Malcolm G. Keeping

Silicon as a plant defence against insect herbivory: response to Massey, Ennos and Hartley

1. Silicon (Si) has received increased attention as a nutrient capable of providing some measure of defence for plants against fungal pathogens, and insect and mammalian herbivores. 2. On the basis of a study including two generalist insect folivores and a phloem feeder, Massey, Ennos & Hartley (2006; Journal of Animal Ecology, 75, 595-603) have drawn attention to a putative distinction between the effects of plant Si in defending against folivorous and phloem-feeding insects. On the basis of their results they imply that phloem feeders are less likely to be adversely affected by increased plant Si than folivores. 3. However, in making this suggestion, Massey et al. have ignored many previous studies demonstrating a clear effect of plant Si on a range of phloem-feeding and some xylem-feeding insects, and that this effect stems not only from leaf mechanical properties based on opaline silica, but also from induced chemical defences seemingly mediated by soluble Si. 4. Furthermore, Massey et al. cannot claim that their study was the first demonstration of a direct effect of Si on insect herbivore preference and performance; there have been numerous earlier studies demonstrating this from folivores, stem borers, and phloem and xylem feeders. 5. We contend that current evidence indicates that Si is likely to be involved to a similar extent in enhancing resistance to all four insect feeding guilds and that any conclusion to the contrary is, at this stage, premature.

Social organization and division of labour in colonies of the polistine wasp, Belonogaster petiolata

SummaryFoundresses in pre-emergence and post-emergence nests of Belonogaster petiolata were organized into linear dominance hierarchies according to their level of physical aggression towards cofoundresses. The female at the top of the hierarchy became the queen, while foundresses ranked below her became worker-like subordinates. In pre-matrifilial colonies, worker offspring were socially subordinate to both their queen and the subordinate foundresses. Queens of matrifilial colonies retained full social dominance over their workers. Queens were reproductively dominant over subordinates and workers, and laid the majority of, if not all, surviving eggs. Subordinate-laid eggs were invariably discovered and destroyed by the queen through oophagy; workers in pre-matrifilial and matrifilial colonies never laid or ate eggs. Colonies contained a single (and the same) functional queen throughout the pre- and post-emergence periods and were, therefore, long-term monogynous.In both established pre-emergence colonies and post-emergence colonies, virtually all foraging for food and nesting material was performed by the subordinates and workers. More dominant subordinates generally foraged less than low-ranked subordinates. Queens were more active builders than subordinates or workers in pre-matrifilial and matrifilial colonies. Queens enjoyed a surplus in exchanged food loads, while subordinates and workers did not. Among cofoundresses, an advantage in food exchange was also positively associated with dominance rank. Queens solicited larvae for their salivary fluid more often than did subordinates and workers. Among cofoundresses, frequency of adult-larva trophallaxis was positively associated with rank. Queens rested most often in the central zones of the nest containing late instar larvae and pupae, while subordinates and particularly workers spent more time in the peripheral, broodless regions of the nest.

Rubbing behavior and morphology of van der Vecht's gland inBelonogaster petiolata (Hymenoptera: Vespidae)

Females of the social wasp, Belonogaster petiolata,rub the secretion of van der Vechts gland, located on their terminal gastral sternite, onto the nest pedicel. In bioassays, the secretion was repellent to two species of ants, while shortchain acids were effective releasers of rubbing behavior. Rubbing was associated with pedicel enlargement and departure from the nest in preemergence colonies. Its frequency was high where wasps were often exposed to ants and low where ants were rare or absent. Rubbing also decreased significantly from the pre-to the postemergence stage of the colony cycle. In both stages, subordinate foundresses rubbed more often than queens or workers. These observations support the hypothesis that rubbing behavior and the secretion of van der Vechts gland function in chemical defense of the nest against ant predation. The general morphology of the gland in B. petiolataresembles that of the four other independent-founding polistine wasp genera.

Silicon reduces impact of plant nitrogen in promoting stalk borer (Eldana saccharina) but not sugarcane thrips (Fulmekiola serrata) infestations in sugarcane.

The stalk borer Eldana saccharina Walker (Lepidoptera: Pyralidae) is a major limiting factor in South African sugarcane production, while yield is also reduced by sugarcane thrips Fulmekiola serrata Kobus (Thysanoptera: Thripidae). Borer management options include appropriate nitrogen (N) and enhanced silicon (Si) nutrition; the effect of N on sugarcane thrips is unknown. We tested the effects of these nutrients, in combination with resistant (N33) and susceptible (N27) sugarcane cultivars, on E. saccharina and F. serrata infestation. Two pot trials with three levels of N (60, 120, and 180 kg ha-1) and two levels each of calcium silicate and dolomitic lime (5 and 10 t ha-1) were naturally infested with thrips, then artificially water stressed and infested with borer. Higher N levels increased borer survival and stalk damage, while Si reduced these compared with controls. Silicon significantly reduced stalk damage in N27 but not in N33; hence, Si provided relatively greater protection for susceptible cultivars than for resistant ones. High N treatments were associated with greater thrips numbers, while Si treatments did not significantly influence thrips infestation. The reduction in borer survival and stalk damage by Si application at all N rates indicates that under field conditions, the opportunity exists for optimizing sugarcane yields through maintaining adequate N nutrition, while reducing populations of E. saccharina using integrated pest management (IPM) tactics that include improved Si nutrition of the crop and reduced plant water stress. Improved management of N nutrition may also provide an option for thrips IPM. The contrasting effects of Si on stalk borer and thrips indicate that Si-mediated resistance to insect herbivores in sugarcane has mechanical and biochemical components that are well developed in the stalk tissues targeted by E. saccharina but poorly developed in the young leaf spindles where F. serrata occurs.

