Alfalfa weevil larvae are present in many south Missouri fields with some alfalfa fields requiring insecticide applications for control of weevil larvae. Producers in southern and central Missouri counties are encouraged to scout for weevils and damage at this time. Alfalfa weevil larvae grow through four instars or worm stages as they develop into adults. First observed damage from this pest is usually seen as small feeding holes in alfalfa leaflets caused by initial feeding of first instars inside plant terminals. As the leaf material grows out of the plant terminal, the damage becomes visible. Second through fourth instars feed directly on leaf tissue with defoliation increasing as the size of larvae increase. Although most larvae in southern Missouri are small to medium in size, economic damage has resulted in the need for use of control measures such as an insecticide application. Damage in more northern parts of Missouri are expected as eggs continue to hatch and larvae develop. Although numbers of alfalfa weevil eggs are high in most areas of the state, high egg numbers do not always result in high numbers of larvae or heavy yield loss of alfalfa. In years with cool, wet springs an insect fungal pathogen, Zoophthora phytonomi, often infects and quickly kills alfalfa weevil larvae. Recent rainfall and cool nighttime temperatures throughout the state certainly increase the potential for the development of this fungal pathogen. Infect alfalfa weevil larvae change from their normal green to more yellow in color, become slowed in movement, and generally die within 2-3 days after infection occurs. At present the fungal pathogen has not been found infecting alfalfa weevil larvae in the state.

Scouting for alfalfa weevil is accomplished by randomly collecting 50 alfalfa stems (10 stems at 5 different locations) and tapping them into a white bucket. Larvae will generally be dislodged by this action and allow for an average number of larvae per alfalfa stem to be calculated. Caution should be used when collecting stems as larvae can be easily dislodged from the growing tip of the plant stem by rough handling. It is recommended that the top of the alfalfa stem be cupped in one hand while the plant stem is removed by cutting with a knife near the base of the stem. If an average of one or more larvae per stem is found (50 or more larvae per 50 stems), then the economic threshold has been reached and control is justified.

Management Options
The main management option for early infestations of alfalfa weevil larvae on small alfalfa is an application of a labeled insecticide. Early harvest of the alfalfa by either machine or livestock may be viable options for some producers in Missouri. If early harvest of alfalfa by machine is selected as a control strategy, then the crop is harvested approximately 7-10 prior to the normal plant growth stage of 1/10^nth bloom. Data from a Missouri study indicate that alfalfa weevil larval numbers may be reduced by about 98 percent with mechanical harvest and about 90 percent by cattle grazing in a management intensive grazing system. Producers using grazing as a control strategy must be aware of the bloat risk to cattle grazing green alfalfa and risk to the alfalfa stand due to trampling during wet conditions. If an insecticide application is selected, a list of insecticides recommended for alfalfa weevil control follows. Rates are given as amount of product applied per acre. The pre-harvest interval lists the minimum number of days before harvest that an insecticide application can be applied. If an insecticide application is selected, a list of insecticides recommended for larval alfalfa weevil control follows. Be sure to read and follow all label directions, restrictions and precautions.

Insecticides recommended for alfalfa weevil larval control on Alfalfa
Chemical Name	Product Name	Rates Amount of Product	Preharvest Interval (PHI)
Cyfluthrin	Baythroid 2*	1.6 to 2.8 fl oz/acre	7 days
Methyl Parathion	Chemnova Methyl 4EC*	See label rates	15 days
Carbofuran	Furadan 4F*	1/2 to 2 pts / acre	7 - 28 days depending on rate
Phosmet	Imidan 70W*	1.3 lbs. / acre	7 days
Chlorpyrifos	Lorsban 4E*	1 to 2 pts / acre	14 - 21 days
Chlorpyrifos	Nufos 4E*	1 to 2 pts / acre	14 - 21 days
Chlorpyrifos	several formulations*	See specific labels	See specific label
Malathion	Malathion	1.25 lb a.i. / acre	See specific label for rates/PHI
Zetacypermethrin	Mustang Max*	2.24 to 4.0 fl oz / acre	3 days
Permethrin	Pounce 3.2EC*	See label rates	Variable results in MO trials
	Ambush 2E*	See label rates	Variable results in MO trials
Gammacyhalothrin	Proaxis*	2.56 to 3.84 fl oz / acre	1 day forage, 7 day hay
Lambdacyhalothrin	Warrior*	2.56 to 3.84 fl oz / acre	1 day forage, 7 day hay
Lambdacyhalothrin	Several formulations*	See specific labels	See specific labels
* indicates Restricted Use
a.i. = active ingredient

