Journal of the Department of Agriculture, Journal of the Department of Agriculture, Western Australia, Series 4 Western Australia, Series 4
Volume 4
Number 10 October, 1963 Article 2
1-1-1963
The webworm. 4. Control of the webworm The webworm. 4. Control of the webworm
J A. Button
Follow this and additional works at: https://researchlibrary.agric.wa.gov.au/journal_agriculture4 Part of the Agronomy and Crop Sciences Commons, and the Entomology Commons
Recommended Citation Recommended Citation
Button, J A. (1963) "The webworm. 4. Control of the webworm," Journal of the Department of Agriculture, Western Australia, Series 4: Vol. 4: No. 10, Article 2.
Available at: https://researchlibrary.agric.wa.gov.au/journal_agriculture4/vol4/iss10/2
This article is brought to you for free and open access by the Agriculture at Digital Library. It has been accepted for inclusion in Journal of the Department of Agriculture, Western Australia, Series 4 by an authorized administrator of Digital Library. For more information, please contact library@dpird.wa.gov.au.
The webworm. 4. Control of the webworm The webworm. 4. Control of the webworm
Cover Page Footnote Cover Page Footnote
The work described here is part of a general investigation of the webworm problem in Western Australia, financed by a grant from the Wheat Research Committee of Western Australia. This assistance is gratefully acknowledged.
This article is available in Journal of the Department of Agriculture, Western Australia, Series 4:
https://researchlibrary.agric.wa.gov.au/journal_agriculture4/vol4/iss10/2
4. Control of the Webworm
By J. A. BUTTON, B.Sc. (Agric), Entomologist
E
ARLIER articles in t h i s series have provided a broad oatline of t h e biology, life history a n d ecology of t h e webworm. In this article chemical a n d cultural methods of control a r e considered in relation to this background information.INSECTICIDES
Most caterpillars can be controlled with DDT, and t h e webworm is no exception.
Experience over m a n y years h a s shown t h a t DDT gives consistently good control of webworm w h e n properly applied.
No alternative m a t e r i a l to DDT can be recommended a t this stage. Dieldrin and other chlorinated hydro-carbon insecti- cides are less satisfactory. Most of t h e o r g a n o p h o s p h o r o u s insecticides such as malathion, have insufficient residual action and the systemic insecticides have not given adequate protection when used as a seed dressing or as a spray.
When to Use Insecticides
The indiscriminate use of insecticides should always be avoided.
Chemical sprays should only be applied when it is anticipated t h a t t h e loss by insect damage will exceed t h e cost of treatment. I n m a n y instances this loss may be quite low (it m a y for example be only the equivalent of 1 bushel of wheat.)
The calculation of how m u c h increase in yield would be required in order to defray the cost of t r e a t m e n t is s t r a i g h t - forward. It is m u c h more difficult to decide how much d a m a g e is likely to occur if the treatment is n o t applied. To spray or not to spray is, of course, t h e crucial question.
Severe Infestations
When t h e a p p e a r a n c e of extensive bare areas or a heavily t h i n n e d crop indicates
a severe infestation the course is clear, but in the case of only slight thinning and a n uneven and patchy distribution of t h e insects, the decision is more difficult. U n - fortunately it is not possible to provide any simple formulae to assist t h e farmer in this situation because too m a n y variables operate to make straightforward prediction feasible.
Field experience has shown t h a t in certain circumstances moderate infesta- tions in cereal may disappear within days if the weather conditions become u n s u i t - able. Unfavourable weather has been observed to destroy webworm populations in crops almost as quickly a n d effectively as chemical methods. Unfortunately these effects c a n n o t be predicted because they depend upon so many variables, for ex- ample t h e severity of t h e conditions (dry or wet periods), the stage of development of the caterpillars and t h e stage of growth of the crop.
All t h a t can therefore be said is t h a t crops planted in circumstances where an infestation is likely (such as on turned in grassy p a s t u r e s ) , should be watched very closely during the emergence and early post emergence periods for signs of damage.