Social behavior and brood decline in reproductive-phase colonies ofBelonogaster petiolata (Degeer) (Hymenoptera: Vespidae)

Colonies ofBelonogaster petiolata in Gauteng (South Africa) produced reproductive offspring (gynes and males) in late January and early February of each nesting season; their appearance was associated with a decline in worker and brood numbers. Brood decline could commence in the presence of a dominant, reproductively active queen, and loss or removal of the queen was not followed directly by cessation of nest growth and brood care. An older worker usually took over the α-position in queenless colonies. Several factors appear to contribute to brood decline and, ultimately, termination of the colony cycle in this species. These include (1) cessation of the supply of solid food to colonies (and particularly their larvae) during the reproductive phase, (2) a decrease in the worker/larva ratio during the latter phase due to the progressive loss of workers, (3) increasing number of gynes and males, and (4) an adult priority over food reception from foragers.

Liming an acid soil treated with diverse silicon sources: Effects on silicon uptake by sugarcane (Saccharum spp. hybrids)

ABSTRACT Highly weathered soils are typically acid, with low plant-available silicon (Si), and may also be high in soluble aluminum (Al). We tested whether pre-liming an acid soil prior to silicate fertilizer application improved Si uptake by sugarcane (Saccharum spp. hybrids), feasibly through elimination of reactive Al. Three trials with potted sugarcane were grown in an acid soil. Treatments included 11 different Si sources and 1- or 3-month pre-liming periods. Soil and plant tissue Si content and yield were determined. Pre-liming for a 1- or 3-month period did not increase plant Si uptake and reduced available soil Si throughout. Alkaline Si sources produced significantly more plant-available Si and greater plant uptake than non-alkaline sources. Silicon and lime treatments did not significantly increase yield. We conclude that pre-liming an acid soil before Si application is unlikely to significantly improve Si uptake by sugarcane.

Uptake of silicon by sugarcane from applied sources may not reflect plant-available soil silicon and total silicon content of sources

Soils of the tropics and sub-tropics are typically acid and depleted of soluble sources of silicon (Si) due to weathering and leaching associated with high rainfall and temperatures. Together with intensive cropping, this leads to marginal or deficient plant Si levels in Si-accumulating crops such as rice and sugarcane. Although such deficiencies can be corrected with exogenous application of Si sources, there is controversy over the effectiveness of sources in relation to their total Si content, and their capacity to raise soil and plant Si concentrations. This study tested the hypothesis that the total Si content and provision of plant-available Si from six sources directly affects subsequent plant Si uptake as reflected in leaf Si concentration. Two trials with potted cane plants were established with the following Si sources as treatments: calcium silicate slag, fused magnesium (thermo) phosphate, volcanic rock dust, magnesium silicate, and granular potassium silicate. Silicon sources were applied at rates intended to achieve equivalent elemental soil Si concentrations; controls were untreated or lime-treated. Analyses were conducted to determine soil and leaf elemental concentrations. Among the sources, calcium silicate produced the highest leaf Si concentrations, yet lower plant-available soil Si concentrations than the thermophosphate. The latter, with slightly higher total Si than the slag, produced substantially greater increases in soil Si than all other products, yet did not significantly raise leaf Si above the controls. All other sources did not significantly increase soil or leaf Si concentrations, despite their high Si content. Hence, the total Si content of sources does not necessarily concur with a products provision of soluble soil Si and subsequent plant uptake. Furthermore, even where soil pH was raised, plant uptake from thermophosphate was well below expectation, possibly due to its limited liming capacity. The ability of the calcium silicate to provide Si while simultaneously and significantly increasing soil pH, and thereby reducing reaction of Si with exchangeable Al3+, is proposed as a potential explanation for the greater Si uptake into the shoot from this source.

Provision of nitrogen as ammonium rather than nitrate increases silicon uptake in sugarcane

Silicons role in ameliorating a range of biotic and abiotic plant stresses is beyond doubt, yet means to maximise its uptake via the roots from applied silicon sources and thereby enhance crop yields have not been fully explored. Our study found that reduction of rhizosphere pH through provision of nitrogen fertilizer to sugarcane as ammonium rather than nitrate increased silicon uptake from a low-silicon soil amended with calcium silicate slag. We propose that ammoniacal fertilizers have potential for enhancing the solubilisation of silicate slags by acidifying the rhizosphere and increasing silicic acid solubility and availability for plant uptake.