Crops and Soils Agent - Fond du Lac County
University of Wisconsin - Extension
Potato Leafhopper (PLH) causes more damage to alfalfa on an annual basis than any other insect pest. This year is starting out to be one of those where damage could be above average. Alfalfa growers will need to stay on top of the situation to prevent significant damage to alfalfa stands. Perhaps now is an appropriate time to reflect on what impact these little critters can 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 have often seen where non-treated alfalfa fields with threshold levels of PLH don't grow with the vigor of similar fields that have been treated for PLH control. 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 have 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. The earlier in the growth cycle that PLH infest a stand, the greater reduction in forage yield if left untreated. 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.
To preserve forage yield and quality, alfalfa growers will need to monitor regrowth from now until about mid-August. Once the visible damage is seen (i.e. hopperburn), the damage has been done and it's typically too late to impart control measures. Treatment thresholds are based on stand height and the number of PLH found per sweep of the net. Most ag chemical dealers can assist you in determining if fields have reached economic threshold levels for treatment. Remember, stands left unattended this year will likely suffer losses in stand persistence and yield in future years.
For more information contact Mike Rankin