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).
|Treatment||Foliar Predators (per 50 Sweeps) (2)||Ground Predators (per 2 pitfall traps) (3)|
|Untreated control||68.0 a||330.5 a|
|Foliar Btt (4)||71.0 a||282.3 a|
|Conventional insecticide (5)||23.0 b||119.3 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 stage||DD per stage||Accumulated DD|
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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