Session 3: Non-Traditional Biological Control Agents 102 Session 3 Non-Traditional Biological Control Agents XIII International Symposium on Biological Control of Weeds - 2011 Session 3 Non-Traditional Biological Control Agents 103 The Case for Biological Control of Exotic African Grasses in Australia and USA Using Introduced Detritivores D. Sands1 and J. A. Goolsby2 1 CSIRO Division of Ecosystem Sciences, PO box 2583, Brisbane, Queensland 4001 Australia Email: [email protected] 2 U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS), Beneficial Insects Research Unit, 2413 E Hwy 83, Weslaco, Texas 78596 USA Email: [email protected] Abstract Many species of African grasses were introduced into Australia and the USA to improve the quality and biomass of green pastures for domestic livestock. However, a range of non-target environmental impacts eventuated from these introductions, including spatial displacement of indigenous ecosystems, and induced changes to soils and nutrient re-cycling. The accumulation in biomass of flammable and senescing grasses predisposes grassland and woodland ecosystems to increasing impacts from wildfires, threatening indigenous plants and animals, and the recovery after fires of natural ecosystems. Very few detritivores have adapted to feed on and decompose detritus from African grasses in Australia and the USA, resulting in accumulation of dead leaves and a build-up of fuel increasing the risks from wildfires. In grasslands and woodlands of the Northern Hemisphere, epigeic detritivores and leaf-shredders include groups of invertebrates such as earthworms and isopods. In Australia, larvae of leaf-shredding moths, beetles and several other insects are important detritivores including oecophorid moths, cryptocephaline beetles, termites and cockroaches; some specifically adapted to breakdown of sub-surface plant materials in dry and moist ecosystems. In grasslands and woodlands the range of epigeic insects are likely to reduce accumulating dead biomass and fuel loads that contribute to flame height and intensity of fires. We propose that detritivores of African grasses may be potential biological control agents for senescing and dead biomass and meet the specificity requirements as agents, to control invasive African grasses in the USA and Australia. Introduction indigenous grasses have co-existed with grazing by abundant large herbivores and respond positively to regular schedules of burning. After fires and rain African grasses were introduced into Australia copious re-growth referred to as “green pick” and (Tothill and Hacker, 1983), the USA and other favoured by livestock, is more palatable and contains countries in the mid to late 1900s, to improve the higher concentrations of nutrients when compared quality of pastures and forage for grazing by livestock. with unburned, unpalatable and senescing grass or Introductions aimed to increase green biomasses its detritus. to enable high stocking rates, or maintain seasonal African grasses spread into natural ecosystems growth of pastures with drought tolerant species, in Australia and USA where layers of senescing to suppress weeds and produce protective covers to leaves and detritus accumulate, accompanied by reduce soil erosion and nutrient losses. In Africa, little or no decomposition attributable to indigenous XIII International Symposium on Biological Control of Weeds - 2011 104 Session 3 Non-Traditional Biological Control Agents herbivores. Grass seeds or vegetative fragments whole natural ecosystems (Everitt et al., 2011). In the are often carried into natural areas by livestock, USA, east African guineagrass, Megathyrsus infestus on clothing or on vehicle tires. They spread from (Andersson) B. K. Simon & S. W. L. Jacobs, and P. rangelands into native grasslands, woodlands, and ciliare are similarly invasive, as are other invasive rainforests (Table 1), where they displace non- species in many parts of Australia (Table 1). flammable or weakly flammable plant communities with understories of highly flammable grass mono- Impacts of African grasses on invertebrate stands. Invasive grasses (especially signal grass, biodiversity Brachiaria decumbens Stapf.) advance growth from the margins of paths and roads, into undisturbed natural vegetation. The high flammability of some African grasses Introductions of African species have been (e.g., molassesgrass, M. minutiflora) (Tothill and mostly beneficial as fodder for livestock but several Hacker, 1983) adds to detrimental impacts on have detrimental impacts or have become serious ecosystems, but the combination of competition weeds, e.g., molassesgrass (Melinis minutiflora and flammability (e.g., of guineagrass, M. maximus; Beauv.) is unattractive to stock (Elliott, 2008) buffelgrass