If the initial working is late, the p a s t u r e should be closely examined for webworm before ploughing (see "Precautionary T r e a t m e n t " below.)
An a t t e m p t should be made to assess t h e general level of any infestation a n d whether it is confined to only one or two 625
Journal of Agruculture Vol 4 No 10, 1963
The effect of D.D.T. on webwonn activity. The strip on the left foreground was untreated. It Is covered with weeds and there are almost no wheat plants. On the right Is a plot treated with DJJ.T.
areas within the paddock or spread more generally. Rough estimates should be made of the numbers of plants affected per square yard, and grubs should be dug up in order to assess the stage of develop- ment which has been reached. When this has been done the farmer will be in no doubt, in cases of more severe infestation, t h a t immediate treatment is warranted.
When the infestation is heavy and general treatment of the whole crop is required, the condition of the soil and con- sideration of time, labour and plant will determine whether the aircraft or ground equipment is used.
Patchy Infestations
Where the infestation is confined to patches and only a small proportion of the crop is affected, spot spraying of these patches will give satisfactory results. This is only practical if the paddocks are dry enough for ground plant to be used.
Close inspection will normally indicate what areas should be treated in this manner and a buffer of approximately 1 chain beyond the area of damage should also be sprayed.
The distribution of webworm will closely correspond to the distribution of the grass component, so a knowledge of the condition
of the paddock and some knowledge of the distribution pattern of the grasses before working will be helpful in deciding whether to spot or blanket spray the paddock.
In paddocks which were heavily and uniformly grassed before cropping spot spraying is normally inadvisable.
Light Infestations
When the infestation is obviously light it may be difficult to decide whether spray- ing is worthwhile. The decision will be based on the following considerations:—
• The proportion of plants affected.
Assuming the destruction of these plants, what effect is this likely to have upon yield?
• How many additional plants are likely to be destroyed if the in- festation is not treated?
The second question is far more difficult to answer, as it depends upon:
(a) The Stage of Development of the Caterpillars.—Some natural mor- tality always occurs and is norm- ally highest in the smaller stages;
therefore, if they are small they will not only consume less in a given period but more are likely to die from natural causes.
626
(b) The Stage of Growth of the Crops.
—A vigorously growing and ad- vanced crop will withstand injury from webworm better than an un- healthy or retarded crop.
(c) The Weather Conditions.—A pro- longed period of wet conditions may kill large numbers of web- worm, particularly if they are still small. A period of dryness may also take its toll of the population at this stage.
If no decision has been reached after these factors have been considered, the area should be watched closely for several days. Should the crop loss increase steadily during this period a point will be reached where treatment can no longer be avoided.
Sometimes, however, when the infestation is light the reduction in numbers through natural mortality may make it unnecessary to spray.
Application Rates
The minimum rate of DDT which will provide adequate control depends mainly upon the stage of development of the grubs, the stage of growth of the crop and both the method of application and the weather conditions at the time.
The larger the caterpillars the more difficult they are to kill. The more ad- vanced the crop and the denser it is the less satisfactory will be the insecticide blanket. The windier the conditions the greater the drift and the less satisfactory is the insecticide placement; this is especi- ally the case when aircraft or equipment other than the low level boom is used.
Under such conditions the full DDT application rate is to be preferred:
DDT at equivalent of 1 quart of 20 per cent, emulsiflable concentrate per acre.
(i lb. DDT active ingredient per acre).
When the grubs are generally small and the conditions for spraying very favour- able, lower rates will usually give adequate control:
DDT at li to li pints of 20 per cent, emulsiflable concentrate acre. (5 to 6 oz.
DDT active ingredient per acre).
When it is desired to control red-legged earth mite and lucerne flea as well as web- worm, 1 pint of 20 per cent. DDT may be added to the DDT/Malathion mixture normally used to control these insects:
DDT at 1 pint 20 per cent, emulsiflable concentrate plus i pint DDT/Malathion.
(emulsion containing 20 per cent. DDT and 20 per cent. Malathion).
