Reference Wisconsin Purple Loosestrife (Lythrum salicaria) Biocontrol. The message below is from Wisconsin Wetland Invasive Plant Program Coordinator Brock Woods. It is intended for all who are rearing beetles this year, and have not yet released them.
If you are not rearing this year, please consider this as a reminder to check any past release sites soon for progress.
The Weather and PL Plants
The weather this year as been very difficult. Average temps early on brought beetles out, but the cool, cloudy conditions slowed Purple Loosestrife (Lythrum salicaria) (PL) plant growth, especially in the North. Thus, your PL plants may not have been ideal size when you added your propagation stock beetles. This is especially the case if you also did not dig your roots as early as possible and/or if you got your plants where you had been putting produced beetles in past years. Plants dug late do not get the warmth and full sun typical of more upland growing sites, and plants continually fed on by adequate beetle populations will not usually grow fast or taller than about three feet. Another issue may be too few stems in any caged pot (5-6 is ideal for 6 foot stems).
Hungry Beetle Larvae
The result of all this may be your larvae consuming your PL plants’ foliage (turning the leaves brown, similar to desiccation) so fast the plants are not able to supply enough food for the larvae to complete their life cycle and become new reproductive adults. Thus, doing nothing now will greatly limit the number of new reproducing adults you will have on your release sites next spring. This will slow or even limit growth for large enough populations to control your local loosestrife!
Releasing the Beetles
Remember — the ideal release timing criteria includes either seeing the first new adult beetles (tan, not dark brown) in your cage(s), or as described above, seeing very quick window-paning of all your plant leaves (loss of green foliage due to heavy larval leaf mining), or both. Only a few new adults suggests most insects are still pupating in the pot’s soil. But they will be out soon, so it’s best to put the pots out –sans net cages. Then leave the pots for a few weeks, so all pupae can emerge in the wetland. If only a few adult beetles have emerged, it is not a good idea to just cut and move the plant tops since this will likely leaves most insects behind.
If your PL plant leaves are turning fast it means little additional food for either larvae or new adults, and they will start to die without it. Immediate action is required, especially when very hot. The best solution is to take the pots out to PL infested wetlands ASAP, and intertwine their tops with field plant tops so the larvae or new adults (that don’t fly for a day or two after pupation) can crawl onto the new stems to feed. Touching field and potted plant tops is crucial if most beetles are still larvae in order to give them food to mature.
If transporting pots is not possible for a time, putting fresh PL cuttings into a cage will help, but cannot be done for long. As a last resort, if there is still some food and new adults are emerging and putting pots out is not possible, the cage tops can be opened to allow flying adults to escape. They will try to find more PL, but most will perish if a long distance flight is necessary.
While early release is not ideal, rest assured you will get new beetles this early, and will allow a careful release. If you get them safely past the early stages of mating, egg laying, eggs and early larval development, your release will be successful. A few beetles will fall to predators without continued protection, but most will likely survive. And you can still put them wherever you need them most.
Call us if you have questions (608-225-5858 best for Brock for now). Thank you for all your efforts!
Wisconsin Purple Loosestrife Biocontrol & Wetland Invasive Plant Programs
Wisconsin Wetland Invasive Plant Program Coordinator
608-266-2554 (FAX: 608-267-2800)
(Transitioning to) PL Biocontrol Program Coordinator!
Also see a PowerPoint presentation by Brock Woods at Taming a “Beautiful Killer:” Wisconsin’s Purple Loosestrife (PL) Biological Control (BC) Program.