November 24, 2017

Harvesting and pests

 

Flower cluster with fruit at various stages of maturity.

Flower cluster with fruit at various stages of maturity.

 

We’ve just finished up our 5th week of harvest here on the St Paul site, and it’s been an interesting year so far. It took awhile longer for the plants to start producing at their normal high rates, and total yields this year aren’t what we’re used to. There are multiple reasons for this: 1) the project this year is using less than half the number of plants than we did in 2013-2014, 2) the plants haven’t accrued as many growing degree units at this point in the season that they have before, and 3) pest pressure this year is enormous on the St Paul site. This post will focus on the third point.

Not all insects are unwelcome on our plots. I love seeing pollinators like this bee going to work on my flower clusters.

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But there are some insects who are very detrimental to growers. The first insect pest we dealt with showed up in late July, and is known as the spider mite. Spider mites are actually a blanket name for more than 1,200 species in the family Tetranychidae that display the same characteristics. They hang on the undersides of leaves, disrupt photosynthesis, and when in large enough numbers spin protective silk webs for themselves. Gross.

Underside of strawberry leaf with spider mite web.

Underside of strawberry leaf with spider mite web.

While unsightly, they don’t cause much damage to the fruit themselves and can be controlled easily, even with organic measures. We used an organically approved insecticide that contains pyrethrins. Pyrethrins are organic compounds created by chrysanthemum plants that have insecticidal properties. While effective against spider mites, pyrethrins are fairly unstable and break down easily under UV light. Its wise to spray on dry, late evenings to ensure the compound stays on the plant as long as possible.

The second bug we’re currently dealing with is a much bigger menace- Lygus lineolaris, known as the tarnished plant bug, or TPB. TPBs hang out on flowers as they develop into fruit and use their sucking mouth parts as a straw to drain the fruit of sugars. TPB damage creates ‘cat-faced’ fruit, and severe damage reduces both total and marketable yields by a considerable degree.

Strawberry with TPB damage.

Strawberry with TPB damage.

We’ve dealt with TPB before, but for some reason this year the damage is much more severe. The strawberry producer in me laments this damage, but the strawberry researcher in me has noticed an interesting distinction. While TPB damage is present in both the low tunnel and non-low tunnel treatments, the damage is markedly more severe in the non-low tunnel plants. We’ve observed this trend for the past 5 harvests, and it implies that the low tunnel coverings provide some protection from TPB; an important observation especially for organic growers who have limited insecticidal options.

Two trays of strawberries. Low tunnel fruit on the right, non-low tunnel on the left.

Two trays of strawberries. Low tunnel fruit on the right, non-low tunnel on the left.

In the picture above you can see the fruit harvested under low tunnels (right tray) are much larger and have less TPB damage than fruit harvested without low tunnel protection (left tray). In the next post I’ll share any differences i’ve noticed in yield or quality based on our different fertility treatments.