attended >95% of time); (III) low "professional" specialization (ants "on duty" constantly attending aphid colonies); (IV) medium and (V) high "professional" specialization (clear division of tasks: honeydew collecting by "shepherds" and protection of trophobionts by "guards"; and honeydew transportation by "transporters" in V). Task specialization of the honeydew foragers is facultative: different ant taxa demonstrate a certain range of the honeydew collecting strategies of different complexities (Formica - I-V, Lasius - I-II, Camponotus - III, Myrmica -I-II) depending on the needs of their colony. The strategy used by ants did not depend on the species of aphid attended, but is strongly dependent on the species of ant, their colony size, available food resources and seasonality. In summer, the aphid milker specialization becomes more complex as ant colony size increases at both intra-and inter-specific levels and when food is scarce. In autumn Formica s. str. ants, which have the most advanced foraging strategy, adopt a simpler honeydew collecting strategy. Overall, the variety of honeydew foraging strategies seems to reflect the unequal contribution of different ants in forming trophobiotic interactions with aphids.

Novgorodova has been carrying out behavioral and foraging studies of honeydew gathering ants in Russian temperate forests for many years. The following is from is Novgorodova (2015b) and summarizes how colony size and other factors play a role in the sophistication of honeydew foraging behavior, the level or organization, and the level of protection of aphids by the ants. The summary of the findings of this particular study, which is applicable to what follows, is given just above: The foraging strategies of ants gathering protein food vary widely and range from solitary hunting to different levels of co-operation during search and food retrieval (Hölldobler and Wilson, 1990). There is little information in the literature, however, on the behaviour of ants foraging for honeydew. Aphid milkers of the genus Formica are described as passive individuals with a few functions, the collection and transport of honeydew (Dobrzańska, 1959), and a strong route/site fidelity (Dobrzańska 1959, Rosengren 1971).

Observations of marked aphid milkers of 12 species of ants belonging to four genera (Formica, Lasius, Camponotus and Myrmica) indicate that honeydew foragers usually visit definite aphid colonies. The groups of aphid milkers attending separate aphid colonies, however, can be quite heterogeneous: the number of groups with different tasks varies from one (lack of functional differentiation) to four among the ants studied and are not affected by species of aphid tended. The groups of specialized aphid milkers include both “passive” and “active” foragers (Novgorodova 2008). The functions of the first (e.g. “shepherds” and “transporters”) are usually restricted to collecting and transporting honeydew. “Active” foragers (e.g. ants “on duty”, “guards” and “scouts”) are usually multifunctional. In addition to the common functions (collecting and transporting honeydew) the aphid milkers can guard trophobionts, search for new aphid colonies and in the case of scouts even mobilize passive “shepherds” to work at newly found aphid colonies (Reznikova and Novgorodova 1998, Novgorodova 2008).

The ants studied used five different strategies to collect honeydew from aphids [from unspecialized foragers in “unprotected” aphid colonies (I) to a high level of “professional” specialization (V)], which reflect the variability in ant behaviour from solitary to group foraging. In the case of the III–V strategies the groups of aphid milkers that tend aphid colonies work as “teams” (Anderson and Franks 2001) and coordinate their activities. In the case of I and II honeydew collecting strategies unspecialized aphid milkers rarely encounter one another. The only exceptions are ants of the genus Lasius (Lasius fuliginosus and Lasius niger), which use strategy II. They also coordinate their activities to a slight extent, e.g. if a honeydew forager is alone in an aphid colony, before leaving the plant to carry honeydew to the nest it usually waits until another aphid milker arrives (Novgorodova 2005). This feature of the genus Lasius sometimes leads to the wrong conclusion on whether an aphid colony is exploited or not. For instance, Devigne and Detrain (2005) consider an aphid colony is exploited by L. niger if they observed at least one ant foraging on it, which ignores the possibility that some aphid colonies are attended by ants using strategy I.

