ISSUE 7 June 21, 2007
GRASSHOPPER EMERGENCE UNDERWAY
Grasshoppers are emerging and there has been a few reports of increased activity. Time to get out and scout field edges and entire fields of agricultural crops for young grasshopper nymphs. Young grasshoppers are tiny about the size of a wheat kernel. Crop injury consists of leaf stripping and defoliation (cupping when in first instar). Grasshoppers are more easily and economically controlled while they are in the nymph stage and still near hatching sites, such as roadsides and fencerows. Fortunately, we’ve had some heavy rains throughout much of North Dakota. Heavy rains during emergence kills young grasshoppers by drowning and physically washing them away. If cloudy, wet weather continues, natural occurring diseases that attack grasshoppers will be favored. One naturally-occurring fungus disease is called Entomophaga grylli or "Summit disease." It is easy to observe in the field, particularly later in the summer. Grasshoppers have the habit of crawling to the tops of the plants and dying with their heads pointing upwards and legs wrapped tightly around the stalks (see photo).
Grasshoppers become infected from spores which stick to the bodies as they seek food. These spores germinate and penetrate the insect cuticle. The fungus then multiplies in the blood and grows on internal organs. At about the time the grasshopper dies, its body is full of several million resting spores. As the cadaver disintegrates, these resting spores are disseminated on the ground, germinate, and produce more sticky spores, thus spreading the disease. This disease is capable of causing high mortality in grasshopper populations, but these epizootics (outbreaks) are usually sporadic and localized and generally occur late in the season after economic damage has occurred.
DIAMONDBACK MOTHS BLOWN IN FROM SOUTHERNLY WINDS
Recent field observations indicate large numbers of adult Diamondback moth in canola fields in the North Central Region of North Dakota (source: M. Hutter, N. Ag Management).
These moths migrated into North Dakota each year. It’s life cycle takes about 32 days to complete from egg to adult. The adult is small, about ½ inch long, drab brown in color and at rest the forewings of the male moth form three diamonds - hence the name diamondback (see photo). Females lay up to 160 eggs during the night. Eggs hatch in five to six days into pale yellowish-green caterpillars with a forked posterior end. The newly emerged larvae burrow into the leaf and mine the leaf for several days to a week. Then, the larvae exit the leaf and feed externally for another 7 to 14 days. When disturbed, the larvae thrash backwards violently and often drop from the plant on a strand of silk. The larvae pupate for 5 to 15 days in a white net-like cocoon attached to the leaves, stems or pods.
(photo by W. Cranshaw, CO State Univ., Bugwood.org)
The recent heavy rainfalls can drown adult moths and larvae. Humid conditions associated with rainfall can also favor the development of fatal fungal diseases like Entomophthorales. It is always advisable to scout early flowering fields for the number of larvae present. Extensive feeding on the flowers will delay plant maturity, cause the crop to develop unevenly, and significantly reduce seed yield. Insecticide applications are likely required at larval densities of 10-15 larvae per square foot (one to two larvae per plant).
ALFALFA WEEVIL FEEDING CONTINUES
Problems with alfalfa weevil larval feeding continues. In some areas, the alfalfa is cut and trying to dry (weather permitting). Typically, larvae would dessicate and die in the cut alfalfa. However, this year with all the moisture the larvae are continuing to feed underneath the cut alfalfa swath. Spraying an insecticide now will not control the larvae since they are protected. The best advice is to hay the alfalfa as soon as possible and then scout regrowth for any feeding injury. If 4-8 larvae per square foot are present in regrowth, an insecticide application would be advisable. The degree day development indicates that most larvae are close to pupal stage (non-feeding stage) at 596 accumulative degree day (see map). No insecticide is advisable then.
WHEN TO COLLECT LEAFY SPURGE FLEA BEETLES?
There has been several questions about how many degree days are required for collecting leafy spurge flea beetles. The accumulated growing degree days (AGDD) for sunflower (base of 44F) can be used as a guide to determine when to begin scouting for adult flea beetles. Begin scouting for adult flea beetles when the AGDD approaches 1,000. Likewise, the flea beetle population and egg laying by females begins to decline when the AGDD reaches 1,600 or more. Collect flea beetles between 1,200 and the 1,600 AGDD.
Most of North Dakota is above 1,000 AGDD with the southeast region near 1,200 AGDD (see map)Peak emergence of Aphthona spp. flea beetles also corresponds to the flowering of the prairie wild rose and the ripening of garden strawberries in North Dakota. See current map of AGDD in North Dakota. Use the sunflower degree days/growth stage application in NDAWN and enter "2007-03-01" for planting date and select "degree day" for map type.