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Mirid invasions: what can be learned from the experiences of crop consultants

Justin Cappadonna

Green mirids, Creontiades dilutus, are small plant bugs (Miridae) found only in Australia. They can persist on isolated patches of native vegetation in the remote deserts of the continental interior, but they also do quite well on cotton planted across vast areas of Queensland and New South Wales. This is problematic for cotton-growers because feeding damage by mirids leads to the loss of fruit (the cotton bolls), and severe infestations reduces the total yield.

Every spring, cotton is planted. Every summer, the mirids arrive. Infestations are part of a typical cotton season, but the pathways in which these endemic pests invade cotton remain elusive.

Members of the Walter-Furlong Lab have investigated the ecology of green mirids since the 1980s, with a recent focus on the movement of these bugs across different non-crop host species. Meanwhile, cotton-growers have employed consultants to monitor crops for mirid infestations. Which is to say, researchers have identified several mechanisms describing how mirids use hosts, and consultants know when these bugs infest cotton farms.

There is no better way for researchers to design a series of surveys for mirids across vast regions of eastern Australia, than to first ask the consultants that best know the pest dynamics on their farms about the invasion patterns of these bugs.

In a recent study published in Agricultural Systems, we evaluated the responses by consultants to questionnaires, reports of seasonal pest pressure, as well as results of field surveys published in scientific journals and unpublished theses to determine if there were any similarities in mirid invasions across a 21 year period. Our primary aim was to determine the environmental conditions and timing of the first observations of green mirids on cotton farms in each cropping season. The results were then used to inform where and when future field surveys should be conducted, as well as which specific ecological relationships between cotton and mirids would be useful for consultants to know more about.

Key findings include:

  1. Mirids initially arrive at different areas of farms and at different times in the season, before numbers gradually build-up within cotton fields. This suggests that mirids do not move into cotton en masse as a single invasion.

  2. In general, invasions are associated with storms from arid environments west of farms, but many consultants suspect local mirid sources come from legume crops and native vegetation growing near water sources.

  3. The most severe mirid infestations in each cotton district in Queensland and northern New South Wales occur at the same time as neighbouring districts. Infestations in the southernmost districts tend to be less severe. This suggests that infestations are likely influenced by local environmental conditions.

The findings of this study have helped shape the design of field surveys in remote arid environments following rain events, as well as surveys of local hosts growing in riparian zones. We are also conducting direct tests of the movement of bugs between cotton and

potential legume pest sources that were identified by consultants. Check back later for updates.

We would like to thank The University of Queensland, and the Cotton Research and

Development Corporation (CRDC) for financial support. The CRDC was also crucial distributing the questionnaires. We are also grateful to all the consultants that responded to the questionnaires, and to The Australian Cotton grower for originally publishing the seasonal pest reports used in the analyses described above.



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