Journal of the Department of Agriculture, Journal of the Department of Agriculture, Western Australia, Series 3 Western Australia, Series 3
Volume 3
Number 5 September-October,1954 Article 6
9-1954
Insect pest - the potato moth Insect pest - the potato moth
C F.H Jenkins
Department of Agriculture
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Recommended Citation Recommended Citation
Jenkins, C F.H (1954) "Insect pest - the potato moth," Journal of the Department of Agriculture, Western Australia, Series 3: Vol. 3: No. 5, Article 6.
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THE POTATO MOTH
(Gnorimoschema operculella, Zell.)
T
HE potato m o t h is almost a world-wide pest and one which under favourable con- ditions can cause great havoc to both growing and stored potatoes. The term"potato fly" is almost universally applied to this pest by growers, but the insect is a true moth and so the n a m e fly is quite inaccurate.
The adult m o t h is a n inconspicuous creature measuring only a little over i i n . across the spread wings. The front pair of wings are grey a n d slightly speckled with darker markings. In some lights they exhibit a distinct metallic sheen. The hind wings are a uniform light grey and fringed with extremely fine hairs.
LIFE HISTORY
From 100 to 300 tiny white eggs may be laid by each female and, under field conditions, these will be laid on t h e under- side of t h e leaves or upon exposed tubers.
The eggs may take from five days to several weeks to h a t c h according to weather conditions.
The fully-fed caterpillar is pale green- ish-white in colour, measures a little over i i n . in length and takes about 17 days under favourable circumstances to reach t h e pupal stage. Before pupating, t h e caterpillar spins a light silken cocoon and these cocoons may be located, according to whether potato tops or tubers are i n - fested, either in curled dry leaves, pro- truding from feeding tunnels, in t h e eyes of potatoes, attached to t h e sides of bags in which potatoes are stored or under clods in t h e field. The l e n g t h of t h e pupal period is a little over a week under good conditions, but may be protracted for a m o n t h or more.
x
Pig- 1. Potato Moth (magnified six times) Photo, Virginia Experiment Station Bulletin.
TYPE OF INJURY In Growing Plants.
I n the case of foliage infestations, t h e young caterpillars quickly eat into t h e leaf and feed between t h e upper and lower surfaces causing colourless parchment-like
5 S & ~ 5 5 £ . u p?e r P |c t u r e shows potato leaflets showing various stages of "mining" by potato moth caterpillars. In the lower picture, terminal Injury to young potato planta
Is shown
Photo, Virginia Experiment Station Bulletin.
532
p a t c h e s to appear. From t h e leaves, the caterpillars may work down the leaf stalks into the stems causing various portions of the plant to wilt and die.
I n Tubers.
When tubers are attacked, the cater- pillars usually commence working near an eye and t h e first indication of trouble is -often the appearance of black specks of excreta in this region. When t h e attack i s of some standing, irregular worm-like tracks may be seen b e n e a t h t h e potato skin showing t h e feeding movements of t h e caterpillars, and t h e cutting of t h e tuber will reveal deep tunnels r u n n i n g in a l l directions.
HABITS AND SEASONAL OCCURRENCE The potato moth is nocturnal in its liabits and so is often overlooked when light infestations only are present. I n the field, the eggs laid are mainly upon t h e growing tops, but if any tubers are exposed
or any deep cracks allow the moths to p e n e t r a t e the soil, t h e n the developing potatoes may be attacked.
It is undoubtedly during dry summer conditions t h a t t h e potato moth is most
•destructive for it is t h e n t h a t the life cycle is completed most quickly and t h a t t h e dry, cracked soil provides t h e most ready
access to the s u b t e r r a n e a n portion of t h e p l a n t . Not only will t h e moths enter
cracks in t h e soil in search of tubers, but when t h r o u g h drought a n d "grub" a t - tack, t h e potato tops wither and die, the caterpillars will leave t h e drying foliage and crawl under the clods to a t t a c k t h e crop waiting to be dug.
CONTROL In the Field.
Cultural Methods.
Tubers should be planted as deeply as possible so t h a t , in the event of soil cracks, moths and caterpillars will not have easy access, and so t h a t developing tubers will not break surface as they grow and m a t u r e .
The constant hoeing and cultivation of t h e ground and t h e hilling up of t h e soil along t h e rows will also prevent cracking a n d keep the tubers well protected. A plentiful water supply is probably t h e
main cultural factor in checking t h e pest, for moist soil seldom cracks and offers considerable resistance to penetrating
"grubs" or moths and is usually favour- able to sturdy plant growth.
