F. MURESAN, D.MUSTEA
Agricultural Research Station Turda, Str. Agriculturii, 27, 3350, Turda, jud. Cluj, ROMÂNIA

In this paper are presented some methods for estimating of yield losses caused by European Corn Borer (Ostrinia nubilalis Hbn.)  to maize crops. All the methods are based on a large number of weighing and notations during 1998-2000. The 4 methods presented are: frequency of damages plants (K=0,219), mean number larvae of plants damaged (K=0,180 for one larva), degree of damaged plants in according to rating scale (1-5): (2=0,15; 3=0,30; 4=0,50; 5=0,80). In maize crops from Transylvania, the yield losses depending on attack frequency, attack degree and the number of larvae were between 15,4-23,7%, at 10 maize hybrids created at Agricultural Research Station Turda.
This damages caused by Ostrinia nubilalis Hbn. at maize crops, determinated the application of some biological and chemical methods unpolluting the medium and the feeding products. Such as biological methods in this pest control, were used a few species of Trichogramma, the biological products, the chitin inhibitory and the some chemical selective products. The efficiency of chemical products was between 25-71%, of biological products was between 40-65% and of Trichogramma species between41-75%, regarding the reduction attack of Ostrinia nubilalis Hbn.

Key words: Maize, Ostrinia nubilalis Hbn., the yield losses, the treatments

INTRODUCTION
The consequence of application of some incomplete or incorrect technologies, practising monocultures, unrespecting the measures of cultural hygiene as maize stems remained in the field after harvesting, permitted the maintaining of one of the most pest-corn borer (Ostrinia nubilalis Hbn.). The attack of this pest in many ecological areas in Transylvania is about 30-95%(MURESAN et al.,1995; MUSTEA et al.,1995; MURESAN et al.,1999), and the damages produced are considerables, till 40% from beans yield (MUSTEA et al.,1975).
In reducing the population of this pest a special attention was paid to chemical and biological methods that form a tehnological part with an efficiency more than satisfactory in realizing the mentioned obiective. Thus in the countries from UE through aplying the biological and chemical treatments, the corn borer attack was reduced with 40-60 % and yield increase realized was about 2,0-6,0 q/ha(BAICU,1992;GRINBERG  et al,1993).The researches in this stream were performed by MUSTEA(1971);MURESAN (1997);MURESAN et al,(1999);MURESAN(2000);TANCIK et al.,(1999);OZTEMIZ et al., (2000);MARK et al., (2000).
The aspects pointed in this paper refer to assessment of chemical(insecticides) and biological(products and entomophagues) treatments efficiency depending on: attack frequency, the number of holes/plant, the number of larvae/plant, the number of cavities/stem, the cavities lengh, obtained yield.

MATERIALS AND METHODS
Between 1998-2000 were performed studies for 10 maize hybrids created and cultivated at the Agricultural Research Station Turda and in the influenced area of this, regarding the natural attack of corn borer(Ostrinia nubilalis Hbn.).The studied aspects were refered to:
a) the frequency and degree of attack;
b) the number of larvae/plant;
c) the yield losses regarding these aspects.
In an experience with 11 variants, three repetitions ,located after randomizated blocks method, where the size of one lot was 7m² ,it was in view the efficiency of biological and chemical treatments in reducing of corn borer population, through different methods of assessment:
a) number of holes/plant
b) number of larvae/plant
c) number and length of cavities / stem
d) the yield increased
The chemical treatments were done using :Regent 200 SC-0,1 l/ha, Bulldock 025 EC-0,3 l/ha, Conquest 25 WG-0,1 l/ha, the biological ones using products were done using: Thuringin 6000-4.0 kg/ha, Dipel 2x-1,5 kg/ha, Match 050 EC-1,2 l/ha, Insegar 25 WP-0,3 kg/ha and those with entomophagues using: T.maidis,T.evanescens,T.dendrolimi-200000 individuals/ha. The chemical treatments and the biological ones were applied from the appearance of the first adults of corn borer at the sexual pheromons traps (plant phenophasis of 6-10 leaves) and at the maximum flight of the pest. The notes referring to the all studied aspects mentioned were done at the harvesting.
The utilized calculation methods were: the variant analysis, the  simple regressions, the Abbot formula for efficiency, and for calculation of crops losses depending on the frequency and the degree of attack, the number of larvae, were utilized the next formulas: P % = F % x Kf  (0,219); P % = F % x K l (0,18) x no. larvae/plant and P % = [ F% x K i₂ (0,15)] + [ F % x K i₃(0,30)] + [ F% x K i₄(0,50)] + [ F % x K i₅ (0,80)], where Kf, Kl and K i_2-5 are values for each element.

