Vegetable Crops News

Timing Bacillus thuringiensis var. tenebrionis Applications for Colorado Potato Beetle Control on Potato

The Colorado potato beetle (CPB) has become the major pest of potatoes in many production areas over the past decade. Biorational insecticides, such as those derived from Bacillus thuringiensis var. tenebrionis (Btt), offer unique advantages for CPB management. The pest-specificity of Btt enables growers to effectively manage CPB populations without disrupting the broad complex of arthropod natural enemies associated with potatoes (Table 1). This compatibility with natural biological control significantly increases the potential for biological regulation of other pest insects, such as aphids, in the system.

Table 1. Natural enemy populations in Russet Burbank potatoes treated with foliar Btt and conventional insecticides, Hancock, Wisconsin, 1993 (1).
TreatmentFoliar Predators (per 50 Sweeps) (2)Ground Predators (per 2 pitfall traps) (3)
Untreated control68.0 a330.5 a
Foliar Btt (4)71.0 a282.3 a
Conventional insecticide (5)23.0 b119.3 b
(1) Means followed by the same letter are not significantly different at the 5% level.
(2) Includes lady beetles, green lacewings, nabid bugs, and minute pirate bugs.
(3) Includes ground beetles, rove beetles, harvestmen, and spiders.
(4) M-Trak , 4 applications.
(5) Ambush , 4 applications; Monitor, 1 application.

Btt foliar sprays are most effective against first and second instar larvae. Foliage consumption by first and second instar larvae is minimal, so targeting small larvae will also avoid significant feeding damage. Since persistence of Btt is very short (1 to 2 days), applications must be precisely timed to target the vulnerable stages to achieve the maximum potential. Effective crop scouting can pinpoint the onset of egg laying in the spring, and day degree accumulation can be used to predict egg hatch and larval development for timing Btt sprays (Table 2).

Table 2. Colorado potato beetle development (Fahrenheit scale, 52F base).
Developmental stageDD per stageAccumulated DD
Eggs120120
1st65185
2nd55240
3rd60300
4th100400
Pupae275675

When eggs are first discovered by scouts, day degree accumulations should begin. The first spray should be applied at 150 day degrees (DD): 120 DD for egg hatch + 30 DD for larval development. Since egg laying is likely to continue for two or three weeks, additional sprays at 5-7 day intervals will ensure that no larval development past second instar will occur without exposure to Btt. Day degrees accumulate more rapidly in warm weather and shorter spray intervals will be necessary under these conditions. In situations where growers are unable to calculate accumulated day degrees, the first eggs found should be flagged and revisited daily until 30% of the eggs have hatched. The first spray should be applied at this time. If effective larval control is achieved in the first generation, damage from second generation adults will be minimal, resulting primarily from beetles flying into fields.

Btt is toxic to CPB larvae only when ingested and thus complete plant coverage with the spray is important to maximize efficacy. When high populations are present, ground application at shorter intervals with higher gallonage, increased pressure, and spreader-stickers may be required to achieve effective control.

- Jeff Wyman, University of Wisconsin-Madison

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