Pests affecting Snowdrops01/05/2009
Snowdrops are generally remarkably healthy and trouble free. As a protection against grazing animals, the bulbs and leaves contain poisonous alkaloids and most things seem to leave them alone; including rabbits, deer and geese.
We have found a few things, however, that are potentially serious problems. They are dealt with below and include a summary of our findings and experiences to date.
Moles disturb snowdrop bulbs, breaking up the clumps, pushing some to the surface and inadvertantly distributing others around via their network of tunnels. This is only a minor problem unless it affects valuable bulbs or leads to adjacent stocks becoming mixed. We avoid this by growing all important snowdrops in plunged mesh containers.
If there are moles in an area then the ground will be full of their tunnels. The mole-hills which appear in late winter and early spring normally denote the maintenance and extension of the existing network rather than the sudden appearance of new moles. This is clearly demonstrated whenever we dig a new planting pocket. Within days the new tree or shrub is joined by several mole hills. In reality, the new planting hole has cut through several existing mole tunnels and the mole-hills represent re-instatement of the network and perhaps some gratuitous new burrowing in an area of easy digging.
There are various ways of trapping moles and if you have only a few you may well be successful. Bear in mind, however, that although moles are solitary and territorial, their tunnels are inter-connected. When a mole dies or is killed, the vacant burrows are quickly taken over by adjacent moles as the population re-distributes.
We adjoin mature permanent pasture and young deciduous woodland, both with healthy mole populations. For a long time we waged war on our moles and killed them in large numbers. We did not feel particularly good about this and there was no evidence that we were having any permanent effect on the presence of moles.
Long ago, we learnt to live in peace with our moles; better for them and us. We now plant all important snowdrops in plunged mesh containers, as explained elsewhere on this site.
Pheasants and other birds
Many birds seem attracted by the appearance of white snowdrop buds and flowers and peck them off. Pheasants are our chief culprits but other birds do it as well. Short of shooting all the culprits there is little that can be done.
The problem seems worse early in the season when the few first flowers are making their appearance; Jack Mead, Mrs. Macnamara and Faringdon Double are frequently targetted. If the affected plants are an important variety that you require for seed or a photograph, then a small mesh cage will protect them.
If leafless snowdrop bulbs become spilt, mixed or separated from their label then they have to be potted individually and grown on to allow identification and re-labelling in the next growing season prior to re-unification with their brethren.
We have used labels of all colours and the result is always the same. Birds pull them out and scatter them around. Unlike flower pecking, their interest in this pastime doesn't abate as the season progresses. The culprits are unknown; we have no sparrows, I suspect blackbirds.
There are a number of solutions:-
- Push the label completely below ground level. Make absolutely sure that the label is buried in the pot and not the adjoining ground.
- Use large (25 cm) labels pushed virtually all the way in.
- Make a written plan of the bed in question. This sounds good but is easily confounded if the pots are not lifted in exact sequence or if one or more pots has lost its occupant in the meantime.
- Cover the bed with string or netting.
Two species of insect lay their eggs on or near dormant snowdrop bulbs. The emerging larvae burrow into the bulb and feed on it, causing great damage. We only have experience of large narcissus fly.
Large narcissus flies (Merodon equestris) look like a cross between a bee and a hover fly. They are attracted by chemicals given off by dying snowdrop leaves in much the same way that carrot fly is attracted by the smell of bruised carrot foliage. Egg laying adults are active from late April until June. This pest was unknown to us until recently but is now a serious problem. It would be interesting to know if climate change is increasing the range or frequency of this insect.
As a snowdrop dies back, there is a small tunnel in the soil where the shoot pushed the ground apart as it forced its way to the surface. The insects lay their eggs in this area. The emerging larvae make their way to the dormant bulb and burrow through the base plate, leaving a characteristic entry hole. They eat the inside of the bulb over a number of months before leaving and pupating in the surrounding soil. Besides the entry hole, infected bulbs feel soft and if cut open contain a large greyish maggot.
The larvae are killed by the hot water treatment to which all commercial daffodil bulbs are subjected; for snowdrops growing in a garden it is a different matter. Conventional advice is to rake soil around the dying foliage thus filling in the passage to the bulb and denying the insect access. For us this is not an option; with different varieties with different die-back dates growing close together.
Conventional wisdom also says that plants in sunny locations are more likely to be attacked. This has certainly been our experience. Large established clumps of snowdrops under trees and shrubs are never bothered but in the last growing season the sunniest section of our propogation beds has lost virtually all its plants to this pest.
This year we will be covering all our propogation beds with insect excluding mesh and will report results here in due course.
Experiment with mesh exclusion
The experiment with insect excluding mesh was interesting, informative and ultimately not very successful.
A synthetic fine meshed netting was draped over the propogating beds and on sunny days was covered in considerable numbers of narcissus fly. The insects were very active ( in the presence of the attracting scent presumably) and constantly moving, flying and re-landing in an attempt to gain access to the foliage.
Our difficulty was in ensuring all edges were insect proof and yet requiring access to the plants for various reasons. Short of digging the mesh in on all sides it vwas not possible to achieve this. We have reluctantly turned to chemicals instead. We spray the entire propogating bed once with a systemic insecticide and have had no further losses. We attempt to spray just before the leaves begin to senesce; so that there is time for the chemical to be taken up.