P. ciliare) is not well documented. Root and guineagrass, Megathyrsus maximus (Jacq.) R. balls and rhizomes of African grasses modify the Webster (= Panicum maximum, Urochloa maxima) soil texture, chemistry and inhibit activity by sub- (Henty, 1969) can be poisonous (cyanogenic). surface invertebrates. buffelgrass (Pennisetum ciliare (L.) Link) is a host Some indigenous animals use African grasses, and potential transmitter of sugarcane whitefly particularly in the absence of indigenous species, (Neomaskella bergii Signoret) in Australia (Palmer, for example in Australia, wallabies and kangaroos 2009) while gamba grass (Andropogon gayanus will feed on and use dense stands for alluding Kunth) produces dense, tall and flammable stands predators. Small vertebrate animals and birds will (Table 1) that limits mustering or location of stock use the grasses for food or shelter, for example, some in northern Australia. Several grasses (e.g M. finches will feed on seeds of M. maximus and wrens maximus, B. decumbens) invade forestry plantations will construct nests in the dense thickets. A few in Australia and New Guinea (Henty, 1969) and are polyphagous invertebrates feed on African grasses difficult to manage. In eastern Australia, African and maintain their abundance in the absence of grasses increase the access to hosts by the paralysis indigenous plant hosts, for example, grass-feeders tick (Ixodes holocyclus Neumann), facilitating the including the larvae of moths and butterflies (Braby, higher transfer of nymphs and adults from tall grass 2004; Zborowski and Edwards, 2007) will feed on species to humans and livestock. African grasses; especially when the densities of indigenous grass food plants cannot alone sustain African grasses displace plant communities breeding. While having serious impacts on many plants, In Australia and USA in the absence of natural invasive grasses reduce or prevent invertebrates enemies, African grasses spread and will out- occupying the shrub and ground cover plants, compete most indigenous grasses, shrubs and low using their fallen limbs and rock shelters, or the plant communities. African grasses inhibit vegetative decomposing organic materials. Most insects, for growth and reduce or prevent seedling recruitment, example, occupy and breed in the understory on by shielding them against entry of light. Those shrubs, “sub-surface” plants, under bark or logs grasses with rhizomes compete for root space and and in fallen leaves. Representing the majority moisture, replacing indigenous grasses and sedges of invertebrate species in temperate and tropical with surface roots. For example, African lovegrass ecosystems, they regulate the architecture of plants, (Eragrostis curvula (Schrad.) Nees) in southern break down decomposing vegetation and re-cycle Australia, and Buffelgrass (P. ciliare) in Australia nutrients, and form part of the food webs and food and southern Texas, USA, displace beneficial and chains for small vertebrates. Some grasses (e.g., B. indigenous plants and often destroy the integrity of decumbens) appear to be repellent to indigenous XIII International Symposium on Biological Control of Weeds - 2011 Session 3 Non-Traditional Biological Control Agents 105 invertebrates that normally breed on, visit or shelter and Hacker, 1983) adds to detrimental impacts on in grasslands. ecosystems, but the combination of competition Most indigenous insects require spatial access and flammability (e.g., of guineagrass, M. maximus; to their understory food plants for oviposition and buffelgrass P. ciliare) is not well documented. Root particular phenotypic expressions of their food balls and rhizomes of African grasses modify the plants. In Australia, phytophagous Lepidoptera of soil texture, chemistry and inhibit activity by sub- grasslands are mostly monophagous or oligophagous surface invertebrates. and require particular densities of their food plants. After each fire event, re-growth of exotic grasses They are often very ‘local’ and restricted in their is invariably more aggressive than prior to being breeding sites and patrolling sites, such as hilltops, burned; African grasses out-compete the slower re- to undisturbed small areas containing their food growth and recruitment by the indigenous plants. plants or supporting specific vegetation types. For After being burned and re-sprouting from root example, some Hesperiidae always congregate on stocks, African grasses grow more rapidly than isolated hilltops with particular rock formations or indigenous grasses and can prevent re-sprouting of plant communities. Hilltops and slope ecosystems indigenous plants or germinating seedlings. The most are particularly vulnerable to displacement from serious impacts on ecosystems