Michelle A. Rafter

The leaf-tying moth Hypocosmia pyrochroma (Lep., Pyralidae), a host-specific biological control agent for cat's claw creeper Macfadyena unguis-cati (Bignoniaceae) in Australia

Abstract:  Cats claw creeper, Macfadyena unguis‐cati, a major environmental weed in coastal and sub‐coastal areas of Queensland and New South Wales, Australia is a target for classical biological control. Host specificity of Hypocosmia pyrochroma Jones (Lep., Pyralidae), as a potential biological control agent was evaluated on the basis of no‐choice and choice larval feeding and survival, and adult oviposition preference tests, involving 38 plant species in 10 families. In no‐choice tests, larval feeding and development occurred only on cats claw creeper. In choice tests, oviposition and larval development was evident only on cats claw creeper. The results support the host‐specificity tests conducted in South Africa, and suggest that H. pyrochroma is a highly specific biological control agent that does not pose any risk to non‐target plants tested in Australia. This agent has been approved for field release by relevant regulatory authorities in Australia.

Species limits, quarantine risk and the intrigue of a polyphagous invasive pest with highly restricted host relationships in its area of invasion.

Scirtothrips aurantii is a generalist horticultural pest in its native African range and recently established quite widely in Australia on the invasive succulent weed Bryophyllum delagoense. Paradoxically, this thrips is not polyphagous in its incursive range. The issue is principally one of quarantine. Will the thrips in Australia shift, perhaps adaptively, to citrus, and should the primary focus be on containment around Australian citrus, or does the real quarantine risk exist offshore with thrips present on citrus in Africa? We examined the phylogenetic relationships between Bryophyllum‐associated thrips populations in Australia and populations sampled from various host plant species in South Africa (including Bryophyllum) using both CO1 and 28s markers. Eight variable microsatellite markers were developed to assess the extent of gene flow between the thrips on different hosts in South Africa. The COI phylogeny resolved S. aurantii into three distinct clades with samples collected from B. delagoense in South Africa and Australia representing a single clade, a second clade associated with Gloriosa lilies and the third with horticultural hosts. The microsatellite analysis confirmed that the populations associated with citrus and Bryophyllum do not hybridize with one another in sympatry. We conclude that the citrus‐damaging thrips are not currently present in Australia and remain a serious quarantine concern in relation to Australian horticulture.

Flight directionality of Tribolium castaneum soon after take-off under glasshouse and field conditions

Flight directionality of the rust‐red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae), was investigated under glasshouse and field conditions using sticky traps placed around dense experimental infestations of T. castaneum derived from field‐collected samples. Although beetles of this species are known to fly quite readily, information on flight of beetles away from grain resources is limited. Under still glasshouse conditions, T. castaneum does not demonstrate strong horizontal or vertical trajectories in their initial flight behaviour. Flight was significantly directional in half of the replicates, but trapped beetles were only weakly concentrated around the mean direction of flight. In the field, by contrast, emigration of T. castaneum was strongly directional soon after flight initiation. The mean vector lengths were generally >0.5 which indicates that trapped beetles were strongly concentrated around the calculated mean flight direction. A circular‐circular regression of mean flight vs. mean downwind direction suggested that flight direction was generally correlated with downwind direction. The mean height at which T. castaneum individuals initially flew was 115.4 ± 7.0 cm, with 58.3% of beetles caught no more than 1 m above the ground. The height at which beetles were trapped did not correlate with wind speed at the time of sampling, but the data do indicate that wind speed significantly affected T. castaneum flight initiation, because no beetles (or very few; no more than three) were trapped in the field when the mean wind speed was above 3 m s−1. This study thus demonstrates that wind speed and direction are both important aspects of flight behaviour of T. castaneum, and therefore of the spatio‐temporal dynamics of this species.

Sublethal exposure to phosphine decreases offspring production in strongly phosphine resistant female red flour beetles, Tribolium castaneum (Herbst).

