Wilson, L. M. and C. B. Randall. 2003. Biology and Biological Control of Knapweed. USDA-Forest Service FHTET-2001-07. 2nd Edition.
This guide is intended to assist those who encounter problems when
establishing a biological control program. It identifies the probable
cause of a typical problem and offers solutions.
| Problem |
Probable Cause |
Solution |
| Bioagents unhealthy |
Physical damage to agents |
Prevent containers from colliding; use crush-proof
containers. |
| Drowning |
Do not put water in containers. Prevent acumulation of
excess moisture in the collection containers. |
| Excess or prolonged heat or cold |
Keep containers cool at all times; use coolers and blue ice
packs; avoid exposure to direct sunlight while in transit. |
| Starvation |
Put knapweed foliage (no flowers, seeds, or roots) in
containers. |
| Redistribution time |
Transport or ship agents immediately after collection.
Release agents at new site immediately upon arrival or receipt of agent. |
| Parasitism and/or disease |
Check source agents. Ensure the insect population is
disease-free when collecting or receiving shipment. |
| Number of eggs low |
Agents past reproductive stage |
Collect at times of peak activity (i.e., insects are
mating). |
| Sex ratio: not enough males or females |
Observe mating among bioagents before collecting; males
often emerge earlier than females. |
| Synchrony |
Agents not synchronized with the knapweed growth stage;
bioagnets require knapweed to be at specific growth stage for optimal
oviposition. |
| Few bioagents collected |
Wrong method used |
Refer to Tables 11 and 12 for recommended collection time
and technique. |
| Collection done at wrong time |
Refer to Tables 11 and 12 for recommended collection time
and technique. |
| Collection technique |
Bioagents can be killed during sweeping or aspirating. Use
vacuum aspirator if aspirating by mouth is not working.
Practice sweeping. |
| Conditions at time of collection wrong |
Refer to "Collecting Biocontrol Agents" and "Monitoring
Biocontrol Agents" for guidelines on desirable weather conditions. |
| Agents not found after release |
Site is unsuitable |
Refer to "Collecting Biocontrol Agents." |
| Site too small |
Select a larger
site with a dense, uniform stand of knapweed. |
| Pesticide used in area |
Select pesticide-free site. |
| Cannot locate release site |
Permanent location marker not obvious |
Use bright-colored wooden, metal, or plastic stake. |
| Map poorly or
incorrectly drawn |
Check map; redraw with more detail or add landmarks. |
