Overwintering insects on high tunnel strawberry and impacts on yield

Overwintering strawberries in high tunnels can jumpstart plant growth in the spring and protect developing flowers from frost damage and disease. However, this protected environment is also ideal for some pests, like spider mites and aphids, that can successfully overwinter and build their populations. We conducted an experiment on ‘Chandler’ strawberry where it was grown in high tunnels from September 2022 for a May 2023 harvest at the Throckmorton and Southwest Purdue Agricultural Research Stations. Both crops were overwintered under row covers that were left in place until they were removed in Feb / Mar 2023 when flowers were forming. We identified the most common pests at both farms, and used a degree-day approach (i.e., quantifying heat accumulation) to measure the impact of aphid accumulation (‘aphid-days’) on strawberry yield at TPAC only.

Pest presence. The overwhelming number of pests we identified at TPAC were aphids (over 99 %), followed by spider mites (Fig 1). However, this trend was flipped at SWPAC where spider mites were most common (75 % of counts) followed by aphids (23 % of counts) (Fig 1). This was not surprising as the previous crop at SWPAC (cucumber) had high spider mite infestation before strawberries were transplanted that was not observed at TPAC. At both farms, aphids peaked in density during mid-February, and at SWPAC spider mites were at low pressure until mid-April after which they peaked in abundance by late May. Pest pressure was considerably higher at SWPAC (9 pests per strawberry leaflet) compared to TPAC (2 pest per leaflet). There could be many factors that led to higher pest pressure at SWPAC, including surrounding landscape complexity, crop management, and higher ambient temperatures. This indicates that farms are likely to vary in the relative density of pests on overwintering strawberries, but aphids and spider mites are likely to be the most common.

Pest impacts on strawberry yield. Strawberry yield per plant was negatively correlated with increasing aphid accumulation that started in Jan 2023 at TPAC (Fig 2). However, strawberry marketability was not impacted by increased aphid accumulation. This indicates that aphids through their piercing and sucking feeding habit reduced overall plant vigor and productivity early in the season, but their excrement, often called honeydew, did not significantly impact marketability later in the season. By the time we harvested, however, aphid populations were lower than at the beginning of the season. Unfortunately, such an analysis could not be performed for spider mite pressure at SWPAC. Among the unmarketable berries, most were small and malformed that may be an indication of poor pollination or severe tarnished plant bug damage. Indeed, when strawberry flowers were bagged and therefore pollinators were excluded, fruit set was significantly lower compared to unbagged flowers (Fig 3).

Aphid management. We released several biocontrol agents against aphids at TPAC, including Adalia lady beetles, green lacewings, and minute pirate bugs, and none of them had a significant impact on aphid accumulation on strawberry. However, the biorational spray PyGanic appeared to provide 3 weeks of protection against aphids (Fig 4). After 3 weeks, aphid populations started to increase indicating that another application may be necessary. It is important to note that pollinators can be impacted by insecticide sprays, and especially PyGanic, so all insecticide applications should be made carefully when pollinators are not active (i.e., during the evening and early in the spring season if possible). Follow all label instructions and greenhouse application rates if such information is present.