Potato Leafhopper
...............post-mortem

Now that the summer is over, the battle with the 1997 flock of potato leafhoppers (PLH) has all but come to a close. A large number of acres were treated for threshold populations and an even larger number were not. For those who chose to treat fields, it was an unneeded expense. For those who did not treat, there were some consequences. Perhaps now is an appropriate time to reflect on what impact these little critters have had and will have on the alfalfa crop ---- both harvested and in the field.

How the PLH does its damage

Recall that the PLH feeds on alfalfa by inserting its stylet into the alfalfa stem or leaf. At this point, the PLH starts sucking sap (carbohydrates) out of the plant like a dehydrated construction worker sucking a fountain drink at the corner convenience store. Research has determined that saliva from the PLH causes the phloem tissue (the plant's pipeline for moving carbohydrates from the leaves to the roots) to compress and restrict flow within the tubes. It's thought that stem feeding, and the subsequent build-up of leaf sugars because of phloem tissue closure, is actually more responsible for the leaf yellowing (hopperburn) than the leaf feeding.

Impact on plant growth and persistence

Heavy infestations of PLH can impact both plant growth and persistence. In one study at Iowa State University, PLH-infested field plots matured 30% slower than uninfested plots (Hutchins and Pedigo, 1990). Indeed, we heard many times this past growing season the complaint that alfalfa fields just didn't seem to be growing or the observation that treated fields were much taller than those not treated. However, research has also documented that the shorter, PLH-damaged plants tend to have a higher leaf-to-stem ratio. Damaged plants will have both reduced rates of nodal development and stem elongation.

Just as important as the above ground part of the plant, is the below ground portion. Several studies have looked at the impact of PLH feeding on the alfalfa plant's root system. In a Wisconsin trial (Hogg et al., 1997), PLH was either controlled or left untreated in the establishment year and the residual effects were measured for two years afterward. Both at Arlington and Hancock, root weights were significantly lower for untreated plots at the end of the establishment year and for each of the two subsequent years. In a Purdue University trial, researchers measured the impact of PLH on carbohydrate root reserves (Shaw and Wilson, 1986). They found significantly lower concentrations of carbohydrates in untreated plots versus those where PLH had been controlled. In this same study, measured regrowth after harvest was shorter for the PLH infested plots. Although smaller roots and lower carbohydrate reserves don't guarantee a loss of persistence, they most certainly don't help the cause in years where other stresses like poor winter/spring survival environments, diseases, and intensive cutting schedules are working against alfalfa plant survival. Even where it is not a case of significant or measurable stand loss, research has shown PLH damaged stands may suffer in terms of yield potential, stand vigor, and longevity.

Impact on forage yield

Many producers had first-hand experience with the negative impact of PLH on forage yields. How much impact depends largely on the level of infestation (how many) and when the little guys moved into the stand. Based on these two factors, research studies have validated yield losses from 0 to 95%. This doesn't include subsequent yield reductions in cuttings and years following the initial infestation.

Impact on forage quality

The impact of PLH on forage quality is interesting and somewhat surprising if you think it's all bad. First, recall that damaged stands are delayed in maturity, shorter, and have a higher leaf to stem ratio. These factors may actually be positive contributors to forage quality. In an Iowa State University study (Hutchins et al., 1989), PLH-damaged whole plants and plant parts (leaves and stems) were analyzed for several forage quality parameters. Some of their findings included: 1) PLH feeding had little effect on whole plant or plant part in-vitro digestibility, 2) PLH feeding had little effect on whole plant neutral detergent fiber (NDF) concentrations but there was a trend toward lower stem NDF% and a higher leaf NDF%, and 3) PLH feeding reduced both whole plant and leaf crude protein (CP) concentrations but tended to raise stem CP%. Based on this study and others, the greatest forage quality impact that PLH feeding may have is when measured on a nutrient yield basis (pounds per acre) rather than on a concentration basis.

Management considerations

For alfalfa stands subjected to PLH feeding in 1997, those also subjected to additional stresses (i.e. diseases and environmental stresses) will suffer the greatest loss in terms of stand persistence and yield reductions in 1998 and beyond. Fields with uncontrolled PLH feeding this year are very risky candidates for any type of a fall cutting regime. Although producers shouldn't see major differences in forage quality from 1997 PLH pressure, alfalfa from some heavily infested fields may be lower in CP% than non-damaged fields and dairy rations should be adjusted accordingly.