Scattered Problems With Cowpea And Pea Aphids In Alfalfa Have Been Reported From Southwest Missouri
The cowpea aphid is a dark colored to black aphid which was first found in Missouri in the early 1990s. This insect tends to feed on the tips of alfalfa during early spring and can cause yellowing of plant leaflets from the bottom upward. Although no formal economic threshold is available, the one used for pea aphid would be a good starting point. If an average of 50 or more aphids per alfalfa stem are present, then control may be justified. If plants are under drought stress or growing slowly due to cool weather, then the threshold number would be reduced. Treatment also may be justified if plants are yellowing and aphids are present. The pea aphid is larger, green in color, and can be identified by a dark band around the base of the antennal segments. Pea aphid problems are most severe on slow growing alfalfa during early spring. Later infestations of the pea aphid during spring may cause economic problems, but generally plants 10 inches or more in height can withstand higher numbers of aphids. As with cowpea aphids, pea aphids can cause yellowing and sometimes wilting of plants due to their removal of plant juices. The tables that follow list insecticides labeled for control of these pests in Missouri. Be sure to follow all label directions, precautions, and restrictions.

 

Insecticides recommended for cowpea aphid control on alfalfa
Chemical Name	Product Name	Rates (Amount of Product)	Preharvest Interval (PHI)
Cyfluthrin	Baythroid 2*	2.8 fl oz / acre	7 days
Methyl Parathion	Chemnova Methyl 4EC*	See specific label	15 days
Chlorpyrifos	Lorsban 4E*	1 to 2 pts / acre	14 - 21 days
Chlorpyrifos	Nufos 4E*	1 to 2 pts / acre	14 - 21 days
Chlorpyrifos	Several formulations*	See specific labels	See specific label
Malathion	Malathion	1 lb a.i. / acre	See specific label for rates/PHI
Zetacypermethrin	Mustang Max*	2.24 to 4.0 fl oz / acre	3 days
Gammacyhalothrin	Proaxis*	2.56 to 3.84 fl oz / acre	1 day forage, 7 day hay
Lambdacyhalothrin	Warrior*	2.56 to 3.84 fl oz / acre	1 day forage, 7 day hay
Lambdacyhalothrin	Several formulations*	See specific labels	See specific labels
* indicates Restricted Use
a.i. = active ingredient

Insecticides recommended for pea aphid control on alfalfa
Chemical Name	Product Name	Rates (Amount of Product)	Preharvest Interval (PHI)
Cyfluthrin	Baythroid 2*	2.8 fl oz / acre	7 days
Methyl Parathion	Chemnova Methyl 4EC*	See specific label	15 days
Dimethoate	Dimethoate; Dimate	See specific label	See specific label
Chlorpyrifos	Lorsban 4E*	1 to 2 pts / acre	14 - 21 days
Chlorpyrifos	Nufos 4E*	1 to 2 pts / acre	14 - 21 days
Chlorpyrifos	Several formulations*	See specific labels	See specific label
Malathion	Malathion	1 lb to 1.25 a.i. / acre	See specific label for rates/PHI
Zetacypermethrin	Mustang Max*	2.24 to 4.0 fl oz / acre	3 days
Gammacyhalothrin	Proaxis*	2.56 to 3.84 fl oz / acre	1 day forage, 7 day hay
Lambdacyhalothrin	Warrior*	2.56 to 3.84 fl oz / acre	1 day forage, 7 day hay
Lambdacyhalothrin	Several formulations*	See specific labels	See specific labels
* indicates Restricted Use
a.i. = active ingredient

Wayne Bailey
573/882-2838 office
or 573/864-9905 cell