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Squash Vine Borer Activity Estimates: April 29, 2022

Originally Submitted: April 29, 2022

Squash vine borers can be a serious garden pest in South Dakota. If left unmanaged, their activity can cause a rapid decline in the health and productivity of zucchini, squash, pumpkins and other winter squash. Squash vine borer moths emerge from the soil after the accumulation of approximately 1000 degree days. In South Dakota, this means we typically see squash vine borer moth activity in late June and early July (Table 1). At this point in time, no immediate squash vine borer activity is expected in South Dakota (Table 2).

Predicting Squash Vine Borer Activity With Degree Days

The emergence of squash vine borer moths can be estimated by using degree days with a developmental threshold of 50°F. As mentioned previously, the squash vine borer moths emerge from the soil when 1000 degree days accumulate. Once eggs are laid on the stems of host plants, they require another 245-300 degree days to hatch. After hatching, the new larvae quickly enter the squash stem. The larvae will feed inside the plant for another 660 degree days. The larvae will then emerge from the plants and burrow into the soil to pupate. The pupae require an additional 745 degree days before the next moths emerge. We don’t normally see a second generation of squash vine borers in South Dakota, but it is possible.

As a reminder, the equation for degree days is:

(Maximum daily temperature + Minimum daily temperature) ÷ 2 - The Developmental Threshold

Table 1. Key for squash vine borer activity
based on accumulated degree days.

Accumulated Degree Days Squash Vine Borer Activity
1000 Moths will emerge from soil
1245-1300 Eggs will hatch
1905-1960 Caterpillars will feed within plants
2650-2705 Possible second generation of moths emerge

Table 2. Accumulated squash vine borer degree days
for selected locations South Dakota.

Location
Accumulated Degree Days
Since January 2022
Aberdeen
84
Brookings
118
Buffalo
135
Cottonwood
246
Faith
155
Gettysburg
105
Hot Springs
345
Huron
156
Lemmon
122
Mission
279
Mitchell
191
Newell
186
Pierre
222
Rapid City
252
Selby
81
Sisseton
23
Tyndall
253
Vermillion
173
Winner
272