 Figure 1. External view of a high tunnel system (from Dr. Wallingford’s presentation, used with permission). written by: Makala Nicole Harrison The looming threat of climate change highlights the importance of developing agricultural systems that can stand against the forces of pest arthropods, especially insects, and extreme weather. As part of the UMD Department of Entomology seminar series, Dr. Anna Wallingford – a USDA research scientist in the Invasive Insect Biocontrol & Behavior Lab in Beltsville, Maryland – discussed how the use of high tunnel systems can protect crops and increase their productivity. High tunnel systems, also called “hoop houses” consist of metal hoops covered in plastic or fabric to create a greenhouse-like structure (Fig. 1). High tunnel systems protect crops from rain and extreme weather, both being consequences of climate change, which increases the shelf-life and marketability of the produce while decreasing the occurrence of fungal diseases. While widespread use of high tunnels is fairly recent in the United States, the structures are used worldwide [1]. The structures can utilize varying levels of technology, some have electricity that powers automatic rolling side walls and air conditioning, while others are simple structures that require the farmer to roll up the sides manually. When the tunnels are equipped with passive heating and cooling systems, they are able to extend the growing season by staying warm into the cooler winter months. The tradeoff for using high tunnel systems is that unique pest control issues can arise. Temperatures in high tunnels can be much higher than those outside. Since the temperature inside of the structures can be much warmer than on the outside, pests can be an issue, even in the winter. Common pests in high tunnel systems include spider mites, thrips, and aphids; these pests can occur year-round in the tunnels [2]. There are a few solutions to the pest issues that occur inside high tunnels. The most promising is one that makes use of biological control, which is the use of living organisms to reduce populations of pests. One strategy for reducing outbreaks of pests is releasing generalist walking predators, which are predator species that can survive in a variety of environments and eat a variety of prey species. These predators primarily move around by walking, rather than flying. Accordingly, Dr. Wallingford’s next experiment was aimed at determining how successful three generalist walking predators – lacewing larvae, adult lady beetles, and adult minute pirate bugs – would be when placed in colder temperatures, similar to the environments found in high tunnel systems in the winter. To determine how the predators respond to potentially lethal winter temperatures, they were exposed to freezing temperatures for different lengths of time, ranging from 10 minutes to 90 minutes. The animals were then allowed to recover at room temperature for 30 minutes before mortality rates were assessed; a control group was held at room temperature for 120 minutes. It was found that mortality rates remained fairly low until the insects were subjected to the lethal temperatures for 90 minutes; even then, the mortality rates did not exceed 50% and there was no difference between species (Fig. 2). Therefore, the predators are not threatened by acute freeze injury.  Figure 2. Mortality rates of the three generalist walking predators when exposed to lethal temperatures. There were no differences found between groups and mortality rates did not rise above 50% (from Dr. Wallingford’s presentation, used with permission). Dr. Wallingford also looked into the amount of time it took for the animals to recover from an induced chill coma and the levels of prey consumption after chill coma recovery. The animals were put into small tubes and divided into groups. Some of the groups were put in a freezer to experience subfreezing temperatures for 30 minutes, other groups were put in a cooler of ice to experience freezing temperatures for 24 hours. After exposure to the freezing temperatures, the animals were observed and the amount of time it took for the animals to emerge from their tubes was recorded. Even after the exposure to freezing and subfreezing temperatures, the animals were still able to recover. Animals that emerged from their tubes were then placed in cups with aphid-infested spinach leaves for 24 hours. After the 24 hours passed, the change in the number of aphids per cup was calculated to see how the chill coma affected prey consumption. The number of aphids in the cups with the generalist walking predators was consistently lower than the number of aphids in the cups with no predators. This shows that even though there were varying levels of prey consumption, the animals were still eating prey after they had recovered from the chill coma (Fig. 3). For farmers looking for solutions to winter-time pests, predators that survive chill injuries could prove to be useful.  Figure 3. Aphid consumption by generalist walking predators after chill recovery. The change in aphid populations on spinach leaves in cups after 24 hours with post-chill coma generalist walking predators. The cups with no predators had a consistently higher positive change in aphids than the cups that had the generalist walking predators because there was nothing in the cup to each the aphids (from Dr. Wallingford’s presentation, used with permission). The three experiments, all dealing with temperature change and pest/predator interactions, give helpful insight into how generalist walking predators can be used as biocontrol agents for pests in high tunnel systems over the winter months. While tunnel temperatures can still fall into the lethal range of pest species, they do not seem to be acutely lethal to pests like aphids. Similar trends are seen for different predator species, especially the three generalist walking predators that were used in the experiments. The predation of the three predators was not negatively affected by the freezing treatments that caused chill comas in the animals. These results show that other factors, including drying, starvation, and repeated temperature injuries, are more likely to play into the mortality of both pest and predator species in high tunnels, so it’s important that these scenarios are tested so farmers can decide whether introducing the predators would be helpful. Since these walking generalist predators can be useful for the biocontrol of pests in high tunnels, the next steps are to look into where these predators spend their time in the plant canopy and how they perform over longer periods of time in high tunnels. Dr. Wallingford’s work may give farmers the knowledge they need to become more resilient and resistant in the face of climate change.
[1] Lamont, W. J., Jr. (2009). Overview of the Use of High Tunnels Worldwide. HortTechnology, 19(1), 25-29. Retrieved Oct 26, 2024, from https://doi.org/10.21273/HORTTECH.19.1.25 [2] Willden, S. A., Zablah, A., Wallingford, A., & Ingwell, L. L. (2024). Management of aphids on winter high tunnel crops. Biological Control, 192, 105511. https://doi.org/10.1016/j.biocontrol.2024.105511 Comments are closed.
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