Aphidiid parasitoids (Hymenoptera: Aphidiidae) of aphids generally exploit only a small percentage of the available host resources in the field. This limited impact on aphid populations has often been explained as a consequence of hyperparasitism. We propose that a wasp's reproductive strategy, as opposed to hyperparasitism, is the dominant factor in aphidiid population dynamics. A wasp's foraging efficiency and oviposition decisions are influenced by several variables, including searching behaviour between and within patches, host choice (as modified by the aphids' defensive behaviours), and plant structural complexity. Two broadly different patterns of host exploitation have evolved in aphidiid wasps in relation to ant-aphid mutualism. Firstly, in species that are exposed to predation and hyperparasitism, a female may leave a patch before all suitable hosts are parasitized. Because predators and hyperparasitoids tend to aggregate at high aphid or aphidiid densities, or in response to aphid honeydew, this strategy enables females to reduce offspring mortality by “spreading the risk” over several host patches. Secondly, in species that have evolved mechanisms to avoid aggression by mutualistic ants, females are able to exploit a hyperparasitoid-free resource space. Such species may concentrate their eggs in only a few aphid colonies, which are thus heavily exploited. Although hyperparasitism of species in the first group tends to reach high levels, its overall impact on aphid-aphidiid population dynamics is probably limited by the low average fecundity of most hyperparasitoids. We discuss the foraging patterns of aphidiid wasps in relation to aphid population regulation in general, and to classical biological control in particular. We argue that a parasitoid's potential to regulate the host population is largely determined by its foraging strategy. In an exotic parasitoid, a behavioural syndrome that has evolved and presumably is adaptive in a more diverse (native) environment may, in a more uniform (managed) environment, result in suboptimal patch-leaving and oviposition decisions, and possibly increased resource usage.

Regulation of aphid populations by aphidiid wasps: does parasitoid foraging behaviour or hyperparasitism limit impact?

Salinity is a widespread problem caused by an imbalance between rainfall and transpiration in the dryland cropping systems of southern Australia. The need to use more perennials has been identified and this paper examines the possibility of replacing annual with perennial pasture legumes and the germplasm available to do so. While lucerne is already used widely in eastern Australia it has only recently been adopted in the wheat belt of Western Australia. There are doubts about its adaptation to acid soils and to climates where summer rainfall is low and ambient temperatures are high. There is also a need to diversify the species available to reduce the likelihood of invasion by exotic diseases and insects. Several genera are likely to be of value in this respect, although few will be as widely adapted as lucerne. Perennial legumes are found in environments ranging from alpine to desert. Targeted collections of genera from the dry areas, especially where soils are acid, are likely to yield species of value. These may include perennial species of Astragalus, Hedysarum, Lotus, Onobrychis, Psoralea, and Trifolium. Some Australian genera, for example Swainsona, Glycine, and Cullen may also be of value. Most of these genera are from alkaline soils, and the need to cope with acid soils that are often high in free aluminium is seen to limit their use in southern Australia. However, since virtually nothing is known of the ecology and ecophysiology of species from the dry areas, it is possible that through selection and the use of adapted rhizobia, some at least may be of value in Australian conditions. Cropping in rotation with perennial legumes is likely to involve several changes in farming systems. It is impossible to predict their nature but it is essential that we understand what these changes are before the species are widely introduced. Account must also be taken of their ability to use water. It is entirely possible that perennials from dry areas are dormant in summer despite the fact that there is no evidence in the literature to this effect. It was concluded that although lucerne is suitable for phase farming, alternatives to lucerne are needed. They will have to match the water-using and nitrogen-fixing capacities of lucerne, and farming systems will be required that make full use of the new germplasm. Collaboration with institutions in the Mediterranean basin and elsewhere is needed and a beginning has been made in this direction. Additional keywords: lucerne, Trifolium, Astragalus, phase farming.

Ecology of herbaceous perennial legumes: a review of characteristics that may provide management options for the control of salinity and waterlogging in dryland cropping systems

Alfalfa, Medicago sativa L., is among the most prized of forages, and is grown worldwide as a feed for all classes of livestock. It is one of man's oldest crops, and its cultivation probably predates recorded history. In addition to its versatility as a feed, alfalfa is well known for its ability to improve soil structure and, as a legume, is an effective source of biological nitrogen. As a perennial crop, alfalfa has a lifespan approaching 5 years, but in some areas of the world fields may remain productive for considerably longer. Such a long stand life affords ample time for the establishment and development of a diverse community structure by an abundance of organisms. In spite of system perturbations caused by frequent harvests and occasional pesticide applications, an alfalfa field provides a temporal stability which is uncommon among field crops. As a result of this stability, alfalfa supports an immense diversity of flora and fauna which, at times, exceeds that of riparian ecosystems. While most of alfalfa's inhabitants have little or no impact on it as a crop, a few are capable of causing extensive damage. Arthropods, plant pathogens, weeds, vertebrates, and plant parasitic nematodes can all cause significant yield and/or quality reductions and frequently contribute to shortening the productive life of the stand. This paper reviews the major strategies which have been developed to manage many of these alfalfa pests including: host plant resistance; cultural controls, such as harvest strategies, irrigation management, sanitation, planting schedules, and crop rotation; mechanical and physical controls; chemical control; and biological controls. Multiple pest interactions, e.g. insect-insect; insect-disease; insect-weed, and their management are discussed. Potential conflicts arising from the use of strategies which may reduce one pest but exacerbate others are also examined. A cross index of management strategies and their role in managing multiple pests is provided. Computer models, both ecological and economic, and their role in alfalfa pest management are discussed. Selected information sources on alfalfa and alfalfa IPM available over the Internet are listed. Alfalfa's role in the agricultural landscape, as it relates to pests, natural enemies, and pest management in other cropping systems as well as its role in crop rotation, is considered.