RESULTS AND DISCUSSION
The observation in the mentioned period (1998-2000) demonstrated that the frequency of natural attack of corn borer (Ostrinia nubilalis Hbn.) maize hybrids cultivated at A.R.S. Turda and in the influenced area, was about 35,7 and 69,3 %, this pest appearing in the first decade of June, mentaining in this culture  till august (tab. 1 and fig. 1 ). Also, these  observations   showed that indeed exist a relation between the yield losses and the attack frequency and of  number of larvae/plant, the strongest in the attack, the greatest are the yield losses ( tab .2 ).
The assessment of efficiency of biological and chemical treatments in reducing the corn borer population were done regarding:
a) the number of holes/ plant;
b) the number of larvae/plant;
c) the number and length of cavities/stem;
d) obtained yield
At maize hybrids cultivated in the influenced area of A.R.S. Turda the yield losses depending on frequency of ECB attack was between 11,4 – 15,2 % (fig.2 ). From the statistical calculation performed, resulted that at the hybrids with attack very strong, the yield losses was significantly. The relation between the attack frequency and the yield losses at maize hybrids mentioned very high, the value of correlation  coefficient is significant ( 0,987*** )( fig. 3 ).
Also, the relation between the average number larvae / plant and the yield losses is tigh the value of  correlation coefficient is very significant (  0,993*** )( fig.4 ).
The chemical and biological treatments assured a reduction very significant of number of cavities / plant, of number holes / plant, of number larvae / plant ( fig.5 ) and of length of cavities from the maize plants ( fig.6 ) comparatively with untreated variant.
The yield obtained was very significantly comparatively with untreated variant, at the products: Regent, Match, Dipel, T. maidis, T. evanescens, the efficiency of these products and entomophagues was between 68 – 80 % ( fig.7 ).
The treatments performed with the 3 species of Trichogramma were reduced very strong the attack of ECB comparatively with untreated variant, than the products Dipel, Match, Insegar, Thuringin, and then the chemical treatments ( Conquest, Bulldock, Regent) ( fig.8 ).

CONCLUSIONS
1.) The frequency and the degree of corn borer attack (Ostrinia nubilalis Hbn.) on maize hybrids cultivated at A. R. S. Turda and in the influence area ,of this between 1998-2000 varied between 35,7-69,3 % recording yield losses significantly, which were correlated with the attack frequency and the number of larvae/plant. In this way the yield losses recorded were between 12,7-23,7 % .
2.) Besides these direct losses which consist in reducing the biological productive potential, it is recorded also indirect losses by breaking the attacked plants, their falling, contributing to multiply the animal pests and making difficult the mechanical harvesting.
3.) As assessment methods of chemical and biological treatments in reducing the corn borer population and even the damages were noted the observation regarding: the number holes/plant, the number of larvaes / plant, the number and the length of cavities / stem and the yield obtained.
4.) The efficiency of chemical treatments (Regent 200 SC, Bulldock 025 EC, Conquest 25 WG) was between 25-71 %, of those biological ones using products (Thuringin 6000,Dipel 2x, Insegar 25 WP, Match 050 EC) between 40-65 % and with entomophagues (T.maidis,  T.evanescens, T.dendrolimi) 41-75 % regarding the reducing attack of Ostrinia nubilalis Hbn.

REFERENCES
1.) MURESAN,Felicia,2000 – Efectul tratamentelor cu produse noi în reducerea sfredelitorul porumbului (Ostrinia nubilalis Hbn), Rev., Agricultura Transilvana – cultura plantelor de câmp,Nr.3, p 98-101.
2.) JANO TANCIK,FRANJA BACA,LUDOVIT CAGAN,ŽIVICA RADIN,1999 –    Effect of sweet maize sawing dates on the degree of the European Corn Borer (Ostrinia nubilalis Hbn.) infestation in north – west VOJVODINA, Integrated Protection of field Crops,p.75 – 83.
3.) MARK,G.W.,MICHAEL,P.,H.,2000 – Biological Control of Ostrinia nubilalis using inoculative releases of Trichogramma ostriniae .„Egg  Parasitoid News”, nr.12, p.23, Braunschweig,Germania.
4.)MURESAN,Felicia,MUSTEA,D.,1999 – Efectul aplicarii unor metode biologice în combaterea sfredelitorului Ostrinia nubilalis Hbn. din culturile de porumb.Probl.prot.plantelor XXVII(2);p.195 – 205.
5.) OZTEMIZ,S.,KORNOSOR,S.,2000 – Biological Control of Ostrinia nubilalis Hübner (Lepidoptera;Pyralidae) by Trichogramma evanescens Westwood (Hymenoptera;Trichogrammatidae) and its Natural Parasitization Rate on Maize in Çukurova Region of Turkey.„Egg Parasitoid News” ,nr.12,p.22, Braunschweig, Germania.