Some bark and ambrosia beetles not only have adult helpers at the natal nest, but can also have larvae that cooperate and may engage in division of labor with the adults (Biedermann and Taborsky, 2011). Although data on larval behavior in these beetles are mostly anecdotic, it could be a common phenomenon in species with gregariously feeding offspring and in which adults and larvae can move freely within their nests. Larval cooperation has been experimentally proven only in the ambrosia beetle X. saxesenii (Biedermann and Taborsky, 2011), but observations suggesting larval cooperation come also from the phloem feeding D. micans, D. valens, and D. punctatus (Grégoire et al., 1981; Deneubourg et al., 1990; Furniss, 1995) and from other ambrosia feeding Xyleborini (X. affinis: Biedermann, 2012) and Platypodinae, Platypus cylindrus (F.) (Strohmeyer, 1906), Trachyostus ghanaensis Schedl, T. aterrimus (Schaufuss), T. schaufussi Schedl (Roberts, 1968), Doliopygus conradti (Strohmeyer), and D. dubius (Sampson) (Browne, 1963).
Remarkably, this division of labor between adult and immature stages is almost unique among social insects. Helper or worker castes in insects without metamorphosis (Hemimetabola), like aphids or termites, are always formed by immature individuals, whereas in insects with metamorphosis (Holometabola), such as beetles and Hymenoptera, workers are typically adults, as immature individuals in ant, wasp, and bee societies are largely immobile, helpless, and often dependent on adults to be moved and fed (Wilson, 1971; Choe and Crespi, 1997). There are very few exceptions of cooperatively behaving immatures in Hymenoptera, including nest-building-silk producing weaver ant larvae (Wilson and Hölldobler, 1980) and nutrient and enzyme producing larvae of some wasp and ant species (Ishay and Ikan, 1968; Hunt et al., 1982).
What does larval cooperation in bark and ambrosia beetles look like? In phloem feeders larvae cooperate primarily by feeding side by side, which helps them to overcome plant defenses, and aggregation is effected by pheromones (Grégoire et al., 1981; Deneubourg et al., 1990; Storer et al., 1997). Gregarious feeding is also known from the ambrosia beetle genus Xyleborinus, in which larvae feed not only on fungal mycelia (as typical), but also on fungus-infested wood. Aggregation pheromones have not been studied in ambrosia beetle larvae, but it is likely that gregarious feeding may more effectively control fungal saprobes threatening their primary ambrosia food fungus (Biedermann, unpubl.; Biedermann and Taborsky, 2011). Like gregarious feeding on plants, gregarious feeding on fungi has been repeatedly found to be an adaptation of arthropods to overcome the induction of secondary fungal defenses (Rohlfs, 2005; Rohlfs and Churchill, 2011).
Larvae take part in gallery hygiene, by relocating frass and by grooming eggs, pupae, each other, and adults; these behaviors have been widely reported from different bark and ambrosia beetle species. In X. saxesenii, larvae ball up frass, which can then be more easily removed by their adult siblings (Biedermann and Taborsky, 2011). In D. micans, larvae pack frass at specific locations, allowing free movement within the brood chamber (Grégoire et al., 1981); they also block tunnels to hinder access by R. grandis predators (Koch, 1909). Fifth instar larvae in some Platypodinae also relocate frass to unused gallery parts or for plugging artificial nest openings (Hadorn, 1933; Beeson, 1941; Kalshoven, 1959) and expel frass and parasitoid planidia through the nest entrance (Darling and Roberts, 1999). These larvae have a plug-like last abdominal segment, which can be used both as a shovel and as a device to fully plug the gallery entrance against intruders (Strohmeyer, 1906). These larvae have been observed to overtake the role of entrance blocker during times when their parents are deep inside the nest (Strohmeyer, 1906; Roberts, 1968). In both Platypodinae and many Xyleborini, larvae also engage in excavation of new tunnels or chambers to create more surface for the developing ambrosia fungus (Strohmeyer, 1906; Kent, 2002; Biedermann and Taborsky, 2011). The flat brood chambers that are typically found in the genus Xyleborinus are almost exclusively accomplished by the larval habit of feeding on fungus-infested wood (Biedermann and Taborsky, 2011; De Fine Licht and Biedermann, 2012). The same is true for the long transverse tunnels in nests of several Platypodinae that are bored by fifth instar larvae (Roberts, 1962, 1968; Browne, 1972).
