Host plant relationships of green mirids – is alternative control possible?
Green mirids, Creontiades dilutus (Hemiptera, Miridae), are polyphagous bugs and are endemic to Australia. These bugs feed on a variety of plant species that grow across massive expanses of subcoastal agricultural landscapes in eastern Australia, as well as in the arid continental interior. Molecular evidence, along with field surveys, have demonstrated that C. dilutus bugs move long distances between native vegetation in the arid interior and crops in the eastern states of Queensland and New South Wales. These bugs arrive in the subcoastal agricultural landscapes every summer, and are important pests of cotton (Gossypium hirsutum L.) (Malvaceae). They feed on the soft tissues of cotton plants, including the developing flowers, which results in a substantial loss of fruit (cotton bolls) and the feeding damage delays harvest through the crops taking time to compensate for these losses.
The seasonal invasions of bugs into cotton are influenced by the high mobility of these insects and their ability to use a wide variety of host plant species across arid and agricultural landscapes. Some plant species are relatively good hosts in supporting the production of high numbers of nymphs, whereas others produce few nymphs but may be used incidentally as shelter, and this may aid in the dispersal of the adults across long distances. The timing of invasions of C. dilutus bugs into cotton, and the pathways followed by them are poorly understood. Also, the general mechanisms by which these insects localize appropriate host plants have not been subject to much investigation.
Host plant availability in the arid continental interior is dependent on highly variable localized rainfall, and such areas are separated by large expanses of extremely dry regions containing few green plants. In agricultural systems, non-crop plants along roadsides and the margins of farms occasionally host low densities of bugs, but previous field surveys have not found high densities of C. dilutus that may act as a source of pests that invade cotton. Within farms several legume crops, such as lucerne (Medicago sativa) and pigeon pea (Cajanus cajan) (Malvaceae), routinely support high bug densities, whereas nearby cotton crops host substantially fewer bugs. Gut analyses conducted previously suggest that individual mirids do
move from lucerne into cotton, but they also move in the other direction, despite the differential numbers of bugs across these crops. Similar movement patterns across pigeon pea and cotton “boundaries” were evaluated in this thesis.
The ambiguity in the dispersal and host use patterns of C. dilutus bugs makes it difficult for pest managers to predict invasions of these insects into cotton, with accuracy. Consequently, researchers are not able to design effective management strategies. With a better understanding of dispersal and host use patterns of C. dilutus bugs, it may be possible to reduce the number of insecticide sprays in cotton if bugs could be attracted away from cotton by planting alternative hosts (trap crops), but these alternative hosts may inadvertently become local reservoirs of pests that move into cotton. A particular aim of this study is, therefore, to investigate aspects of the dispersal of these C. dilutus bugs across crop host species and the associated host localization behaviour of these insects. The ultimate goal is to use this information to form a conceptual model for the host localization process of C. dilutus bugs and provide a realistic framework to develop effective pest management decisions in cotton systems.
Specifically this thesis presents the results of: 1) surveys that asked pest managers about their perceptions of invasion patterns, 2) field surveys across vast arid and agricultural landscapes to identify which host species are used most consistently by bugs, 3) molecular evaluations to confirm feeding and movement of individuals across different hosts, and 4) behavioural experiments to identify host-associated cues used by bugs to localize specific plants.
Findings from this study indicate that most pest managers reported that the earliest seasonal infestations into cotton are associated with the proximity of cotton to legume crops and also with storms that move in from the arid regions to the west. Infestation patterns are consistent with multiple invasion events in each season and into each crop, and a gradual increase in bug numbers as nymphs develop into adults within squaring (flowering) cotton. Field surveys
in the arid zone found that the highest densities of bugs were found on Cullen australasicum (Fabaceae) and Goodenia cycloptera (Goodeniaceae), at a time prior to when cotton was planted, and on lucerne and pigeon pea in agricultural systems during the flowering period of cotton.
Similarly, too, bug densities were consistently much higher on pigeon pea than on cotton. Creontiades dilutus bugs were found in the field to feed on both pigeon pea and cotton, and frequently they move back-and-forth between these crops, as found across lucerne-cotton boundaries. Behavioural tests in the laboratory revealed that these bugs are arrested in the vicinity of pigeon pea and cotton by olfactory cues, but there was no evidence that olfactory cues alone attracted bugs to either host beyond a range of 2cm. Also, this is the first behavioural study that observed an increase of insect locomotion at night, suggesting that these bugs are essentially nocturnal.
Collectively, the results of this thesis indicate that C. dilutus bugs are produced in relatively greater numbers on specific plant species than on nearby alternative species (with cotton being a relatively poor host ecologically (although not economically)). Olfactory cues that arrest bug movement to a locality appear to have a stronger influence on settling patterns across host plant species than do olfactory cues that attract bugs towards plants. A general host localization model is proposed in which high densities of bugs develop on host plant species that maintain soft tissues (after receiving rain or growing on irrigated farmlands), then move across the landscape and land on plants until recognizing cues that arrest their movement. Other host species, such as cotton, are used if their primary hosts are not available. The implications of how these bugs persist in highly variable environments are discussed, and the implications for pest management are specified.