FAQ: Pests and Diseases

Are aphids a concern when cutting lucerne for hay?

Aphids are not a concern when cutting hay but….don’t feed that hay to ovulating stock in autumn – see our previous blog onovulation for details. Aphids at high numbers can make the lucerne unpalatable to stock and elevate phytoestrogen levels so beware of the end use of the hay. Also, as they are cut the aphids can drop onto the newly emerging shoots and significantly reduce the regrowth of the following crop through direct damage and the introduction of a virus. If levels are damaging an insecticide spray is recommended.

Does wireworm affect lucerne?

We don’t know! We have searched our resources and asked some of the local entomologists and none can remember ever seeing wireworm attack a lucerne stand. But it is highly likely they will attack young seedlings because they are not that selective at feeding so will eat young seedlings of most things. My entomologist tells me seed treatment is likely to take care of the problem but he’s not sure! So we get two out of 10 for that answer – sorry we can’t be more authoritative.

How does clover root weevil affect lucerne?

Clover root weevil (Sitona lepidus) has minimal impact on lucerne as far as we know – work at AgResearch has indicated a preference of the root weevil for white, red and Caucasian clovers with minimal impact on lucerne. Just another advantage of using the world’s most important pastoral legume.

In contrast, Sitona weevil (Sitona discoideus) does affect lucerne and control was achieved in the 1980s by the introduction of a parasitic wasp. This has probably been the most successful biological control programme developed in NZ. Occasional outbreaks of Sitona weevil can occur but like most insect outbreaks they tend to follow a boom bust cycle and the parasite keeps numbers to levels below an economic threshold most of the time.

The third weevil to watch is white fringe weevil (Naupactus leucoloma) – this can cause economic damage and there is no chemical control option. For white fringe weevil cultural control methods are avoiding grazing until last in a rotation (they move attached to wool and machinery), direct drilling grass into infested paddocks (it reduces their reproductive ability) and cultivating starlings (a less popular biological control agent).

What is the cause of yellow leaves with purple black dots over a whole young lucerne plant?

Without seeing the plant it is difficult to know. However, if it is a single plant in the stand it is likely to be insect damage whereas if if it covers an area of the paddock it could be a nutrient deficiency. It may also be a fungāl issue which typically occurs in warm and humid conditions. Sending the sample to Plant Diagnositics Ltd could give a diagnosis.

One of my established lucerne paddocks has come up very patchy this year. What could have caused this to happen?

It is difficult to know what has caused the problem without seeing it – there are several reasons.

It could be eel worm also known as stem nematode (Ditylenchus dipsaci). If so it will eventually take over the paddock – (over the next few years rather than in a season). This would be most common if you have sown ‘Wairau’ as the cultivar. Most of the other cultivars (e.g. ‘Kaituna’) have been bred with resistance. For a confirmation of this you could take a sample and send it to Plant Diagnostics Ltd.

If it is stem nematode there is not much you can do about it. Try to avoid making hay from this paddock and carting that hay around – the nematodes live inside the stems and there is no viable chemical control which is why resistance is bred into them. The symptoms are usually short stunted internodes (which you seem to have with the stems shorter than the rest of the paddock) in patches and they tend to spread in the direction of any water movement and out in a circular pattern.

Other causes of patchy growth in paddocks that previously grew well include if this is where water ponded during the winter and there is some soil compaction that is restricting root growth or has caused some death of the root systems.

As for slugs – yes they can be a problem mostly in a wet spring and especially if the paddock was direct drilled. Slug bait should take care of the problem.