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(BASIC) Mowing Alfalfa Hayfield
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Dietrick Institute for Applied Insect Ecology

Mowing Alfalfa Hayfield Next to Sanders’ Cotton

Excerpts from Kern County 2001 Field Reports

Alfalfa hay is both a resource for pests as well as their natural enemies––a resource for NEC to help cotton. When hay fields mature and are harvested all adult (winged) insects and spiders are forced to find shelter and food in adjacent cotton fields. Lygus is particularly worrisome. There are presently many lygus in the alfalfa that will move at cutting time into cotton and almost immediately (7 to 10 days) back out of cotton as soon as the hay is harvested and watered back. Cotton can sustain low levels of lygus so long as sufficient bolls are setting without damage. The increased plantings of alfalfa in this geographic area will help grow back the trillions of predatory and parasitic insects that used to protect all of California agricultural, particularly in Kern County where one–sixth of the farming was alfalfa was grown for beef.

Much of the research supporting our point of view about the value of unsprayed alfalfa was carried out in this very area of southern San Joaquin Valley in and around Old River Road. Our experience has shown that alfalfa still supports the necessary natural enemies of all pests of vegetable and field crops, especially cotton in this area. We will be closely monitoring alfalfa that is adjacent to cotton to help predict the movement of these insect populations in relation to cotton.

Field Observations 6/7/01:

Alfalfa hay has been cut leaving 7 strips of uncut alfalfa in bloom starting from the south to half way is solid cut. 5 ft. strip swept with net. Each uncut strip is slightly wider going north until the last strip which is a full border. The insects have moved away from the cotton plot migrating into strips, 1 through 7 with increasing population density. (most in #1 and least dense in final N Border(.

D–Vac Samples 6/7/01:

Taken to see where the NEC sought refuge from the stubble field. The cotton was being irrigated in the meantime. The alfalfa will be watered after the second cotton field. It appears that the insects that could fly went to the cotton. The concentration in the seven strips forced them to follow the hay before going to the cotton. We have sampled the cotton and the alfalfa adjacent to the cotton through the first cutting of alfalfa and the cotton is ok so far. The extraordinary numbers of cabbage looper moths observed flying and lesser numbers of bollworm and beet armyworm moths is a concern.

D–Vac Samples 6/13/01:

Our samples of alfalfa have shown naturally occurring adult Trichogramma not to be present except at Sander's unsprayed alfalfa hay. This important parasite normally over–winters on eggs of the painted lady butterfly (this host is common laying eggs all winter on the Malva weeds) and over–summers on alfalfa butterfly eggs. Both of these alternate hosts are present in higher than normal densities around Old River Road than at Sanders' untreated alfalfa, near Maricopa. The difference appears to suggest that the use of CCC (conventional chemical control) in both alfalfa and other crops and the monocultures created by herbicides (the non–availability of few if any flowering plants) in the plantscape as compared to grains and wild sunflowers and blooming alfalfa hay fields that receive no CCC.

Analysis of process of hay cutting on June 13, one week afterward:

Adjacent alfalfa to the south of the plot (Sanders) and other alfalfa hay has been harvested and irrigated. The hay field to the south was cut driving the insects into the unirrigated half of the adjacent cotton where the moths and other insects were congregated; held there by the irrigation process starting on the north side of the cotton plot. There is evidence that tricho releases are best when the cotton is being irrigated since the moths seek the watered cotton. The NEC (natural enemy complex) insects do not all remain in the cotton. Experiments have shown that lygus adults migrate to cotton from cut alfalfa and return to alfalfa when it grows back after irrigation restarts the next alfalfa growth cycle. These insects do some damage since the adults deposit and leave eggs and nymphs but the adults mostly seek alfalfa in preference to cotton and return to alfalfa in 7 to 10 days, depending on the time it takes to start the new growth cycle. There is one generation of lygus per cutting cycle of alfalfa.

It is well known that insect populations move from the cutting process into the adjacent uncut portion in a continuous manner as the field is cut. Thus lygus and the other adult insects can be driven to one end or the other of a field by the cutting pattern or process. The hay harvest at Sanders was one way of many that this farmer devised to drive the insects away from the cotton and at the same time concentrate the insect populations in seven uncut strips at the distant half of the 90 acre field. The first strip left was only a cutting swath wide and the width of each strip of uncut alfalfa increased one swath until the last remaining standing hay was more like a border width. The cutting began on 6/7 and upon our return on 6/13 the uncut strips were finally harvested and all of the bales were removed from the whole field.

The mowing process therefore concentrated the predators on the stubble and within the strips of flowering uncut alfalfa for a time and this favored the predators of the NEC. With less alfalfa for the plant pests to hide in, the preponderance of predators speed up their reproduction and many of the mobile adults find their way to the cotton. The full crop of alfalfa was harvested from the whole field in time to move the sprinklers, ready to irrigate the alfalfa in sequence to other fields on the farm. There was minimum extra effort and no disruption in scheduling of the farming process for the whole ranch and no loss of market quality or yield of hay.

Analysis of effect of hay cutting on June 15, 2001, Sanders:

Essentially, the cutting process has driven all flying insects away from the sample field. The field east of this alfalfa has not been irrigated (start this week). This north end of this check (non–release field) is adjacent to the concentration of insects in the alfalfa. However, this will probably not matter since the lygus like alfalfa better than cotton. Stinkbugs like alfalfa seed and will stay in the alfalfa too. There are no spider mites or aphids and plenty of beneficials to control them as well as the lygus and the other potential pests present in the alfalfa at this time.

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