The strategy used by the honeydew foragers was not significantly associated with the species of aphid attended. It is to be noted that this result concerns, first of all, species of aphids that live in exposed colonies and do not form galls. The degree of functional differentiation of honeydew foragers is, however, closely associated with the lifestyle of trophobionts, namely whether there was direct contact between ant foragers and their symbionts (Novgorodova and Biryukova 2011). During interactions with trophobionts that cannot be directly contacted by ants, e.g. sawfly larvae of Blasticotoma filiceti living in fern fronds, honeydew foragers show a lower degree of functional differentiation, unspecialized foragers (II) and low “professional” specialization, than when attending open aphid colonies (Novgorodova and Biryukova 2011).

The foraging strategy for collecting honeydew shown by ants is dependent on the species of ant. The most complicated strategies (IV, V) with clear divisions between a number of tasks, at least honeydew collection by “shepherds” and protection of trophobionts by “guards”, are recorded for Formica s. str. ants that dominate multi-species communities. Only these ants strongly protect their trophobionts from natural enemies (Novgorodova and Gavrilyuk 2012): the number of aphid colonies with aphidophages among colonies tended by these ants is significantly lower than in colonies tended by other species of ants. The major difference between the high and the medium “professional” specialization of the aphid milkers are the “transporters” that carry the honeydew to the nest. This enables ants from larger colonies (105–106) to station a definite number of foragers (“shepherds” and “guards”) on plants with trophobionts, which seems to increase the efficiency with which they can collect honeydew.

Other species of ants usually have simpler strategies [low “professional” specialization (Camponotus saxatilis), and unspecialized foragers in (I) “unprotected” and (II) “protected” aphid colonies (Formica ants of the subgenus Serviformica, Lasius and Myrmica)] and provide their symbionts less protection from aphidophages (Novgorodova and Gavrilyuk 2012).

The organization of honeydew collection is also greatly affected by ant colony size. The number of task-groups among aphid milkers is strongly positively correlated with increase in the size of ant colonies. Behaviour of social insects, e.g. the choice of foraging strategy and division of labour, is known to be closely associated with the size of their colonies (Anderson and McShea 2001, Mailleux et al. 2003, Jeanson et al. 2007). The highest level of division of labour is typical of the larger colonies of various social insects including wasps (Jeanne and Taylor 2009) and ants (Anderson and McShea 2001, Thomas and Elgar 2003, Holbrook et al. 2011). However, the effect of ant colony size on the behaviour of honeydew foragers is investigated here for the first time.

The greater functional differentiation among the aphid milkers with increase in ant colony size is also revealed at the intra-specific level in Formica cunicularia and Formica candida. Unlike the honeydew foragers from small colonies (102) that only adopt the simplest strategy (I), those from colonies of the next size class (103) adopt more complex strategies (II–IV) in addition to strategy I. As a result, the percentage of the aphid colonies protected by their foragers is much higher for ant colonies of thousands of ants (103). The increase in the colony size of Serviformica ants (Formica cunicularia and F. candida) up to 103 workers is associated with a reorganization of the work of the honeydew foragers within these species. This is confirmed by the fact that the complexity of both the social structure of the ant colony and the territorial behaviour of these ants, increases in colonies of more than one thousand workers (Reznikova, 1983). At this colony size ants begin to build a mound nest and (at least partly) to protect their foraging territory. The foraging territory of F. cunicularia, however, does not enlarge with increase in colony size (Reznikova, 1983). Thus, these ants solve the problem of the increased demand for carbohydrate food not by expanding their territory, but by their honeydew foragers adopting low or medium levels of “professional” specialization in some aphid colonies, which increases the efficiency with which they can collect honeydew.

The increase in the number of ants in a colony of up to one or several thousand appears to trigger quantitative and qualitative changes in ant behaviour. However, it is to be noted that the species-specific peculiarities also play an important role in the choice of the honeydew collecting strategy. This relates, first of all, to ants of the genus Lasius in which task specialization among honeydew collectors is not recorded despite the large size of their colonies (103 – Lasius niger and 105 – Lasius fuliginosus). For honeydew collection they depend on unspecialized foragers visiting “unprotected” and “protected” aphid colonies (I and II). Strategy II is adopted only in aphid colonies located quite close to ant nests, or close to the main trails in the case of Lasius fuliginosus. The effectiveness of aphid milkers improves due to the high number of unspecialized foragers visiting aphid colonies. In the case of L. fuliginosus this is possibly due to the trunk trail system typical of this ant (Beckers et al., 1989). Furthermore, it has been shown experimentally that aphid milkers of both these species of ants demonstrate highly aggressive behaviour towards aphidophages (Novgorodova and Gavrilyuk 2012).