Applying Insecticides with a low volume spray boom.
627
Journal of Agruculture Vol 4 No 10, 1963
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CULTURAL AND MANAGEMENT PRACTICES
Population studies have shown that the number of webworm infesting crops de- pends largely upon the number of eggs laid, and the number of newly hatched larvae which survive in the paddock before it is ploughed.
The numbers of eggs laid and also this level of survival is, to a large extent, dependent upon the amount of grassy residue present during these critical periods.
Summer Fallow
Little egg laying occurs on fallowed areas and little protection is afforded any larvae which do hatch on such areas. Crops planted on clean fallow are therefore unlikely to be damaged by webworm.
Precautionary Treatment
Weather conditions, soils type and other factors may sometimes make it necessary to plough-in grassland and sow without delay. These same factors may also make it impossible to spray with ground equip- ment should the need arise.
From a knowledge of the moth activity or as a result of careful examination of the pasture before it is ploughed, it is usually possible to estimate the potential danger of webworm occurrence. In these circumstances some interest has been ex- pressed in preventative treatment. General application of such preventative measures cannot be recommended at present but when it is known that high populations of advanced larvae already exist, and when the special circumstances discussed above apply, control will be achieved by spraying the pasture with DDT at the standard rate. For maximum efficiency ploughing should be delayed for about a week after spraying.
It is hoped that an opportunity will be found to check the effectiveness of such measures in field trials next season.
Autumn Fallow
When seeding is delayed for at least three weeks after a thorough initial working little trouble is normally experienced with
webworm. The effectiveness of the method depends upon:
(a) Cleanness of the Working.—If the grass residues are not completely covered some shelter is provided for moths and young larvae, and some subsequent infestation could occur. Burning or heavy grazing prior to turning-in wiJl be of value.
(b) The Length of the Fallow Period.
—Any delay is useful, but the longer the period the better the control. Three weeks is generally satisfactory.
(c) The Stage of Development of the Caterpillars.—When the initial working is late in relation to the opening rains many larvae will be quite well advanced and could even be mature. These are more difficult to kill by any method and delayed planting may, in these circumstances, be less effective.
(d) Weed Control.—In addition to the controlling influence of factors associated with exposure and shelter, starvation is often im- portant. The less weed growth during this period the better.
Burning
Any operation which reduces the amount of grassy residue on a paddock during the critical April-May period is of value. The effectiveness of burning depends on the cleanness of the burn and the time at which the burning is done. Burning is most useful when it is done before the main moth emergence (that is preferably early in April); the cleaner the paddock is at the time of egg laying the better.
Burning can never guarantee freedom from webworm attack but if the burn is a good one and done early webworm num- bers will be reduced. Patchy burns (which are often unavoidable) may result in patchy webworm outbreaks within the crop.
Grazing and Mowing
The effects of these operations are still under investigation but they may be effective in reducing the quantity of grass residue which is present during the autumn.
629
Journal of Agruculture Vol 4 No 10, 1963
Rotation
The likelihood of heavy grass dominance in pastures is increased with longer rota- tions. The danger of webworm infestation accordingly tends to increase with the number of years since the paddock in question was last cropped. Webworm out- breaks occur very rarely in crops planted on stubble, particularly if the stubble is burnt early in the season.
Special Situations
As has been stressed previously, webworm density is likely to be highest where very heavy stands of grasses such as barley or silver grass occur. On many properties there exist small areas which are perman- ently grassed, the best example being salt flats which are fenced-off from the re- mainder of the farm and which normally support heavy stands of salt tolerant barley grass. The role of these high density webworm areas on the farm is still being investigated and at present no general recommendation for treating them can be made. However, heavy grazing of these situations in autumn should help reduce webworm numbers.
ACKNOWLEDGMENT
The work described here is part of a general investigation of the webworm problem in Western Australia, financed by a grant from the Wheat Research Com- mittee of Western Australia. This assist- ance is gratefully acknowledged.
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Journal of Agruculture Vol 4 No 10, 1963