For t h e foregoing reasons it is mainly summer grown potatoes t h a t are severely attacked. Cold moist soil is so detri- mental to the development of the pest, t h a t even when infested tubers are planted under such conditions, little fear of sub- sequent losses need be feared.
Chemical Treatment.
The growing crop should be sprayed or dusted with DDT as soon as m o t h activity is observed. A bad infestation of cater- pillars in the tops will not only affect ad- versely the general growth of t h e p l a n t s and so reduce the yield but will also p r o - vide large numbers of grubs to attack t h e tubers as they lie on the ground after digging. Dust or spray preparations may be used to treat field crops. If dust is used, a 2% mixture is recommended a t from 25 to 50 lbs. of dust per acre, de- pending on the size of the plants and t h e severity of the infestation. Where a spray mixture is applied, a 0.1% p r e - paration is suggested at up to 100 gallons to the acre. If a low volume spray unit is used, however, a good cover can be ob- tained with 10 gallons. If a 20% DDT emulsion is used for low volume spray- ing, 2 pints of emulsion per acre will be required for a i lb. application of actual DDT. The emulsion can be diluted in ac- cordance with t h e output of the spray unit.
Where large plants are badly infested with moth or conditions are favourable for moth breeding, t r e a t m e n t s , whether dust or spray, should provide for 1 lb. of actual DDT per acre. On small plants and where infestations are light, less DDT may be effective.
In Store.
After being dug, potatoes should be bagged and removed from the field as soon as possible. If they must be left out over- night, the bags should be sewn a n d turned upside down to make it increasingly dif- ficult for wandering moths to reach the tubers. On no account should potato tops be used as is so often the case, to cover t h e mouth of the filled bag awaiting sew-
Fig. 3.—Left: Severely Infested tuber cut to show potato moth larvae In tunnels Right: External view of potato moth Injury. (Magnified by two.)
Photo, Virginia Experiment Station Bulletin.
ing, for caterpillars will quickly leave t h e wilting foliage a n d crawl down to be a future trouble in t h e stored potatoes.
T h e tubers m a y be dusted in store with 2 % DDT or proprietary derris dust.
How to Dust Tubers.
(a) Dust t h e empty bag on the inside e i t h e r by m e a n s of a dusting machine or by h a n d , a n d as t h e tubers are introduced f u r t h e r puffs or handfuls of dust should be added. Using this method, u p to ilb. of dust is required per bag.
(b) A convenient method of applying d u s t is to g a t h e r potatoes into a bucket before bagging. An ounce of dust should be placed in t h e bottom of each tin, and a s each bucketful is tipped into the bag t h e dust will be spread t h r o u g h the tubers.
A few ounces of dust should be sprinkled i n t o t h e m o u t h of t h e bag before sewing.
(c) When t h e tubers are dug and whilst lying on t h e ground they may be covered w i t h a film of dust applied by means of a h a n d dusting m a c h i n e a t the r a t e of 20- 25 lbs. per acre. T h e potatoes may t h e n be bagged in t h e ordinary way.
T h e storing of potatoes in bags previous- ly dipped in 2 % D.D.T. emulsion and al- lowed to dry will also reduce t h e chance of m o t h infestation.
I m p o r t a n t : Do not use a DDT dust stronger t h a n 2%.
Do not use BHC or " G a m m e x a n e " dust, or composite dusts containing BHC, as t h i s material m a y t a i n t the tubers.
Residue Hazards.
Tests carried out in conjunction with t h e D e p a r t m e n t of Public Health a n d t h e Government Chemical Laboratories h a v e shown t h a t no dangerous residues will be present on potatoes treated in accordance with the above instructions.
Fumigation. •
Where a severe infestation is feared in stored potatoes t h e tubers may be fumi- gated in any gas t i g h t container such a s a tank, room or drum, with carbon bi- sulphide. The liquid fumigant gives off a h e a v i e r - t h a n - a i r gas a n d so should be applied to t h e top of t h e potato stack.
Pieces of bagging soaked in the liquid may be placed on the stack or the fumi- gant allowed to evaporate from shallow trays. The a m o u n t of fumigant required is 2 lb. per t h o u s a n d cubic feet of space o r for small lots about two tablespoonsful to the bag. If serious gas leakages a r e suspected, t h e n more fumigant should be used. The potatoes should be kept sealed for a t least 24 hours. If t r e a t m e n t is being given in t h e summer m o n t h s a n d a t a n k or d r u m is being used, a shady spot should be chosen otherwise: t h e potatoes may be scorched a n d "cooked" by t h e heat.
534
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