March 15, 2008
Biocontrol of black vine weevil in cool temperatures
By Jen Llewellyn,
OMAFRA Nursery Crops Specialist
Back in the June edition of my column for Horticulture Review, I wrote about using entomopathogenic nematodes for reducing populations of black vine weevil larvae in soils and container media (biological control). You may remember that we did a warm temperature media trial and had excellent success with Heterohabditis species nematodes. Just before Christmas, we carried out a short trial, only under cool media temperatures. We wanted to see just how the Steinernema species performed, since they are sometimes marketed as being cold hardy and active under cooler temperatures than 10-12 C.
We chose three readily available species of beneficial nematodes and compared them to a drench with plain water (Control). The nematode species we used were Heterohabditis bacteriophora (Hb, B-Green® from Biobest), Heterohabditis megidis (Hm, Nemasys H from Becker Underwood),) Steinernema kraussei (Sk, Nemasys L from Becker Underwood) and another Steinernema species that hasn’t been named yet. We collected populations of late instar black vine weevil and placed 10 healthy larvae in each one-gallon pot with newly potted Heuchera plugs. The media was a peat-bark mixture (pH 5.15 to 6, EC 619 micro Siemens). Each treatment was replicated over 10 pots. All nematode solutions were examined under a stereoscope for viability prior to application. The expiry date was several weeks away for all species. The nematodes were applied at a rate of 50 million per 100 m2 (which is equal to about 4,700 one-gallon pot surfaces). The nematode solutions were applied at dusk and pots were kept moist in a storage area that received minimal natural light (with media temperatures ranging from 7 to 8 C) for two weeks, followed by analysis.
If we assume that Steinernema species (e.g. Nemasys L, Steinernema kraussei) work well to suppress populations of black vine weevil, but that it takes eight weeks for them to actually kill a significant amount of larvae, this information can help us make better decisions about using these nematode products. If you have a container nursery crop, chances are you are going to want quick knock down because: 1) the larvae affect saleability and 2) larval feeding can have a very significant effect on plant health in a pot in a short period of time. So, for container nursery crops, Heterohabditis species nematodes applied during warm soil temperatures (≥10-12 C) are going to give excellent knock-down of larvae within two weeks (or less). But for field production nursery crops and landscape plantings, where black vine weevil populations can build up over time, Steinernema species can be a blessing. Because we can apply Steinernema species at cooler soil temperatures (i.e. earlier in the spring), soil moisture status will be higher and we won’t have to go back and irrigate to maintain the soil moisture status. Plants in the landscape will likely be able to tolerate infestations of weevil larvae, especially if they are established.
Jen Llewellyn can be reached at (519) 824-4120, ext. 52671 or by e-mail at jennifer.llewellyn@ontario.ca
OMAFRA Nursery Crops Specialist
Back in the June edition of my column for Horticulture Review, I wrote about using entomopathogenic nematodes for reducing populations of black vine weevil larvae in soils and container media (biological control). You may remember that we did a warm temperature media trial and had excellent success with Heterohabditis species nematodes. Just before Christmas, we carried out a short trial, only under cool media temperatures. We wanted to see just how the Steinernema species performed, since they are sometimes marketed as being cold hardy and active under cooler temperatures than 10-12 C.We chose three readily available species of beneficial nematodes and compared them to a drench with plain water (Control). The nematode species we used were Heterohabditis bacteriophora (Hb, B-Green® from Biobest), Heterohabditis megidis (Hm, Nemasys H from Becker Underwood),) Steinernema kraussei (Sk, Nemasys L from Becker Underwood) and another Steinernema species that hasn’t been named yet. We collected populations of late instar black vine weevil and placed 10 healthy larvae in each one-gallon pot with newly potted Heuchera plugs. The media was a peat-bark mixture (pH 5.15 to 6, EC 619 micro Siemens). Each treatment was replicated over 10 pots. All nematode solutions were examined under a stereoscope for viability prior to application. The expiry date was several weeks away for all species. The nematodes were applied at a rate of 50 million per 100 m2 (which is equal to about 4,700 one-gallon pot surfaces). The nematode solutions were applied at dusk and pots were kept moist in a storage area that received minimal natural light (with media temperatures ranging from 7 to 8 C) for two weeks, followed by analysis.
No effect in cool temperatures
At cool media temperatures, kept constant at 7 to 8 C, none of the nematode species were effective at killing black vine weevil larvae and none were significantly better than the water control. When I contacted the suppliers about the performance of Steinernema species under the cool temperatures, they said that at least six to eight weeks would be required for Steinernema to infect and kill the weevil larvae. They also hypothesized that some of the apparently “healthy” recovered weevil larvae might have actually been infected with Steinernema nematodes, but that it would take several more weeks for the weevil larvae to die under those cool temperatures. I have found it to be difficult to recover weevil larvae two weeks after treatment. Larvae that die tend to rot and become unrecoverable. However, under cooler soil temperatures, the rotting process may be a little slower. I think I’ll stop there, just in case you are eating your lunch.If we assume that Steinernema species (e.g. Nemasys L, Steinernema kraussei) work well to suppress populations of black vine weevil, but that it takes eight weeks for them to actually kill a significant amount of larvae, this information can help us make better decisions about using these nematode products. If you have a container nursery crop, chances are you are going to want quick knock down because: 1) the larvae affect saleability and 2) larval feeding can have a very significant effect on plant health in a pot in a short period of time. So, for container nursery crops, Heterohabditis species nematodes applied during warm soil temperatures (≥10-12 C) are going to give excellent knock-down of larvae within two weeks (or less). But for field production nursery crops and landscape plantings, where black vine weevil populations can build up over time, Steinernema species can be a blessing. Because we can apply Steinernema species at cooler soil temperatures (i.e. earlier in the spring), soil moisture status will be higher and we won’t have to go back and irrigate to maintain the soil moisture status. Plants in the landscape will likely be able to tolerate infestations of weevil larvae, especially if they are established.
Jen Llewellyn can be reached at (519) 824-4120, ext. 52671 or by e-mail at jennifer.llewellyn@ontario.ca