The red flour beetle is a cosmopolitan pest of stored grain and stored grain products. The pest has developed resistance to phosphine, the primary chemical used for its control. The reproductive output of survivors from a phosphine treatment is an important element of resistance development but experimental data are lacking. We exposed mated resistant female beetles to 0.135 mg/L of phosphine for 48 h at 25°C. Following one week of recovery we provided two non-exposed males to half of the phosphine exposed females and to half of the non-exposed control females. Females that had been exposed produced significantly fewer offspring than non-exposed females. Females that remained isolated produced significantly fewer offspring than both exposed females with access to males and non-exposed controls (P<0.05). Some females were permanently damaged from exposure to phosphine and did not reproduce even when given access to males. We also examined the additional effects of starvation prior to phosphine exposure on offspring production. Non-exposed starved females experienced a small reduction in mean offspring production in the week following starvation, followed by a recovery in the second week. Females that were starved and exposed to phosphine demonstrated a very significant reduction in offspring production in the first week following exposure which remained significantly lower than that of starved non-exposed females (P<0.05). These results demonstrate a clear sublethal effect of phosphine acting on the female reproductive system and in some individuals this can lead to permanent reproductive damage. Pest population rebound after a fumigation may be slower than expected which may reduce the rate of phosphine resistance development. The results presented strongly suggest that phosphine resistance models should include sublethal effects.

Sampling of South African Citrus Thrips (Scirtothrips aurantii Faure) (Thysanoptera: Thripidae) Across Host Plant Species in South Africa

Scirtothrips aurantii Faure (Thysanoptera: Thripidae), the well-known economic pest of horticulture in Africa (Samways et al. 1987; Lewis 1997; Grove et al. 2000), established adventitiously in Australia around 2002. Although known colloquially as the South African citrus thrips, the species is a well-documented generalist that is reported from 70 to 100 plant species from 32 families (Gilbert 1989; de Villiers et al. 1987). Consequently, the establishment of this species in Australia was a major cause for concern (Anon. 2003), but no reports of S. aurantii attacking horticultural hosts in Australia have surfaced, despite several targeted surveys (Anon. 2003; Rafter et al. 2008).

Polyandry, genetic diversity and fecundity of emigrating beetles: understanding new foci of infestation and selection

The frequency of polyandry and its ecological (and therefore selective) advantages remain unclear, especially in the field, where such tests are seldom conducted. We quantified the fecundity and level of polyandry in field populations of two stored grain beetles, Tribolium castaneum (Herbst) and Rhyzopertha dominica (F.), by comparing resident with dispersing individuals at common sites. Almost all females caught in flight had mated, with paternity analysis from 110 parent–offspring arrays revealing that most females had mated with more than one male (T. castaneum: 90.6, R. dominica: 70.2%). No difference in the extent of polyandry between beetles collected from grain in storage and beetles caught in flight was detected for either species, and the degree of polyandry did not impact overall fecundity. Levels of polyandry were similar to those in recently established (first generation) laboratory cultures of both species. Our results provide the first direct evidence of polyandry in wild populations of T. castaneum and R. dominica, and that females that have mated several times have the potential to produce offspring of multiple genotypes in the field. Females thus invade resource patches (whether previously colonised or not) with considerable genetic diversity to distribute across their offspring, an advantage in a newly colonised site because it increases the chances of her offspring establishing there. This advantageous consequence of polyandry to dispersing individuals, and the fact that most of the individual beetles sampled in the field had mated multiple times, provides justification as to why polyandry should not be viewed as an evolutionary paradox.

Progression of phosphine resistance in susceptible Tribolium castaneum (Herbst) populations under different immigration regimes and selection pressures

Insecticide resistance is an escalating global issue for a wide variety of agriculturally important pests. The genetic basis and biochemical mechanisms of resistance are well characterized in some systems, but little is known about the ecological aspects of insecticide resistance. We therefore designed a laboratory experiment to quantify the progression of phosphine resistance in Tribolium castaneum populations subject to different immigration regimes and selection pressures. Mated resistant females were added to originally susceptible populations under two distinct migration rates, and in addition, half of the populations in each migration treatment were exposed to selection pressures from phosphine fumigation. The progression of phosphine resistance was assessed by screening beetles for the resistance allele at rph2. Phosphine resistance increased slowly in the low migration treatment and in the absence of selection, as expected. But at the higher migration rate, the increase in frequency of the resistance allele was lower than predicted. These outcomes result from the high levels of polyandry known in T. castaneum females in the laboratory, because most of the Generation 1 offspring (86%) were heterozygous for the rph2 allele, probably because resistant immigrant females mated again on arrival. Phosphine resistance was not fixed by fumigation as predicted, perhaps because susceptible gametes and eggs survived fumigation within resistant females. In terms of phosphine resistance progression in populations exposed to selection, the effect of fumigation negated the difference in migration rates. These results demonstrate how species‐specific traits relating to the mating system may shape the progression of insecticide resistance within populations, and they have broad implications for the management of phosphine resistance in T. castaneum in the field. We specify and discuss how these mating system attributes need to be accounted for when developing guidelines for resistance management.

