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Forages & Grains
Volume 59 Number 7 Date 06/12/2014 ALFALFA WEEVIL - Larval counts remain low in both first and second crop alfalfa. The average this week was 0.3 per sweep (or 13 per 50 sweeps) in 48 alfalfa fields sampled from Iowa County in the southwest to Calumet County in the east-central area. Leaf tip feeding was below 25% in all surveyed fields, but failure to harvest the first cutting in the week ahead could result in avoidable damage by the larger and more destructive third- and fourth-instar larvae. Alfalfa should be regularly scouted through harvest and until new growth of the second crop is established. POTATO LEAFHOPPER - Surveys in the southern and east-central areas yielded very low counts of 0-0.1 leafhoppers per sweep. Economic populations of 1.0 per sweep for alfalfa 8-11 inches and 2.0 per sweep for alfalfa 12 inches or taller have not been detected in any field sampled as of June 11. Nymphs were collected from two of 48 sites surveyed this week. ALFALFA BLOTCH LEAFMINER - The distinctive comma-shaped leaf mines caused by this insect were noted on 2-10% of alfalfa trifoliates in a few alfalfa fields in Iowa, Juneau, Monroe, Richland and Sauk counties. These percentages are very low in comparison to the economic threshold of 30-40% of leaves with pinholes or mines. PLANT BUG - Nymphs have become more abundant than the adults in most fields. Combined counts of the tarnished and alfalfa species range from 0.2-1.5 per sweep and average 0.6 per sweep, a noteworthy increase over 0.2 per sweep last week but still well below the 5.0 per sweep threshold for plant bugs in alfalfa. The high count of 1.5 per sweep was found near Random Lake in Sheboygan County. The tarnished plant bug continues to be the more common of the two species. PEA APHID - Densities have increased sharply since early June and counts now vary from 0.2-19.5 per sweep. The average is about 2-3 per sweep. Counts of this level are considered non-economic in alfalfa, but aphid populations can escalate rapidly when natural enemies are eliminated by alfalfa weevil insecticide sprays. Harvesting fields on time, thus reducing the need for insecticidal control, is important for preserving natural enemies. -- Krista Hamilton, DATCP Entomologist 
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