The behaviour of ants is also known to be affected by the availability and abundance of carbohydrate resources (Sakata, 1995). The availability of an alternative source of carbohydrate can result in ants eating their symbionts (Offenberg 2001). A chance observation in the field indicates that an insufficient supply of food can lead to the reorganization of the work of the aphid milkers, which results in the constant protection of aphid colonies by F. fusca, F. cunicularia and L. niger. This was recorded when the majority of the aphid colonies tended by these ants were destroyed when the grass was mowed: the number of unspecialized foragers and their aggressiveness significantly increased on those plants where a few aphid colonies survived. As a result, these aphid colonies were attended almost all of the time by foragers with strategies II or III (F. fusca – 2 aphid colonies, F. cunicularia – 3, L. niger – 5) rather than type I. In the laboratory a decrease in the number of the aphid colonies (from 10 to 1) available for F. cunicularia results in this ant adopting more complex strategies of honeydew collection (types III and IV) rather than type I recorded at the beginning of the experiment (Novgorodova 2007, 2008). It is to be noted that all of the above observations and experiments were carried out in summer when the foraging activity of the ants is high.

However, the foraging behaviour of ants is known to depend on the season (Cook et al. 2011). Seasonality appears to be another important factor affecting the honeydew collection strategy used by ants. A simplification of the work organization of honeydew foragers is recorded in autumn even in the highly social red wood ants Formica (F. rufa-group) living in large colonies (105–106workers). In September–October groups of these ants tending aphid colonies characteristically have a smaller number of task groups, with only non-aggressive passive foragers (previously worked as “shepherds” and “transporters”) recorded in aphid colonies, which both collect and transport the honeydew, and protect the aphids.

The simplification of the work organization of aphid milkers in autumn could be a result of changes in the quantity or quality of the honeydew and decrease in the activity of the ants. The production of honeydew by a particular species of aphid is known to be influenced by various factors including their age and physiological condition (Auclair, 1963, Fischer et al. 2005), the quality of the host plant and the aphid’s interaction with ants (Del-Claro and Oliveira, 1993, Fischer et al. 2001, 2005). Despite the large number of papers on this problem (Lundgren 2009), it is still unclear whether the honeydew production by a particular species of aphid feeding on a particular host plant depends on the season. Unfortunately, the sugar composition and the volume of honeydew produced by the species of aphids studied were not recorded in this study. Nevertheless, if the productivity of the aphids was the main reason for changes in ant behaviour, we could expect the marked ants to switch to another food resource located not far from the aphid colonies studied. However, the marked ants which left the aphid colonies were not recorded elsewhere in the feeding territory. They did not switch to another food source or perform other functions.

As for the activity of ants, the traffic of foraging wood ants (Formica rufa group) on trees infested with trophobionts is known to be significantly lower in autumn (September) than at similar temperatures in summer (Domisch et al. 2009). Since the activity of aphids and other Hemiptera also tend to decrease with the autumnal decline in temperature (Richardson et al. 2002) it is assumed that the simplification in the honeydew collecting strategy used by foragers is explained by a decrease in both the activity of foraging ants throughout their territory and the productivity of aphids. However, this hypothesis needs to be tested.

Overall, the functional differentiation of the honeydew foragers is species specific and facultative for the species of ants studied. Ants demonstrate a certain range of honeydew collecting strategies of different complexity and use them according to the circumstances and ant colony needs. The honeydew collecting strategy used by ants is strongly dependent on species and colony size of the ants, available food resources and seasonality (at least in the highly social ant species of Formica s. str.). The aphid milker specialization becomes more complex as ant colony size increases at both intra- and inter-specific levels and when food is scarce. The variety of honeydew foraging strategies seems to reflect the unequal contribution of different ants in the forming of trophobiotic interactions with aphids.