| The economic threshold values are taken from
the "Pennsylvania Agronomy Guide" and "Field Crop Scouting Manual" of
the University of Illinois Cooperative Extension.
|
Table 16. Economic Threshold Values for Insects That Attack Alfalfa in Pennsylvania
| Pest Species | Abbreviation | Economic Thresholds |
| Alfalfa blotch
leafminer Agromyza frontella |
(ABL) | 30% or more of the leaflets have a mine. |
| Alfalfa weevil
(larva) Hypera postica |
(AW) | First crop growth period
- See Table 17 for economic thresholds based on crop height, crop value and insecticide
cost. Stubble after first cutting - 2 or more larvae per crown. Note: If the majority of weevils have formed cocoons their feeding is near completion for the season. |
| Alfalfa weevil
(adult) Hypera postica |
5 or more larvae per crown and leaf damage is evident during early to mid-April | |
| Clover leaf weevil Hypera punctata |
(CLW) | Usually not of economic importance |
| Clover root curculio Sitona hispidula |
None developed, but economic losses do occur from the pest. This pest is difficult to manage because its life cycle is similar to alfalfa weevil and an insecticide application aimed at controlling the adult stage would kill parasites of the alfalfa weevil leading to an outbreak of that pest. | |
| Cutworm (Black) Agrotis ipsilon |
(BCW) | None established, but new seedlings have the greatest potential for injury. |
| Cutworm (Variegated) Peridroma saucia |
(VCW) | None established, but new seedlings have the greatest potential for injury. |
| Grasshoppers Melanoplus spp. |
(GH) | No economic thresholds have been
established for GH during the growing season. It is estimated that an infestation of
17 grasshoppers per square yard can consume one ton of hay per day in a 40 acre field.
Grasshopper scouting is used to assess the potential for injury to the crop the following
growing season. The following guidelines can be used: 0 to 3 GH per square yard - noneconomic to light. 4 to 6 GH per square yard - light 7 to 14 GH per square yard - threatening 15 to 28 GH per square yard - severe 29+ - very severe |
| Meadow Spittlebug Philaenus spumarius |
(MSB) | One or more spittle mass per stem in mid-May. |
| Pea aphids Acyrthosiphon pisum |
(PA) | 100 or more per sweep of an insect net and beneficials are not present in high numbers. |
| Plant bugs | None established | |
| Alfalfa
plant bug Adelphocoris lineolatus |
(APB) | Not usually at high enough population levels to cause economic damage to the crop. |
| Tarnished
plant bug Lygus lineolarius |
(TPB) | |
| Rapid plant
bug Adelphocoris rapidus |
(RPB) | |
| Potato leafhopper Empoasca fabae |
(PLH) | Established Stands
- See Table 18 for economic thresholds based on crop height, crop value and insecticide
cost. New Seedlings - None established. As a guideline use one half the value in the table for established stands. |
| Spotted Alfalfa
Aphid Therioaphis maculata |
(SAA) | 20 or more aphids per stem. Note: This aphid is capable of injecting a toxin into the plant that causes chlorisis and wilting. If high numbers of beneficials are present, then they will most likely control the pest without the aid of an insecticide treatment. |
| The economic threshold values are taken from
the "Pennsylvania Agronomy Guide" and "Field Crop Scouting Manual" of
the University of Illinois Cooperative Extension.
|
