Thrips management in the green beans industry (VG07017)

This is a final research report from Hort Innovation’s historical archives. Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.

What was it all about?

Thrips in green beans was a continuing problem for bean growers from north Queensland all the way to Tasmania, with the majority of fresh market beans grown in Queensland and the majority of processing beans grown in Tasmania. Thrips damaged the developing pod while it was still enclosed within the flower making it extremely difficult to target the pest with appropriate insecticides. There had been 13 different thrips recorded from bean flowers, some of which may have been using the flowers for shelter, but others that were regularly found within the flower and were known pests of crops. The thrips found to date included;

• Megalurothrips usitatus or bean blossom thrips
• Frankliniella occidentalis or western flower thrips
• F. schultzei or tomato thrips
• Thrips tabaci or onion thrips
• T. imaginis or plague thrips and possibly
• Pseudanaphothrips achaetus or hairless flower thrips

The trial work in Queensland concentrated on finding suitable insecticide control options in addition to those \recommended for use in green beans at the time. Different application methods were also investigated, such as ground application at planting and foliar application at flower bud formation.

Thrips populations in spring, although more than 5 thrips per flower, equated to 10 per cent of unmarketable pods in the untreated control. In contrast, the autumn plants suffered severe pod damage, with on average 50 per cent of pods unmarketable, resulting from on average 1 thrips per flower. The Success™ (spinosad) treatment during the autumn 2008 trial appeared to give the best return on healthy pods, however this was not the case when applied as a mix with dimethoate during the 2010 autumn trial. No other treatment performed significantly better than the unsprayed control treatment. Movento® (spirotetramat) and those treatments with spinosad significantly reduced the larval populations, especially during the autumn trials. This reduced larval count increased the percentage of clean pods slightly but was not significantly better than the other treatments.

The Tasmanian component of this project was designed to determine if ‘wind scorch’ damage in Tasmanian green bean crops was at least partly the result of thrips feeding on the developing pods inside the bean flowers. This type of damage could be as high as 10 per cent scaring which would result in the complete crop being rejected by the processor due to the increased cost associated with sorting at the processing plant. The ideal level of damage would be less than 4 per cent damage whether it was due to thrips and/or wind scorch symptoms. The Tasmanian trials focused on the effect of wind and thrips populations on damage to pods. The results showed that the major cause of wind-scorch was wind and that the incidence of these symptoms may  have been reduced and the yield of marketable pods increased by installing wind-breaks. However, this may have increased the incidence of disease such as Sclerotinia rot, and since there were many variables involved the severity of the disease would be unpredictable. When thrips numbers were high, insecticidal control resulted in only 6.4 per cent thrips damage compared to between 14.2 -22.1 per cent pod damage in untreated plots. The use of strip plantings with taller crops or wind breaks around paddocks could help in reducing the severity of wind scorch on Tasmanian properties.

A better understanding of thrips population dynamics was a must for future research due to the large number of thrips genera and species found within bean flowers as well as the numbers required to trigger a spray decision depending on the species identified at the different times of the year.