The ultimate cause for the larval specialization for tunneling shown by many ambrosia beetles may relate to their repeated molting: mandibles of adults gradually wear down during excavation, and adult females that bore extensively would suffer from substantial long-term costs. In contrast, larval mandibles regenerate at each molt (Biedermann and Taborsky, 2011).
Xyleborinus saxesenii larvae that feed on fungus-infested wood likely fertilize the growing ambrosia fungus with the finely fragmented woody sawdust in their feces, which gets smeared on the gallery walls after defecation (Hubbard 1897; Biedermann and Taborsky, 2011). This larval frass probably also contains enzymes for further wood degradation, as a recent study showed that X. saxesenii larvae possess hemicellulases, which are not found in their adult siblings (De Fine Licht and Biedermann, 2012). Furthermore, bark and ambrosia beetle larvae may spread associated bacterial and fungal symbionts within the galleries, which have been shown to have defensive functions against pathogens, detoxify poisonous plant metabolites, degrade lignocellulose plant cell walls, or fix nitrogen from the air (Cardoza et al., 2006b; Adams et al., 2008; Scott et al., 2008; Morales-Jiménez et al., 2013; Chapter 6). This suggests that cooperation, and division of labor among larvae and adults, goes far beyond behavioral interactions, but may also include microbial, biochemical, and enzymatic processes.
Larval contributions to gallery extension and to hygiene reduce the workload for adults. Indeed, and against the common preconception that larvae only compete for resources among each other, positive effects of larval numbers on group productivity have been observed in X. saxesenii (Biedermann and Taborsky, 2011), D. micans (Storer et al., 1997), and several Platypodinae species, in which females only lay second egg clutches in the presence of fifth instar larval helpers (Roberts, 1968).
In summary, larvae in some bark and many ambrosia beetle species are free to move within the natal nest, and are not confined to small areas or brood cells like those of most hymenopteran social societies (Wilson, 1971; Hölldobler and Wilson, 1990). This, in combination with different capabilities of larvae and adults, predisposes especially ambrosia beetles for division of labor between larval and adult stages. Importance and specific roles of larvae in the galleries appear to vary between species (Biedermann, 2012).
One aspect that has not been studied at all in bark and ambrosia beetles is the possibility of delayed development of larvae. If larvae play such an important role in the nests of many gregarious bark and ambrosia beetle species and there are possibilities for larvae to gain indirect fitness benefits by cooperating in the natal nest, selection may favor prolonged development (e.g., by additional larval instars). Prolonged development or even permanently immature helper/worker castes are the rule in hemimetabolous social insects like termites, aphids, or thrips, in which individuals only mature to become reproductive queens or kings (Choe and Crespi, 1997; Korb and Heinze, 2008). There are two hints for prolonged development also in larvae of bark and ambrosia beetles. First, the number of larval instars varies between two and five among species in bark and ambrosia beetles; it is unknown what factors select for more or fewer instars. The numbers of instars are sometimes, but not always, related to size of the adult (Lekander, 1962; Lekander, 1968a, b). Second, among species with helping larvae (Dendroctonus, Xyleborini, Platypodinae) and for reasons that remain unclear, there appears to be high variability in the developmental periods of larvae (Wichmann, 1927). Koch (1909) observed that from D. micans eggs laid the same day, the progeny pupated over a period of 44 days without any obvious reasons. While the first larval instars are typically short and quite fixed in time, the length of the last instar is highly plastic and in some cases two to four times longer than all previous instars together (Koch, 1909; Baker, 1963; Browne, 1963; Biedermann et al., 2009). Generally, the last instar is typically the one that overtakes most helping and has evolved even some morphological adaptations for helping (see above). The maximum of five instars and the longest development of larvae (which can be several years) relative to the oviposition period of adults are both found in Platypodinae (Kirkendall et al., 1997). Unfortunately, researchers have rarely reported larval numbers when dissecting galleries, and experimental studies are lacking, so prolonged development of larvae as an investment in siblings must remain speculative.