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Integrated Pest management in Forage Alfalfa

Aphids can cause substantial damage to cereals, oilseeds and legumes through direct feeding and through the transmission of plant pathogenic viruses. Aphid-resistant varieties are only available for a limited number of crops. In Australia, growers often use prophylactic sprays to control aphids, but this strategy can lead to non-target effects and the development of insecticide resistance. Insecticide resistance is a problem in one aphid pest of Australian grains in Australia, the green peach aphid (Myzus persicae). Molecular analyses of field-collected samples demonstrate that amplified E4 esterase resistance to organophosphate insecticides is widespread in Australian grains across Australia. Knockdown resistance to pyrethroids is less abundant, but has an increased frequency in areas with known frequent use of these insecticides. Modified acetylcholinesterase resistance to dimethyl carbamates, such as pirimicarb, has not been found in Australia, nor has resistance to imidacloprid. Australian grain growers should consider control options that are less likely to promote insecticide resistance, and have reduced impacts on natural enemies. Research is ongoing in Australia and overseas to provide new strategies for aphid management in the future.

Insecticide resistance and implications for future aphid management in Australian grains and pastures: a review

The aphid Therioaphis trifolii (Monell) is indigenous to the western Palaearctic region, where it colonizes lucerne (alfalfa), clovers and related legumes. A form of the species, T. trifolii f. maculata (the spotted alfalfa aphid, SAA), has been known in Australia since 1977, feeding almost exclusively on Medicago sativa (lucerne). Since 1989, T. trifolii has been found in Australia colonizing Trifolium subterraneum (subclover). We have compared samples of T. trifolii in Australia collected on lucerne and subclover using several techniques: survival and reproduction on different hosts, morphology, cuticular hydrocarbon profile, karyology, and a combination of RAPD-PCR and mitochondrial DNA (cytochrome oxidase) genetic markers. Whereas there were no distinct differences in cuticular hydrocarbons and karyology, we found significant correlated differences between the host on which aphids were collected, some morphological characters, and the genotypes of the aphids. Aphids collected from lucerne were always of one genetic type, and nearly all those from subclover were of another. Both groups of aphids are morphologically distinguishable from the yellow clover aphid (YCA), the other form of the species recognized so far. We conclude that the clover-colonizers (spotted clover aphid, SCA) and the lucerne-colonizers (SAA) are each host-restricted forms (biotypes) of T. trifolii. Estimates of mtDNA divergence are in the range of conspecific to closely congeneric, as compared with other insects including aphids. We developed simple diagnostic DNA tests to distinguish the two pest aphids.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0007485300037433

Biological and genetic characterization of morphologically similar Therioaphis trifolii (hemiptera: Aphididae) with different host utilization

SUMMARY satisfactorily account for it. The capacity of the aphid to infest and increase on susceptible lucerne remains unchanged. The drought, which developed concurrently with the decline and may have reduced source populations on non-irrigated lucerne plants thus lowering immigration rates into irrigated crops, broke in 1983 with no indication of aphid resurgence since. Insecticide use, although high at first, decreased rapidly with the reduction in infestations. Relative to the rate of decline, the replacement of aphid- susceptible lucerne with aphid-resistant varieties proceeded slowly to reach only 10%/o in 1981 and 26%/o in 1984. The majority of lucerne grown in Australia is still susceptible to the aphid. Although native predatory insects became numerous in aphid-infested crops, they did not reduce aphid numbers sufficiently consistently to account for the decline. (4) Of the hymenopterous parasites, only one, Trioxys cornplanatus, became generally widespread and numerous throughout the lucerne-growing areas of Australia. (5) Parasitization by T. complanatus increased from one lucerne growth cycle to the next and higher levels were associated with aphid infestations that did not increase to damaging levels. (6) Observed reductions in the proportion of apterous adult female aphids in field populations were consistent with estimates of parasitization by T. complanatus. (7) When the properties of the aphid and parasite, as measured in the laboratory, were assembled into a simulation model of their interacting life systems on irrigated lucerne crops, outputs from the model closely matched the observed field data. Furthermore, the simulated population at harvest from initial lucerne growth cycles, when used as input for other runs of the model, resulted in good simulations of aphid and parasite populations in subsequent cycles in the same season. (8) It is concluded that Therioaphis trifolii on irrigated lucerne crops was biologically controlled by the introduced hymenopterous parasite Trioxys complanatus. (9) The short-term (say 5-year) value of the biological control, in terms of the reduction in the cost of insecticide applications needed to prevent loss and spoilage of the N.S.W. lucerne-hay crop, has probably been of the order of 1 million dollars a year. In terms of reduced urgency for replanting with aphid-resistant varieties, the value was similar. Over the whole of Australia, the value would be proportionally greater. (10) Although T. complanatus was known to be less effective at high summer temperatures, this did not prevent an overall reduction in immigration rates, although some alates still infest many crops.

Biological control of the spotted alfalfa aphid, Therioaphis trifolii F. maculata, on lucerne crops in Australia, by the introduced parasitic hymenopteran Trioxys complanatus

L. T. Woolcock

Papers

Biological control of the spotted alfalfa aphid, Therioaphis trifolii F. maculata, on lucerne crops in Australia, by the introduced parasitic hymenopteran Trioxys complanatus