Investigating movement in the laboratory: dispersal apparatus designs and the red flour beetle, Tribolium castaneum

The natural dispersal of Tribolium castaneum Herbst (Coleoptera: Tenebrionidae) has been emulated in the laboratory for more than 50 years, using a simple dispersal apparatus. This has typically comprised of a starting container (initial resource or patch) connected by tubing, which contains thread for the animals to climb into a tube and hence to an end container. That is, beetles move to a new viable resource or patch from an inter‐patch zone or non‐viable habitat. We modified this basic apparatus design to test the effect of tubing length and tubing insertion angle on the dispersal rate and proportion of successful dispersers. We expected that the proportion of successful dispersers would be repeatable within each apparatus design, and that increasing tubing length and steepness of the insertion angle would reduce dispersal rate and success across apparatus designs. Dispersal increased linearly through time, similarly so for both males and females. The design with the most vertical tubing insertion angle had a lower proportion of successful dispersers. Tubing length also had a negative relationship with dispersal success (as judged by insects reaching the end container), but a significant reduction in dispersal success was only apparent between the shortest and longest tubing between containers. We suggest that locating and climbing the vertical section of string before they can enter the tubing between containers restricts dispersal and that at higher densities, insects exhibit greater inclination to climb. This type of apparatus has flexible design tolerances and further potential to study the dispersal of other small insect species that primarily use pedestrian locomotion.

Post hoc assessment of host plant use in a generalist invader: implications for understanding insect–plant interactions and weed biocontrol

Structured host-choice and no-choice tests were conducted to help clarify the host plant interactions of an insect herbivore that is simultaneously seen as broadly polyphagous and pestiferous (in Africa) and host restricted/beneficial (in Australia). The research reported here involves specification of the host range of the invasive population of Scirtothrips aurantii found on Bryophyllum in Australia and included tests involving three separate lists of plant species considered to have the potential for thrips attack (plants of horticultural concern, native species at risk of attack and species listed for screening in the search for specialist B. delagoense biocontrol agents). This procedure was developed specifically to deal with the S. aurantii situation in Australia. Because the test species is already present in the field, the conclusions from the tests could be evaluated independently against field sampling results. Host testing revealed that the fundamental host range of the Bryophyllum population of S. aurantii includes Macadamia integrifolia, Mangifera indica and Kalanchoe blossfeldianna. However, the choice tests (involving B. delagoense) and a field survey of Man. indica demonstrated conclusively that the realised host range of S. aurantii in the field is restricted to Crassulaceae. We recommend that host testing of generalist insects not be discounted out of hand (for biological control) because of their perceived polyphagy. Any evidence of populations being strongly associated with the weed species of interest (through quantified host association studies in the native range) suggests further scrutiny of that population is warranted, by means of the host testing methods developed here and in conjunction with appropriate tests of the population’s species status.

Tiny insects against the weather - Flight and foraging patterns of Frankliniella schultzei (Thripidae) not altered by onset of rainfall

To survive in nature, organisms may need to take direct action to mitigate specific dangers from their environmental surroundings. Tiny flying insects are thought to be at particular risk from rainfall that would be of negligible concern to larger animals. The study species Frankliniella schultzei is a thrips that inhabits flowers and feeds mostly on petal tissue and pollen. While found to respond in the laboratory to decreases in atmospheric pressure associated with cyclonic conditions (rather than merely heavy rainfall), their responses to conditions preceding rainfall have not been tested in the field. Initial field sampling investigated the relationship between floral development and sites at which male, female, and larval thrips were generally present on sunny days. We then designed a sampling strategy to test if these thrips can anticipate imminent rainfall or storms and so seek shelter deep within flowers, by sampling host flowers (in sections) on multiple days with different weather conditions. Sticky traps were used to intercept thrips in flight, thus providing a measure of flight behavior across different days. The initial sampling found adult thrips primarily at the petal apex of anthesis‐stage flowers where pollen is distributed. We subsequently found that rainfall, atmospheric pressure change, temperature, humidity and wind had no effect on flight behavior of F. schultzei, or on their positions within flowers. These findings suggest rainfall is not a serious hazard for them. Perhaps thrips can survive raindrop collisions during flight, as impacts with water droplets are not expected to break the surface tension.